Climatology

UNIT-I

CLIMATE AND HUMAN COMFORT

COMPONENTS OF CLIMATE

The climate system, as defined is an interactive system consisting of five major components: the

atmosphere, the hydrosphere, the Cryosphere, the land surface and the biosphere.

Atmosphere

The atmosphere is the most unstable and rapidly changing part of the system. The Earth’s dry

atmosphere is composed mainly of nitrogen (N2, 78.1% volume mixing ratio), oxygen (O2, 20.9%

volume mixing ratio, and argon (Ar, 0.93% volume mixing ratio).

These gases have only limited interaction with the incoming solar radiation and they do not interact

with the infrared radiation emitted by the Earth. However there are a number of trace gases, such as

carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone (O3), which do absorb and emit

infrared radiation. These so called greenhouse gases, with a total volume mixing ratio in dry air of less

than 0.1% by volume, play an essential role in the Earth’s energy exchange.

Moreover the atmosphere contains water vapour (H2O), which is also a natural greenhouse gas. Its

volume mixing ratio is highly variable, but it is typically in the order of 1%. Because these greenhouse

gases absorb the infrared radiation emitted by the Earth and emit infrared radiation up- and

downward, they tend to raise the temperature near the Earth’s surface. Water vapour, CO2 and O3

also absorb solar short-wave radiation.

The atmospheric distribution of ozone and its role in the Earth’s energy budget is unique. Ozone in the

lower part of the atmosphere, the troposphere and lower stratosphere, acts as a greenhouse gas.

Higher up in the stratosphere there is a natural layer of high ozone concentration, which absorbs solar

ultra-violet radiation.

In this way this so-called ozone layer plays an essential role in the stratosphere’s radiative balance, at

the same time filtering out this potentially damaging form of radiation.

Beside these gases, the atmosphere also contains solid and liquid particles (aerosols) and clouds,

which interact with the incoming and outgoing radiation in a complex and spatially very variable

manner. The most variable component of the atmosphere is water in its various phases such as

vapour, cloud droplets, and ice crystals.

Water vapor is the strongest greenhouse gas. For these reasons and because the transition between

the various phases absorb and release much energy, water vapour is central to the climate and its

variability and change.

Hydrosphere

The hydrosphere is the component comprising all liquid surface and subterranean water, both fresh

water, including rivers, lakes and aquifers, and saline water of the oceans and seas.

Fresh water runoff from the land returning to the oceans in rivers influences the ocean’s composition

and circulation.

The oceans cover approximately 70% of the Earth’s surface. They store and transport a large amount

of energy and dissolve and store great quantities of carbon dioxide.

Their circulation, driven by the wind and by density contrasts caused by salinity and thermal

gradients . is much slower than the atmospheric circulation. Mainly due to the large thermal inertia of

the oceans, they damp vast and strong temperature changes and function as a regulator of the Earth’s

climate and as a source of natural climate variability, in particular on the longer time-scales.

Cryosphere

The Cryosphere, including the ice sheets of Greenland and Antarctica, continental glaciers and snow

fields, sea ice and permafrost, derives its importance to the climate system from its high reflectivity

(albedo) for solar radiation, its low thermal conductivity, its large thermal inertia and, especially, its

critical role in driving deep ocean water circulation.

Because the ice sheets store a large amount of water, variations in their volume are a potential source

of sea level variations

Land surface

Vegetation and soils at the land surface control how energy received from the Sun is returned to the

atmosphere. Some is returned as long-wave (infrared) radiation, heating the atmosphere as the land

surface warms.

Some serves to evaporate water, either in the soil or in the leaves of plants, bringing water back into

the atmosphere. Because the evaporation of soil moisture requires energy, soil moisture has a strong

influence on the surface temperature.

The texture of the land surface (its roughness) influences the atmosphere dynamically as winds blow

over the land’s surface. Roughness is determined by both topography and vegetation. Wind also blows

dust from the surface into the atmosphere, which interacts with the atmospheric radiation.

Biosphere

The marine and terrestrial biospheres have a major impact on the atmosphere’s composition. The

biota influence the uptake and release of greenhouse gases. Through the photosynthetic process,

both marine and terrestrial plants (especially forests) store significant amounts of carbon from carbon

dioxide. Thus, the biosphere plays a central role in the carbon cycle, as well as in the budgets of many

other gases, such as methane and nitrous oxide.

Other biospheric emissions are the so-called volatile organic compounds (VOC) which may have

important effects on atmospheric chemistry, on aerosol formation and therefore on climate.

Because the storage of carbon and the exchange of trace gases are influenced by climate, feedbacks

between climate change and atmospheric concentrations of trace gases can occur.

The influence of climate on the biosphere is preserved as fossils, tree rings, pollen and other records,

so that much of what is known of past climates comes from such biotic indicators.

FACTORS AFFECTING CLIMATE

Solar Radiation

Earth sun relationship

Distance from the sea

Presence of ocean currents

Wind

Topography

Proximity to equator

Solar Radiation.

The Sun is the dominating influence on climates,as the earth receives almost all its energy from the

sun.

The radiation incident on a surface varies from moment to moment depending on its geographic

location (latitude and longitude of the place), orientation, season, time of day and atmospheric

conditions.

About 25 % of the solar radiation reaching the atmosphere is reflected back into space, in the form of

refection from ground(5%) and reflected by clouds (20%) another 25 % is absorbed by the

atmosphere, and the rest 50 % in the form of diffuse radiation on the ground(23%) and Direct

radiation on the ground (27%) is absorbed by Earth's surface.

Therefore the total amount of heat absorbed by the earth is balance by a corresponding heat loss.

Earth sun relationship

The earth rotates around its own axis ,each rotation making one 24 hour day. The axis of rotation is

the plane of the elliptical orbit at an angle of 66.50 .i.e. a tilt of 23.50 from the normal.

Maximum intensity is received on a plane normal to the direction of radiation. If the axis of earth were

rectangular to the plane of the orbit,it would always be on equatorial regions which are normal to the

direction of solar radiation.

Due to the tilted position the area receiving maximum intensity moves North and South between the

tropic of Cancer and tropic of Capricorn. This is the main cause for seasonal changes.

On June 21st areas along Tropic of cancer(23.50 N) are normal to the sun's rays and on December 21st

areas along Tropic of Capricorn (23.50 S) are normal to the sun's rays.

On 21st March and 23rd September areas along the Equator are normal to the sun's rays and these are

equinox days (equal day and night) for all parts of the earth.

The earth sun relationship affects the amount of radiation received at a particular point on the earth's

surface in three ways

1. The cosine law: Which states that the intensity of radiation on unit area is lesser as radiation is

distributed over a larger area because the earth is tilted.

2. Atmospheric depletion: The percentage of radiation absorbed by atmosphere varies with

varying altitude angle. The lower the altitude angle ,the longer the path of radiation through

atmosphere ,thus a smaller part of radiation reaches the earth surface .

3. Duration of sunshine, .i.e. the length of daylight period

Distance from sea

The sea affects the climate of a place. Coastal areas are cooler and wetter than inland areas. Clouds

form when warm air from inland areas meets cool air from the sea.

The center of continents are subject to a large range of temperatures. In the summer, temperatures

can be very hot and dry as moisture from the sea evaporates before it reaches the center of the

continent.

Presence of ocean currents

Ocean currents can increase or reduce temperatures. Because ocean currents circulate water

worldwide, they have a significant impact on the movement of energy and moisture between the

oceans and the atmosphere. As a result, they are important to the world’s weather.

The Gulf Stream for example is a warm current that originates in the Gulf of Mexico and moves north

toward Europe.

This means that the air coming from the Gulf of Mexico to western Europe is also warm. However, the

air is also quite moist as it travels over the Atlantic ocean. This is one reason why western europe(ex

Britain) often receives wet weather.

The Gulf Stream keeps the west coast of Europe free from ice in the winter and, in the summer

warmer than other places of a similar latitude.

Wind

Winds are basically convection currents in the atmosphere,tending to even out the differential heating

of various zones. The pattern of movement is modified by the earth's rotation.

Near the equator the winds prevail blowing north easterly winds north of equator and south easterlies

south of equator also known as north east and south east trade winds.

Between 30 and 60 deg North and south of equator strong westerly winds prevail known as mid

latitude westerlies.

Further towards the poles ,pole winds prevail in the north easterly polar winds in the north and south

easterly polar winds in the south.

During the course of each year the global wind patterns shift from North to South and back again .As a

consequence of this annual shift most regions of the earth experience seasonal changes in

temperature,wind direction and rainfall.

Topography

The force direction and moisture contents of air flow are strongly influenced by topography.

Air can be diverted,funneled by mountain ranges. Air flow deflected upwards ,as it cools,releases its

moisture content. A descending air mass will very rarely give any precipitation ,there fore rainfall

characteristics vary sharply between windward and leeward slopes of mountain ranges.

The humidity of air will vary with the rate of evaporation of moisture from the surface below .i.e it

depends on the availability of water to be evaporated.

Proximity to equator

The proximity to the equator affects the climate of a place.

The equator receives the more sunlight than anywhere else on earth so the areas near equator are

the hottest and this reduces gradually as we move away from the equator and is coldest at the poles.

Human Influence

The factors above affect the climate naturally. However, we cannot forget the influence of humans on

our climate.

We have been affecting the climate since we appeared on this earth millions of years ago. In those

times, the affect on the climate was small. ex. Trees were cut down to provide wood for fires.

The Industrial Revolution, starting at the end of the 19th Century, has had a huge effect on climate.

The invention of the motor engine and the increased burning of fossil fuels have increased the amount

of carbon dioxide in the atmosphere.

The number of trees being cut down has also increased, meaning that the extra carbon dioxide

produced cannot be changed into oxygen.

ELEMENTS OF CLIMATE

Air temperature /Temperature

Air temperature is the most important element of climate, measured by the dry bulb temperature

(DBT). This will determine the convective heat dissipation, together with any air movement.

A thermometer mounted inside a louvered wooden box known as Stevenson screen,at a height of 1.2

to 1.8 m above ground level, can be used to take reading at specified times of the day

A Maximum-minimum thermometer can be used to take measurement of the maximum and

minimum temperature reached in the past 24 hours

Thermograph is used to give continuous graphic reading of temperature variations

Air movement/wind

Air movement is measured by its velocity (v, in m/s) and it also affects the evaporation of moisture

from the skin, thus the evaporative cooling effect.

Humidity

Humidity of the air also affects evaporation rate. This can be expressed by relative humidity (RH, %),

absolute humidity or moisture content (AH, g/kg), or vapour pressure (p, in kPa).Air humidity, which

represents the amount of moisture present in the air, is usually expressed in terms of ‘relative

humidity’.

Relative humidity is defined as the ratio of the mass of water vapour in a certain volume of moist air at

a given temperature, to the mass of water vapour in the same volume of saturated air at the same

temperature; it is normally expressed as a percentage.

It varies considerably, tending to be the highest close to dawn when the air temperature is at its

lowest, and decreasing as the air temperature rises.

Humidity is measured with a wet and dry bulb Hygrometer

The wet bulb and dry bulb readings are measured the difference between the two is called 'wet-bulb

depression' .Having made these two readings corresponding RH can be found out by using a

psychrometric chart or a special slide rule

Vapour pressure is another indicator for atmospheric humidity but rarely used in practical work.

Solar Radiation

Solar radiation is the intensity of sun rays falling per unit time per unit area and is usually expressed in

Watts per square meter (W/m2).

It is the intensity of sun rays falling per unit time per unit area and is usually expressed in Watts per

square meter (W/m2).

A simple sunshine recorder can be used to register the duration of sunshine which can be expresssed

in mumber of hours per day as an average for each month

Sophisticated instruments such as solarimeter, heliometer ,actinometer and pyranometer are also

used.

Expressed in W/ m2 and for measurements over a longer period J/m2

Radiation exchange

Radiation exchange will depend on the mean temperature of the surrounding surfaces (weighted by

the solid angle subtended by each surface), referred to as the mean radiant temperature (MRT) or on

the presence of strong mono directional radiation, e.g. from the sun.

The mean radiant temperature cannot be measured directly, but it can be approximated by globe

temperature measurements.

The globe thermometer is a mat black copper sphere, usually of 150 mm diameter, with a

thermometer located at its center . Positioned in a room, after equilibrium is reached (in 10-15

minutes) the globe will respond to the net radiation to or from the surrounding surfaces.

If radiation is received, then GT>DBT; GT<DBT indicates that the surrounding surfaces are cooler than

the air, radiation is emitted. In still air MRT = GT, but a correction for air movement of v velocity (in

m/s) is possible

Precipitation

Precipitation includes water in all its forms rain, snow, hail or dew deposited from the atmosphere. It

is usually measured in millimeters (mm) by using a rain gauge.

If rain is associated with strong wind it is called driving rain, as exposure of surfaces and fenestration

to this will have a marked effect on performance of a building.

The driving rain index characterizes a given location and expresses the degree of exposure. It is a

product of annual rainfall and the annual average wind velocity expressed in m2/s.

Up to 3m2/s the location is considered sheltered,between 3 and 7 m2/s exposure is moderate and if

over 7m2/s is considered severe.

Sky conditions

Sky conditions are usually described in terms of presence or absence of clouds. On average, two

observations are made per day ,when proportions of sky covered by cloud is expressed as a

percentage.

A single average figure giving sky conditions for a typical day of a given month may conceal significant

differences e.g. between morning and afternoon conditions ,which may affect the design of

roofs,overhangs and shading devices.

Vegetation

Vegetation is generally regarded as a function of climate,but vegetation can in turn influence the local

or site climate. It is an important element in the design of outdoor spaces providing sun-shading and

protection from glare

CLASSIFICATION OF TROPICAL CLIMATES

It is not possible to define accurately the boundary between climatic zones, as variations are usually

gradual, but it is usually possible to define which zone, or area of transition, a particular site lies in.

In 1953 G.A. Atkinson developed a classification based on two factors,temperature and humidity,

which has since become widely accepted.

Atkinson defined three major zones and three sub-groups:

(1) Warm/humid equatorial; sub-group warm/humid island or trade wind climate.

(2) Hot/dry desert or semi-desert; sub-group hot dry maritime desert.

(3) Composite or monsoon climate; sub-group tropical island climate.

Warm Humid Climate

Warm humid equatorial climates occur in a belt extending roughly 15° either side of the equator.

There is very little seasonal variation, other than the amount of rain, which is high throughout the

year, and the incidence of gusty winds and electric storms.

Mean maximum DBT is between 27 and 32°C.

Night minimum varies between 21 and 27°.

RH is usually high at around 75%, but may vary between 55 and 100%. Sky conditions are fairly cloudy

throughout the year, although luminance can vary considerably.

Solar radiation is partly reflected and partly scattered by cloud and the high humidity. The latter also

reduces outgoing radiation.

Wind velocities are generally low.

Vegetation grows quickly, and the high humidity,encourages algae and rusting. Mosquitoes, usually

malarial, and other insects abound.

Warm humid island climates differ from the above in that night time temperature minima may be

slightly lower, and humidity is more variable.

Skies are more likely to be clear, and solar radiation stronger and more direct.

Winds are frequently constant at 6-7m/s, and provide relief from temperature and humidity.

Salt in the atmosphere encourages corrosion.

The greatest difference is the risk of cyclonic winds, which are frequent, unmitigated, and destructive.

Precipitation may be less, but deluges are similar.

Hot Dry Climate

Hot dry desert climates occur in two belts between 15 and 30° N/S.

There are two seasons, one hot, one cooler.

DBT maxima range from 43-49°C in the hot season and 27-32° in the cool.

Night mean minima are 24-30° hot, 10-18° cool.

RH varies from 10% to 55%.

Precipitation is slight and variable.

Flash storms may occur, although droughts of several years are possible.

Sky conditions are normally clear, with limited luminance which may be further reduced by dust

storms.

High glare and luminance may be caused by white dust haze.

Solar radiation is strong but long wave re-radiation releases heat at night into the cold sky.

Winds are usually local and turbulent.

High day temperatures and rapid night cooling may cause materials to break up.

In maritime desert climates the DBT maxima are usually lower, but humidity tends to remain high, due

to evaporation from the sea.

These climates are generally regarded as among the most uncomfortable. Kuwait is an example

Composite or monsoon climates

Composite or monsoon climates usually occur in large land masses near the tropics of Cancer and

Capricorn.

Approximately 2/3rds of the year is hot/dry, and the other third is warm/humid.

Localities further north and south sometimes have a third season of cool/dry.

Typical DBT values are:

hot dry warm/humid cool/dry

Day mean max 32-43 27-32 up to 27

Night mean max 21-27 24-27 04/10/11

RH is 20-55% during dry seasons, and 55-95% during the wet.

Monsoon rains can be heavy and prolonged.

Annual rainfall varies between 500-1300mm, with little or no rain during the dry season.

Sky conditions vary considerably with the seasons; heavily overcast and dull during monsoons, clear

and dark blue during the dry. Glare increases towards the end of the dry season, due to dust haze.

Solar radiation varies between conditions found in the warm-humid and the hot-dry desert climates.

Winds are hot and dusty during the dry season, and steady, humid and often strong during the

monsoon.

Seasonal changes in RH tend to weaken building materials. Termites are common.

HUMAN BODY HEAT BALANCE

The human body continuously produces heat. This metabolic heat production can be of two kinds:

• basal metabolism, due to biological processes which are continuous and non-conscious

• muscular metabolism, whilst carrying out work, which is consciously controllable (except in

shivering).

Table below shows some typical metabolic rates, which can be expressed as power density, per unit

body surface area (W/m²), as the power itself for an average person (W) or in a unit devised for

thermal comfort studies, called the met. 1 met = 58.2 W/m². For an average sized man this

corresponds to approximately 100 W.

activity met W/m² W(av)

sleeping 0.7 40 70

reclining, lying in bed 0.8 46 80

seated, at rest 1 58 100

standing, sedentary work 1.2 70 120

very light work (shopping, cooking, light industry) 1.6 93 160

medium light work (house~, machine tool ~) 2 116 200

steady medium work (jackhammer, social dancing) 3 175 300

heavy work (sawing, planing by hand, tennis) up to 6 350 600

very heavy work (squash, furnace work) up to 7 410 700

Table 2: heat produced by human body

The heat produced must be dissipated to the environment, or a change in body temperature will

occur.

The deep body temperature is about 37°C, whilst the skin temperature can vary between 31°C and

34°C under comfort conditions.

Variations occur in time, but also between parts of the body, depending on clothing cover and blood

circulation.

There is a continuous transport of heat from deep tissues to the skin surface, from where it is

dissipated by radiation, convection and evaporation – and to a lesser extent by Conduction.

Convection is due to the heat transmission from the body to the air in contact with the skin or clothing

which then rises and is replaced by cooler air. The rate of convective heat loss is increased by a faster

rate of air movement,by a lower air temperature and a higher skin temperature.

Radiant heat loss depends on the temperature of the body surface and the temperature of opposing

surfaces

Evaporation heat loss is governed by the rate of evaporation ,which depends on humidity in air.(Dryer

the air,the faster the evaporation) and the moisture available for evaporation. Evaporation takes place

in lungs through breathing and on the skin as imperceptible perspiration and sweat.

Conduction depends on the temperature difference between the body surface and the object the

body is in direct contact with.

The thermal balance of the body can be expressed by an equation

Met- Evp (+ or -)Cnd(+ or -) Cnv (+ or -) Rad =ΔS

Met-Metabolism

Evp- evaporation

Cnd-Conduction

Cnv-Convection

Rad-Radiation

ΔS=Change in heat stored

Thermal balance exists when ΔS is Zero

If ΔS is positive, the body temperature increases, if negative, it decreases.

The heat dissipation rate depends on environmental factors, but the body is not purely passive, it is

homoeothermic: it has several physiological regulatory mechanisms.

To warm conditions (or increased metabolic heat production) the body responds by vasodilation:

subcutaneous blood vessels expand and increase the skin blood supply, thus the skin temperature,

which in turn increases heat dissipation.

If this cannot restore thermal equilibrium, the sweat glands are activated, the evaporative cooling

mechanism will operate. Sweat can be produced for short periods at a rate of 4 L/h, but the

mechanism is fatigable.

The sustainable rate is about 1 L/h. Evaporation is an endothermic process,it absorbs heat at the rate

of some 2.4 MJ/L (= 666 Wh/L).

When these mechanisms cannot restore balance conditions, inevitable body heating, hyperthermia

will occur.

When the deep body temperature reaches about 40°C, heat stroke may develop.

This is a circulatory failure (venous return to the heart is reduced) leading to fainting.

Early symptoms are: fatigue, headache, dizziness when standing, loss of appetite, nausea, vomiting,

shortness of breath, flushing of face and neck, rapid pulse rate (up to 150/min), glazed eyes, as well as

mental disturbances, such as poor judgment, apathy or irritability.

At heat stroke the temperature rapidly rises to over 41°C, sweating stops, coma sets in and death is

imminent.

Even if a person is saved at this point, the brain may have suffered irreparable damage. At about 42°C

death would probably occur.

To cold conditions the response is firstly vasoconstriction: reduced circulation to the skin, lowering of

skin temperature, thus reduction of heat dissipation rate. (Associated with this goose-pimples may

appear, an atavistic phenomenon: the erection of hair, which would make the fur a

better thermal insulator.)

If this is insufficient, thermogenesis will take place:muscular tension or shivering, thus increased

metabolic heat production.

Shivering can cause up to tenfold increase in heat production. The deep body tissues remain at the

normal 37°C.

Body extremities, fingers, toes, ear lobes may be starved of blood and may reach temperatures below

20°C, or in severe exposure may even freeze, before deep body temperature would be affected.

When these physiological adjustments fail to restore thermal equilibrium, hypothermia, i.e. inevitable

body cooling will occur.

The deep body temperature may drop to below 35°C. Death usually occurs between 25 and 30°C

(except under medically controlled conditions).

Even if hypothermia is not reached, continued exposure to cold conditions, which require full

operation of vasomotor and thermogenetic controls, can cause mental disturbances (insufficient

blood supply to the brain); willpower is “softened” and conscious control gives way to hallucinations,

drowsiness and stupor.

Table below summaries the critical body temperatures. The skin should always be at a temperature

less than the deep body, and the environment should be below the skin temperature, in order to allow

adequate, but not excessive heat dissipation.

The environmental conditions which allow this,would ensure a sense of physical well-being and may

be judged as comfortable.

Factors of comfort

The variables that affect heat dissipation from the body (thus also thermal comfort) can be grouped

into three sets:

environmental personal contributing factors

air temperature metabolic rate (activity) food and drink

air movement clothing acclimatization

humidity body shape

radiation subcutaneous fat

age and gender

state of health

The personal factors include the metabolic rate (activity level) - as discussed above, which in turn may

be influenced also by food and drink, and the state of acclimatization.

Short-term physiological adjustment to changed conditions is achieved in 20 - 30 minutes, but there

are also long term, endocrine adjustments which may extend beyond six months, which constitute the

acclimatization process.

Both the vasomotor and evaporation regulation mechanisms are subject to acclimatization.

In hot climates - for example - the volume of blood circulating can be increased by up to 20%, to

maintain a constant vasodilation.

Sweat secretion rate also increases over a period of several weeks.

It is believed that the forward section of the hypothalamus gland regulates these changes through a

complex neuro-endocrine process.

Body shape and subcutaneous fat are important factors. Heat production is proportionate to the body

mass, but heat dissipation depends on the body surface.

A thin person would have a greater surface-to-volume ratio than someone with a more rounded body

shape, so a proportionately greater heat exchange with the environment.

The more rounded person would prefer a lower temperature, partly because of the lower surface-to

volume ratio, but also because subcutaneous fat is a good insulator.

Age and gender also affect thermal preferences: older people tend to have a narrower comfort range

and women usually prefer a temperature 1 K higher than men.

Clothing is one of the dominant factors affecting heat dissipation. For the purposes of thermal comfort

studies a unit has been devised, named the clo.

This corresponds to an insulating cover over the whole body of a transmittance (U-value) of 6.45

W/m²K (i.e. a resistance of 0.155 m²K/W).

1 clo is the insulating value of a normal business suit, with cotton underwear.

Shorts with short-sleeved shirts would be about 0.25 clo, heavy winter suit with overcoat around 2 clo

and the heaviest arctic clothing 4.5 clo.

Table below gives the clo-values of various pieces of garments. The total clo value of an ensemble is

0.82 times the sum of individual items.

HEAT LOSS IN VARIOUS THERMAL ENVIRONMENTS

Heat exchange in human body and hence human comfort is influenced in some way by four factors

viz.air temperature,humidity,air movement and radiation

The effect of these four variables in heat exchange process needs to be understood by an architect to

design comfortable environments.

Calm,Warm air,Moderate humidity:

In a temperate climate,indoors,when air temperature is around 180C ,when the air is calm ,i.e. air

velocity does not exceed 0.25m/s and when the humidity is between 40% and 60% a person engaged

in sedentary work will dissipate surplus heat without difficulty in the following ways,if the

temperature of bounding surfaces is approximately, the same as the air temperature.

by Radiation 45%,by convection 30% and by evaporation 25%

Hot air and considerable radiation:

The normal skin temperature is between 31 and 34 0C.As the air temperature approaches skin

temperature ,convective heat loss gradually decreases.

Vasomotor regulation will increase the skin temperature,to higher limit(340C),but when air

temperature reaches this point there will be no more convective heat loss.

As long as the average temperature of opposing surfaces is below skin temperature,there will be some

radiation heat loss,but as the surface temperature increases,radiation losses are diminished. Radiant

heat from the sun or a hot body can be a substantial heat gain factor.

When both the convective and radiant heat exchange process are positive ,bodily thermal balance

may still be maintained by evaporation up to a limit,provided the air is sufficiently dry to permit high

evaporation rate.

Hot air, radiation and appreciable air movement

When the air is hot (equal to or above skin temperature) so that the convection element is

positive,when the surface temperatures are warm or there is a substantial radiant heat source, so that

the radiant element is also positive,and when the air is humid(but less than 100% RH) the movement

of air will accelerate evaporation,thus increase heat dissipation,even if its temperature is higher than

that of skin.

The mechanism is as follows :if air is at approximately 90% RH,it will take on some humidity by

evaporation from skin,but the thin layer of air in immediate contact with the skill will soon become

saturated and this saturated air envelope will prevent any further evaporation from skin.

Moving air will remove this saturated air envelope and the evaporation process can continue.It has

been estimated that over 2000 n/m2 vapour pressure ,every 1m/s increase in air velocity will

compensate for an increase of 300 N/m2 in vapour pressure.

When the air is completely saturated and warmer than skin,air movement would only increase

discomfort and heat gain. Fortunately such conditions are seldom met in nature

Even in Warm-humid regions the highest humidities are experienced when air temperature is below

skin temperature ,whilst the highest temperature are accompanied by moderate humidities.

Saturated still air,above body temperature.

Assuming air temperature and the temperature of surfaces are above skin temperature (over 340C)

,where there is no appreciable air movement (less than 0.25m/s) and the relative humidity is near

100%.

Sweating would be profuse ,but there would be no evaporation. There will be convective and radiation

heat gain;therefore however small is the metabolic heat production,all the elements in the thermal

balance equation would be positive.

The body temperature would begin to rise and when the deep body temperature has increased to 2 or

3 deg C only,heat stroke would occur. This is a circulatory failure,followed by a rapid increase in deep

body temperature .

When this reaches about 410C coma sets in and death is imminent. At about 450C deep body

temperature ,death is unavoidable.

Such conditions rarely,if ever,occur in nature,but can quite easily be produced inside buildings of poor

design and with bad management.

Effects of prolonged exposure:

Even if the conditions are not bad enough to produce such immediate disastrous effects,prolonged

exposure to discomfort conditions can produce adverse effects.

Even if the physiological control mechanisms can maintain life there is considerable loss of efficiency

work coupled with physical strain

Factors which may provide immediate relief,such as high wind velocity ,may themselves become

causes of irritation and discomfort when of a long duration.

Conditions which are perfectly comfortable,may produce adverse effects if constant and there is not

change at all over prolonged periods. One of the basic needs of humans is change and variation. This

point becomes particularly noticeable in mechanically controlled environments ,such as air

conditioned buildings.

What the designer should aim at is a range of comfort conditions within which considerable variations

are permitted. Fortunately,in buildings without mechanical environmental controls,such variations will

be produced by the diurnal variation of climatic factors.

EFFECTIVE TEMPERATURE

It is defined as the temperature of a still, saturated atmosphere, which would, in the absence of

radiation, produce the same effect as the atmosphere in question. It thus combines the effect of dry

air temperature and humidity.

It is represented by a set of equal comfort lines drawn on the psychrometric chart.

ET overestimates the effect of humidity, at lower temperatures ,also under cool and comfortable

conditions.

A nomograph can be used to find out the effective temperature if the WBT(wet bulb temperature)

and DBT (Dry bulb temperature) are known along with the air velocity.

Globe thermometer (GT)

The Globe Thermometer consist of a hollow 6 inch metal sphere (e.g. painted copper toilet ball cock),

coated with matte black paint, and containing an ordinary thermometer with its bulb at the center of

the sphere.

If the walls and other surfaces which surround the globe are warmer than the air, then the

temperature of the globe thermometer will be higher than air temperature.

Conversely, with the surroundings cooler than the air, the globe thermometer temperature will be

below air temperature. The globe thermometer is also influenced by the velocity of air movement.

The globe thermometer reaches equilibrium with its environment after about 15-20 minutes

exposure. More accurate than direct air temperature readings as it makes allowance for radiant heat,

and assumes importance when two thermal environments are being compared.

When globe thermometer temperature is higher than air temperature, exploration of the surrounding

surfaces for the cause of the radiation is necessary and possible using Surface Thermometers (with

flattened bulbs)

CORRECTED EFFECTIVE TEMPERATURE (CET)

It is defined as the temperature of a still, saturated atmosphere, which would, produce the same

effect as the atmosphere in question.

Corrected effective temperature scale includes radiation effects, along with the three variable

integrated by the ET scale (temperature,Humidity and air movement).

To find corrected effective temperature a Globe thermometer (GT) reading can be used instead of DBT

as it take in to account the radiant heat exchange.

Finding CET using Nomograph

1.Measure the globe thermometer temperature

2.Measure WBT

3.Measure air velocity.

4.Locate GT on the left hand vertical scale

5.Locate WBT on the right hand vertical scale

6.Connect the two points with a line

7.select a curve appropriate to the air velocity

8.Mark the point where the velocity curve intersects the line drawn

9.Read off the value of the short inclined line going through the same point,this is the CET value.

COMFORT ZONE

The range of conditions within which 80% of the people would feel comfortable can be termed as

'comfort zone'.

For tropical climates a value between 22 0Cand 27 0C is considered to be the comfort zone.

The comfort zone must be limited in terms of air velocities. Below 0.15 m/s even if all other conditions

are satisfactory most people would complain of stuffiness.

Above 1.5 m/s air velocity,air movement can produce secondary effects as paper blown about,dust

stirred up.

Under hot and humid conditions such minor annoyances may be put up with, for the sake of some

thermal relief.

UNIT III

HEAT FLOW THROUGH BUILDING ENVELOPE CONCEPTS

THERMAL QUANTITIES

Conductivity

The rate of heat flow through varies with different material and is described as a property of the

material-its thermal conductivity or k-value

Conductivity is defined as the rate of heat floe through unit area of unit thickness of the material

when there is unit temperature difference between the two sides.

The unit of measurement is W/m deg C

Its value varies between 0.03 W/m deg C for insulating materials and up to 400 W/m deg C for metals

The lower the conductivity the better the insulator a material is.

Resistivity is the reciprocal of this quantity (1/k) measured in units of m degC/W.Better insulators will

have higher resistivity values.

Density is often an indicator of conductivity,higher density materials have higher k-value but it might

not always be true.

The relationship is due to the fact that air has very low conductivity value ,and as lightweight materials

tend to be porous,thus containing more air,their conductivity tends to be less.

Conductance

Conductance is the heat flow rate through a unit area of the body when the temperature difference

between the two surfaces is 1 deg C. The unit measurement is W/m2 degC

Thermal conduction is the process of heat transfer from one part of a body at a higher temperature to

another (or between bodies in direct contact) at a lower temperature.

This happens with negligible movement of the molecules in the body, because the heat is transferred

from one molecule to another in contact with it.

Whilst conductivity and resistivity are properties of a material, the corresponding properties of body

of a given thickness are described as conductance ( C ) or its reciprocal, resistance ( R ):C=1/R

Resistance of a body is the product of its thickness and the resistivity of its material and its unit

measurement is m2 deg C/W

R=bx1/k= b/k

where b is the thickness in meters.

Surface conductance

In addition to the resistance of a body to the flow of heat,a resistance will be offered by its

surfaces,where a thin layer of film separates the body from the surrounding air.

A measure of this is the surface or film resistance,denoted by 1/f (m2 deg C/W) f being the surface or

film conductance (W/m2 deg C)

Surface conductance includes the convective and the radiant components of the heat exchange at

surfaces.

If the heat flow from air on one side ,through the body,to air or the other side is considered,both

surface resistances must be taken in to account.

The overall air-to-air resistance ( Ra)is the sum of the body's resistance and the surface resistances.

Ra =1/fi +Rb+1/f0

where 1/fi= internal surface resistance

Rb = Resistance of the body

1/f0 =External surface resistance

Transmittance

The reciprocal of this air-to -air resistance is the air-to-air transmittance or U-value

U=1/Ra

Its unit of measurement is the same as for conductance- W/m2 degC.

This is the quantity most often used in building heat loss and heat gain problems, as its use greatly

simplifies the calculations.

Cavities

If an air space or cavity is enclosed within a body,through which heat transfer is considered,this will

offer another barrier to the passage of heat.

It is measured as the cavity Resistance (Rc).

The reciprocal of this value is the cavity conductance.

At most the value of for an empty cavity may be the sum of internal and external surface resistances

.It is less if the cavity is less than 50 mm or if strong convection currents can develop inside the cavity.

Its value can be significantly improved by hanging an aluminium foil freely inside the cavity.

An unventilated cavity is a good insulator (R=0.15 m2 deg C/W),equal to about 180 mm brick wall.

The inner leaf of the wall should be the main mass (ex.230mm brick wall) as the insulation should

happen outside the main mass and the outer leaf should be the lesser dense mass (ex hollow blocks)

reducing the conductivity thereby improving insulation

Convection

Convection,(The transfer of heat by bodily movement of a carrying medium )may be due to thermal

forces alone (Self generating) or may be propelled by an applied force.

The rate of transfer in convection depends on three factor:

1. Temperature difference

2. The rate of movement of the carrying medium in terms of kg/s or m3/s

3. The specific heat of the carrying medium in J/kg deg C or J/m3 deg C

These quantities will be used in ventilation heat loss or cooling calculations.

Radiation

The rate of heat flow depends on temperatures of the emitting and receiving surfaces and on certain

qualities of these surfaces:the emittance and absorbance.

Radiation received by a surface can be partly absorbed and partly reflected:the proportion of these

two components is expressed by the coefficients absorbance (a) and reflectance ®.The sum of these

two coefficients is always 1

Light coloured ,smooth and shiny surfaces tend to have a high reflectance and dark surfaces tend to

have high absorbance.

The theoretical white body is a perfect reflecter with coefficients a=0 and r=1 while the theoretical

black body is a perfect absorber with coefficients a=1 and r=0

The practical significance of this is that both light coloured surface and drak coloured surface when

exposed to solar radiation will reflect and absorb same amount of heat but the light coloured surface

will re-emit much of the absorbed heat where as the dark surface will not and therefore will attain a

high temperature.

Sol-Air temperature

In the design of buildings,for surfaces exposed to solar radiation,to calculate heat gain, it is useful to

combine the heating effect of radiation incident on the building with the effect of warm air.This can be

done using the sol-air temperature concept.

A temperature value is found out which would create the same thermal effect as the incident

radiation in question and this value is added to the air temperature.

Ts=To + lXa/fo

Where Ts= Sol-air temperature in 0C

To= Outside temperature in 0C

l=Radiation intensity in W/m2

a=absorbance of the surface

fo=Surface conductance (outside) in W/m2 deg C

Solar gain factor

Solar gain factor(θ) is defined as the heat flow rate through the construction due to solar radiation

expressed as a fraction of the incident solar radiation.

It is expressed as q/l=axU/ fo (Non dimensional)

where q= Extra heat flow rate per unit area cause by radiation

U= Transmittance value in W/m2 degC.

l=Radiation intensity in W/m2

a=absorbance of the surface

fo=Surface conductance (outside) in W/m2 deg C

This is useful in calculating heat flow rate through openings and windows,which are galzed or

protected by some material.

Heat flow through an unglazed aperture will be calculated by multiplying the area of opening with the

radiation intensity (l).For a glazed surface this value will be reduced by solar gain factor (θ ).

The lesser the solar gain factor the lesser the heat transfer through windows or openings.

HEAT EXCHANGE PROCESSES

The heat exchange processes with the outdoor environment happens in the following ways

Conduction of heat may occur through the walls either inwards or outwards the rate of which will be

denoted as Qc

The effects of solar radiation on opaque surface can be included in the above by using sol-air

temperature concept, but through transparent surface the solar heat gain must be considered

seperately and denoted Qs

Heat exchange in either direction may take place with the movement of air .i.e ventilation and the rate

of this will be denoted as Qv

An internal heat gain may result from the heat output of human bodies,lamps motors and appliances.

This may be denoted as Qi

There may be a deliberate introduction or removal of heat using some form of outside energy supply.

The heat flow rate of such mechanical controls may be denoted as Qm

Evaporation takes place on the surface of the building or within the builidng and the vapours are

removed this will produce a cooling effect,the rate of which will be denoted as Qe

The thermal balance equation is

Qi + Qs( + or –) Qc (+ or –) Qv (+ or –) Qm- Qe = 0

If the sum of above this equation is less than zero ,the building will be cooling and if it is more than

zero the temperature in the Building will increase.

Conduction heat flow rate

Conduction heat flow rate through a wall or mass of a given area can be described by the equation

Qc= A x U x ΔT

where Qc =Conduction heat flow rate

A= Surface area in m2

U= Transmittance value,in W/m2 deg C

ΔT= Temperature difference

If heat loss from a building is considered ΔT = Ti-To

If heat gain is considered ΔT = To-Ti

(ex:Heat transfer in a bedroom from the common wall between a kitchen and a bed room)

If a surface is exposed to solar radiation ΔT = Ts-Ti

where Ts is sol-air temperature .

Convection

Convection heat flow rate between the interior of a building and the open air,depends on the rate of

ventilation, i.e. air exchange.

This may be an unintentional air infiltration or deliberate ventilation.

The rate of ventialtion may be given in m3/s and the rate of ventialtion heat flow is given by the

following equation

Qv = 1300 x V x ΔT

Where Qv = Ventilation heat flow rate,in W

1300 = Volumetric specific heat of air ,J/ m3 deg C

V= Ventilation rate in m3/s

T= temperature difference, deg C

If the number of air changes per hour (N) is given the ventilation rate can be found as:

V= N x room volume / 3600

(3600 is the number of seconds in an hour)

Radiation through windows:

To get heat flow rate through a window,the intensity of solar radiation (l) incident on the plane of

window is multiplied by the area of aperture (m2) .This will be the heat flow rate through an unglazed

aperture.

For glazed windows this value will be reduced by solar gain factor (θ) which depends on the quality of

glass and on the angle of incidence.

The solar heat flow equation is

Qs= A x l x θ

where A= area of window in m2

l=radiation heat flow density ,in W/m2

θ= Solar gain factor of window glass.

Internal Heat gain

The heat out put rate of human body (given in table 2) inside a building is the heat gain for a room.

Thus the heat output rate appropriate to the activity to be accommodated must be selected and

multiplied by the number of occupants

The total rate of energy emission of electric lamps can be taken as internal heat gain. (internal heat

gain will be 100 W for a 100 W bulb)

Evaporation

The rate of cooling by evaporation can only be calculated if rate of evaporation is known. The

estimation of evaporation rate is a difficult task and can be rarely done with any degree of accuracy,

unless the environment is mechanically controlled.

Usually evaporation heat loss is ignored for the purpose of calculations except for mechanical

installations.

Thermal mass and Thermal capacity

The thermal mass of the house is a measure of its capacity to store and regulate internal heat.

Buildings with a high thermal mass take a long time to heat up but also take a long time to cool down.

As a result they have a very steady internal temperature. This is sometimes called the thermal

flywheel effect because, like a flywheel, the thermal mass can store and even out fluctuations in

temperature.

Buildings with a low thermal mass are very responsive to changes in internal temperature- they heat

up very quickly but they also cool down quickly. They are often subject to wide variables in internal

temperature.

Everything inside the house contributes to its thermal mass according to its capacity to absorb and

store heat, known as its 'thermal capacity'.

The best materials for storing heat are those that are very dense, heat up slowly, and then give out

that heat gradually. Brick, concrete and stone have a high thermal capacity and are the main

contributors to the thermal mass of a house.

Water has a very high thermal capacity, so it is well suited for climates that have high diurnal

variations.

Air has a very low thermal capacity- it warms up fast but cannot stay warm for long. Only when the

walls and floors in a building have warmed up will the air stay warm.

Low thermal capacity or quick response structures warm up quickly but also cool rapidly.

Large thermal capacity structures will have a longer 'heat-up time' and also take a longer time to

dissipate heat.

Time lag and decrement factor

In nature the diurnal variations produce an approximately repetitive 24 hour cycle of increasing and

decreasing temperature.

The effect of this on a building is that in hot period the heat flows from the environment in to the

building,where some of it is stored and at night during cool period the heat flow is reversed :from the

building to the environment.

In the morning the outdoor temperature increases ,heat starts entering the outer surface of the wall.

Each particle in the wall will absorb a certain amount of heat for every degree of rise in temperature

depending on the specific heat of the wall material. Heat to the next particle will only be transmitted

after the temperature of the first particle has increases. Thus the corresponding increase of the

internal temperature will be delayed.

The out door temperature will have reached its peak and started decreasing before the inner surface

temperature has reached the same level. From this moment the heat stored in the wall will be partly

dissipated to the outside on partly to the inside. As the out door air cools ,an increasing proportion of

this stored heat flows outwards and when the temperature falls below the indoor temperature the

direction of heat flow is completely reversed.

The two quantities characterizing this periodic change are time lag (Ø) and decrement factor (µ)

The time delay due to the thermal mass is known as a time lag (Ø). The thicker and more resistive the

material, the longer it will take for heat waves to pass through.

The reduction in cyclical temperature on the inside surface compared to the outside surface is knows

and the decrement(µ). Thus, a material with a decrement value of 0.5 which experiences a 20 degree

diurnal variation in external surface temperature would experience only a 10 degree variation in

internal surface temperature.

A rule of thumb for massive masonry ,earth and concrete walls is Ø=10 hours for each 0.3 m

thickness.

This effect is particularly important in the design of buildings in environments with a high diurnal

range. In some deserts, for example, the daytime temperature can reach well over 40 degrees. The

following night, however, temperatures can fall to below freezing. If materials with a thermal lag of 10-

12 hours are carefully used, then the low night-time temperatures will reach the internal surfaces

around the middle of the day, cooling the inside air down. Similarly, the high daytime temperatures

will reach the internal surfaces late in the evening, heating the inside up.

In climates that are constantly hot or constantly cold, the thermal mass effect can actually be

detrimental. This is because both surfaces will tend towards the average daily temperature which, if it

is above or below the comfortable range, will result in even more occupant discomfort due to

unwanted mean radiant gains or losses. Thus in warm tropical and equatorial climates, buildings tend

to be very open and lightweight.

In very cold and sub-polar regions, buildings are usually highly insulated with very little exposed

thermal mass, even if it is used for structural reasons.

Insulation

Insulation can help increase time lag and decrement factor and hence help in the heat balance of a

building.

The position of insulation relative to the high thermal mass has a very significant effect on the time-

lag and decrement factor.

With a 100 mm concrete slab,the pacing of 40 mm glass wool insulation gives the following variation:

Time-lag:h Decrement factor

Under the slab 3 0.45

On the top of slab 11.5 0.05

The reason for this is because

1.insulation on the outside reduces the heat flow rate in to the mass-it will take much longer to fill up

the storage capacity of the mass.

2. Insulation inside will not affect the filling up process and although it will reduce the he at emmission

to the inside space it will not change the periodicity.

In hot climates the aim is not only to reduce the heat entering the space but also to dissipate much of

the stored heat to the outside.

An insulation on the outside or top of a slab will reduce the time taken for the heat to pass through

but does the same to dissipation of heat to the outside,so there is a possibility that by the morning

when heat gain states again,all heat gained the previous day might have not been dissipated ,that will

lead to discomfort,this can be avoided by good ventilation of the inner surface.

UNIT IV

IMPACT OF AIR MOVEMENT DUE TO NATURAL AND BUILT FORMS

Wind

Winds are basically convection currents in the atmosphere,tending to even out the differential heating

of various zones. The pattern of movement is modified by the earth's rotation.

Near the equator the winds prevail blowing north easterly winds north of equator and south easterlies

south of equator also known as north east and south east trade winds.

Between 30 and 60 deg North and south of equator strong westerly winds prevail known as mid

latitude westerlies.

Further towards the poles ,pole winds prevail in the north easterly polar winds in the north and south

easterly polar winds in the south.

During the course of each year the global wind patterns shift from North to South and back again .As a

consequence of this annual shift most regions of the earth experience seasonal changes in

temperature,wind direction and rainfall.

Topography

The force direction and moisture contents of air flow are strongly influenced by topography.

Air can be diverted,funneled by mountain ranges. Air flow deflected upwards ,as it cools,releases its

moisture content. A descending air mass will very rarely give any precipitation ,there fore rainfall

characteristics vary sharply between windward and leeward slopes of mountain ranges.

The humidity of air will vary with the rate of evaporation of moisture from the surface below .i.e it

depends on the availability of water to be evaporated.

Ventilation

Ventilation is generally defined as the replacement of stale air by fresh air. It also provides cooling by

air movement. Hence, it would be appropriate to define the term ventilation as the supply of outside

air to the interior for air motion and replacement of vitiated air. An indoor air speed of 1.5 – 2.0 m/s

can cause comfort in warm and humid regions where the outdoor maximum air temperature does not

exceed 28 – 320C

Providing proper ventilation in buildings calls for due consideration in the design phase of buildings. A

faulty design resulting in inadequate ventilation will result in higher energy consumption in the

building for creating comfortable indoor conditions. Therefore, the ventilation requirements of

different seasons, for different types of occupancies should be determined first. A ventilation system

should then be suitably designed to meet the required performance standards.

There are many ways in which ventilation can improve comfort. For example, opening the windows to

let the wind in, and thus providing a higher indoor air speed, makes people inside a building feel

cooler. This approach is termed as comfort ventilation. In hot environments, evaporation is the most

important process of heat loss from the human body for achieving thermal comfort. As the air around

the body becomes nearly saturated due to humidity, it becomes more difficult to evaporate

perspiration and a sense of discomfort is felt.

A combination of high humidity and high temperature proves very oppressive. In such circumstances,

even a slight movement of air near the body gives relief. It would, therefore, be desirable to consider a

rate of ventilation which may produce necessary air movement. If natural ventilation is insufficient,

the air movement may be augmented by rotating fans inside the building.

Functions of Ventilation:

Natural ventilation and air movement could be considered under the heading of ‘structural controls’

as it does not rely on any form of energy supply or mechanical installation, but due to its importance

for human comfort, It deserves a separate section.

It has three distinctly different functions:

1 supply of fresh air

2 convective cooling

3 physiological cooling

There is a radical difference in the form of provisions for 1 and 2 and for 3: therefore, the first two

functions will be considered as ‘ventilation’ but the last function is considered separately as ‘air

movement’.

Supply of fresh air:

The requirements of fresh air supply are governed by the type of occupancy, number and activity of

the occupants and by the nature of any processes carried out in the space – as explained in

connection with mechanical ventilation.

Requirements may be stipulated by the building regulations and advisory codes in terms of m3/h, or

in number of air changes per hour, but these are only applicable to mechanical installations.

Nevertheless, they can be taken as useful guides for natural ventilation.

For natural ventilation usually certain limited solutions are prescribed and not the expected

performance. The provision of ‘permanent ventilators’, i.e. of openings which cannot be closed, may

be compulsory.

These may be grilles or ‘air bricks’ built into a wall, or may be incorporated with windows. The size of

openable windows may be stipulated in relation to the floor area or the volume of the room.

The aim of all these rules is to ensure ventilation, but the rigid application of such rules may often be

inadequate. To ensure a satisfactory performance of the principles involved must be clearly

understood.

Convective Cooling:

The exchange of indoor air with fresh out-door air can provide cooling, if the latter is at a lower

temperature than the indoor air. The moving air acts as a heat carrying medium.

A situation where this convective cooling is a practical proposition, can arise in moderate or cold

climates, when the internal heat gain is causing a temperature increase, but also in warm climates,

when the internal heat gain or solar heat gain through windows would raise the indoor temperature

even higher than the out-door air temperature.

Provision for ventilation: stack effect

Ventilation i.e. both the supply of fresh air and convective cooling, involves the movement of air at a

relatively slow rate. The motive force can be either thermal or dynamic (wind).

The stack effects relies on thermal forces, set up by density difference (caused by temperature

differences) between the indoor and out-door air. It can occur through an open window (when the air

is still): the warmer and lighter indoor air will flow out at the top and the cooler, denser out-door sir

will flow in at the bottom. The principle is the same as in wind generation.

Special provision can be made for it in the form of ventilating shafts. The higher the shaft, the larger

the cross-sectional area and the greater the temperature difference: the greater the motive force

therefore, the more air will be moved.

The motive force is the ‘stack pressure’ multiplied by the cross-sectional area (force in Newtons – area

in m2). The stack pressure can be calculated from the equation :

Ps = 0.042 x h x ΔT

Where Ps= stack pressure in N/m2

H= height of stack in m

ΔT temperature difference in degC

(the constant is N/m3 degC)

Such shafts are often used for ventilation of internal, windowless rooms(bathrooms and toilets) in

Europe. Fig above shows some duct arrangements for multistory buildings, with vertical or horizontal

single or double duct systems.

These systems operate satisfactorily under winter conditions when the temperature difference is

enough to generate an adequate air flow.

Physiological Cooling:

The movement of air past the skin surface accelerates heat dissipation in two ways :

1) increasing convective heat loss

2) Accelerating evaporation

Both the bioclimatic chart and the ET nomograms show the cooling effect of air movement, i.e. how

much higher temperatures can be tolerated with adequate air velocity

For example, from Fig 30: 30⁰C DBT and 25⁰C WBT will give and ET of 27⁰C with still air (less than 0.1

m/s); and 22⁰C with a 7.5 m/s air velocity.

In very low humidities (below 30%) this cooling effect is not great, as there is an unrestricted

evaporation even with very light air movement.

In high humidities (above 85%) the cooling effect is restricted by the high vapour pressure preventing

evaporation, but greater velocities (above 1.5 to 2 m/s) will have some effect. It is most significant in

medium humidities ( 35 to 60%).

Cooling by air movement is most needed where there are no other forms of heat dissipation available,

when the air is as warm as the skin and the surrounding surfaces are also at a similar temperature.

Provision for air movement : wind effects

Thermal forces will rarely be sufficient to create appreciable air movements. The only ‘natural’ force

that can be relied on is the dynamic effect of winds. When the creation of air movements indoors is

the aim, the designer should try to capture as much of the available wind as possible. Negative control

– when the wind is too much – is easy, if windows and openings can be shut.

Air – although light – has a mass (around 1.2 kg/m3), and as it moves, has a momentum, which is the

product of its mass and its velocity(kg m/s). When moving air strikes an obstacle such as a building,

this will slow down the air flow but the air flow will exert a pressure on the obstructing surface. This

pressure is proportionate to the air velocity, as expressed by the equation:

Pw = 0.612 x v2

Where Pw = wind pressure in N/m2

V = wind velocity in m/s (the constant is NS2/m4)

This slowing down process effects a roughly wedge-shaped mass of air on the windward side of the

building, which in turn diverts the rest of the air flow upwards and sideways.

A separation layer is formed between the stagnant air and the building on the one hand and the

laminar air flow on the other hand.

The laminar air flow itself may be accelerated at the obstacle, as the area available for the flow is

narrowed down by the obstacle. As shown in the fig below

At the separation layer, due to friction, the upper surface of the stagnant air is moved forward, thus a

turbulence or vortex is developed.

Due to its momentum, the laminar air flow tends to maintain a straight path after it has been diverted,

therefore it will take sometime to return to the ground surface after the obstacle, to occupy all the

available ‘cross-section’.

Thus a stagnant mass of air is also formed on the leeward side, but this is at a reduced pressure. In

fact, this is not quite stagnant: a vortex is formed, the movement is light and variable and is often

referred to as ‘wind shadow’.

Consequently vortexes are formed wherever the laminar flow is separated from the surfaces of solid

bodies.

On the windward side such vortexes are at an increased pressure and on the leeward side at a

reduced pressure.

If the building has an opening facing a high pressure zone and another facing a low pressure zone, air

movement will be generated through the building.

Air flow through buildings:

As no satisfactory and complete theory is available, air flow patterns can only be predicted on the

basis of empirical rules derived from measurements in actual buildings or in wind tunnel studies

On the basis of such experimental observations the following factors can be isolated which affect the

indoor air flow (both patterns and velocities) :

Orientation

External features

Cross-ventilation

Position of openings

Size of openings

Controls of openings

Orientation:

The greatest pressure on the windward side of a building is generated when the elevation is at right

angles to the wind direction, so it seems to be obvious that the greatest indoor air velocity will be

achieved in this case. A wind incidence of 45⁰ would reduce the pressure by 50%.

Thus the designer must ascertain the prevailing wind direction from wind frequency charts of wind

rose and must orientate his building in such a way that the largest openings are facing the wind

direction.

It has, however, been found that a wind incidence at 45⁰ would increase the average indoor air

velocity and would provide a better distribution of indoor air movement.See fig below

It often happens that the optimum solar orientation and the optimum orientation for wind do not

coincide. In equatorial regions a north-south orientation would be preferable for sun exclusion but

most often the wind is predominantly easterly.

The usefulness of the above findings is obvious for such a situation – it may resolve the contradictory

requirements.

Effect of direction on width of wind shadow

External Features:

Wind shadows created by obstructions upwind, should be avoided in positioning the building on the

site and in positioning the openings in the building.

The wind velocity gradient is made steeper by an uneven surface, such as scattered buildings, walls

fences, trees or scrub – but even with a moderate velocity gradient, such as over smooth and open

ground, a low building can never obtain air velocities similar to a taller one. For this reason (or to

avoid specific obstructions) the building is often elevated on stilts.

External features of the building itself can strongly influence the pressure build-up. For example, if the

air flow is at 45⁰ to an elevation, a wing-wall at the downwind end or a projecting wing of an L-shaped

building can more than double the positive pressure created.

A similar ‘funnelling’ effect can be created by upward projecting eaves. Any extension of the

elevational area facing the wind will increase the pressure build-up. If a gap between two buildings is

closed by a solid wall, a similar effect will be produced.

The air velocity between free-standing trunks of trees with large crowns can be increased quite

substantially due to similar reasons.

The opposite of the above means will produce a reduction of pressures: if a wing wall or the

projecting wing of an L-shaped building is upwind from the opening considered, the pressure is

reduced or even a negative pressure may be created in the front of the window.

Cross-Ventilation:

Figure below shows that in the absence of an outlet opening or with a full partition there can be no

effective air movement through building even in case of strong winds.

With a windward opening and no outlet, a pressure similar to that in front of the building will be built

up indoors, which can make condition even worse, increasing discomfort. In some cases oscillating

pressure changes, known as ‘buffeting’ can also occur. The latter may also be produced by an opening

on the leeward side only, with no inlet.

Air flow loses much of its kinetic energy each time it is diverted around or over an obstacle. Several

right angle bends, such as internal walls or furniture within a room can effectively stop a low velocity

air flow . Where internal partitions are unavoidable, some air flow can be ensured if partition screens

are used, clear of the floor and the ceiling.

Position of openings:

To be effective, the air movement must be directed at the body surface. In building terms this means

that air movement must be ensured through the space mostly used by the occupants: through the

‘living zone’ ( up to 2m high).

As the figure above shows If the opening at the inlet side is at high level, regardless of the outlet

opening position, the air flow will take place near the ceiling and in the living zone.

The relative magnitude of pressure build-up in front of the solid areas of the elevation ( which in turn

depends on the size and position of openings) will, in fact, govern the direction of the indoor air

stream and this will be independent of the outlet opening position.

Figure below shows that Larger solid surface creates a larger pressure build-up and this pushes the air

stream in an opposite direction, both in plan and in section.

As a result of this, in a two storey building (fig bel0w) the air flow on the ground floor may be

satisfactory but on the upper floor it may be directed against the ceiling. One possible remedy is an

increased roof parapet wall.

Size of Openings:

With an given elevational area – a given total wind force(pressure x area) – the largest air velocity will

be obtained through a small inlet opening with a large outlet.

This is partly due to the total force acting on a small area, forcing air through the opening at a high

pressure and partly due to the ‘venturi effect’: in the broadening funnel (the imaginary funnel

connecting the small inlet to the large outlet) the sideways expansion of the air jet further accelerates

the particles.

Such an arrangement may be useful if the air stream is to be directed (as it were focused) at a given

part of the room.

When the inlet opening is large, the air velocity through it will be less, but the total rate of air flow

(volume of air passing in unit time) will be higher. When the wind direction is not constant, or when

air flow through the whole space is required, a large inlet opening will be preferable.

The best arrangement is full wall openings on both sides, with adjustable sashes or closing devices

which can assist in channeling the airflow in the required direction, following the change of wind.

Controls of Openings:

Sashes, canopies, louvers and other elements controlling the openings, also influence the indoor air

flow pattern.

Sashes can divert the air flow upwards. Only a casement or reversible pivot sash will channel it

downwards into the living zone .ref fig below

Effect of sashes

Canopies can eliminate the effect of pressure build-up above the window, thus the pressure below the

window will direct the air flow upwards. A gap left between the building face and the canopy would

ensure a downward pressure, thus a flow directed into the living zone.

Effect of Canopies

Louvres and shading devices may also present a problem. The position of blades in a slightly upward

position would still channel the flow into the living zone (up to 20⁰ upwards from the horizontal)

Effect of louvres

Fly screens or mosquito nets are an absolute necessity not only in malaria infested areas, but also if

any kind of lamp is used indoors at night. Without it thousands of insects would gather around the

lamp. Such screens and nets can substantially reduce the air flow.

A cotton net can give a reduction of 70% in air velocity. A smooth nylon net is better, with a reduction

factor of only approximately 35%. The reduction is greater with higher wind velocities and is also

increased when the angle of incidence.

Air movement and rain:

Exclusion of rain is not a difficult task and making provision for air movement does not create any

particular difficulties, but the two together and simultaneously is by no means easy. Opening of

windows during periods of wind-driven rain would admit rain and spray; while closing the windows

would create intolerable conditions indoors. The conventional tilted louvre blades are unsatisfactory

on two counts :

1) Strong winds will drive the rain in, even upwards through the louvers

2) The air movement will be directed upwards from the living zone

Verandahs and large roof overhangs are perhaps the best tradition methods of protection.

Only type ‘M’ was found to be capable of keeping out water at wind velocities up to 4m/s and the

same time ensuring a horizontal air flow into the building. The air velocity reduction varies between

25 and 50%.

Air flow around buildings

When the architect’s task is the design of more than one building, a cluster of buildings or a whole

settlement, especially in a warm climate, in deciding the layout, provision for air movement must be

one of the most important considerations.

After a careful analysis of site climatic conditions a design hypothesis may be produced on the basis of

general information derived from experimental findings, such as those described below.

A positive confirmation (or rejection) of this hypothesis can only be provided by model studies in a

wind simulator. If the construction of adjustable or variable layout models is feasible, alternative

arrangements can be tested and the optimum can be selected.

If there are tall blocks in mixed developments air stream separates on the face of a tall block, part of it

moving up and over the roof part of it down, to form a large vortex leading to a very high pressure

build-up. An increased velocity is found at ground level at the sides of the tall block. This could serve a

useful purpose in hot climates, although if the tall block is not fully closed but is permeable to wind,

these effects may be reduced.

Air stream separation at the face of buildings

If a low building is located in the wind shadow of a tall block, the increase in height of the obstructing

block will increase the air flow through the low building in a direction opposite of that wind. The lower

(return-) wing of a large vortex would pass through the building.

Reverse flow behind a tall block

If in a rural setting in open country, single storey buildings are placed in rows in a grid-iron pattern,

stagnant air zones leeward from the first row will overlap the second row.

A spacing of six times the building height is necessary to ensure adequate air movement for the

second row. Thus ‘five times height’ rule for spacing is not quite satisfactory

Air flow grid-iron layout

In a similar setting, if the buildings are staggered in a checker-board pattern, the flow field is much

more uniform, stagnant air zones are almost eliminated.

Air flow checker-board layout

Humidity Control:

Dehumidification is only possible by mechanical means; without this, in warm-humid climates, some

relief can be provided by air movement.

In hot-dry climates humidification of the air may be necessary, which can be associated with

evaporative cooling.

In these climates the building is normally closed to preserve the cooler air retained within the

structure of high thermal capacity, also to exclude sand and dust carried by winds. However, some

form of air supply to the building interior is necessary. All these functions:

Controlled air supply

Filtering out sand and dust

Evaporative cooling

Humidification

are served by a device used in some parts of Egypt – the wind scoop.

A wind scoop

The large intake opening captures air movement above the roofs in densely built up areas. The water

seeping through the porous pottery jars evaporates, some drips down onto the charcoal placed on a

grating, through which air is filtered. The cooled air assists the downward movement – a reversed

stack effect.

This device is very useful for ventilation ( the above four functions), but It cannot be expected to

create an air movement strong enough for physiological cooling.

In some parts of India a curtain made of cascas grass is often hung in the front of the windows on the

windward side. This is wetted by throwing a bucket of water against it from time to time. The grass is

highly absorptive and retains the moisture for a long time. The wind passing through the loose

textured mat curtains is both cooled and humidified.

In recent years the cascas mat is often hung from a perforated water pipe which keeps it uniformly

moist all the time.

Architects in Israel have used a porous honeycombed brick grill, with a perforated water pipe at the

top, for a similar purpose. The water pipe ma be supplied by an automatic flush-cistern, of the type

used for urinals.

The ‘desert cooler’ developed in Delhi, is a cube shaped frame, of 500 to 600 mm sides. The top and

bottom are shallow tanks. The sides are covered with cascas mats, the top of which is immersed in the

upper tank.

Water seeps down through the mat and is collected in lower tank. Inside the box is an ordinary table

fan which blows air through the cascas mat, cooling and humidifying it. The fan motor may also drive

a small pump, which lifts the surplus water back to the upper tank. If the box is mounted in or near a

window, it is quite effective during the dry season. It is not used during the monsoon period.

UNIT II

DESIGN OF SOLAR SHADING DEVICES

MOVEMENT OF SUN

When we see the Sun’s position changing in the sky it is of course the Earth that is moving, not the

Sun. While recognizing this, we generally refer to it as the Sun’s 'movement'.

By appreciating how the Sun’s movement throughout the day varies from season to season, we can

predict the performance of solar equipment and buildings. We will know when the Sun is shining on

them and for how long.

The seasonal variation in the times of sunrise and sunset, and the variation in the Sun’s altitude are

caused by the Earth’s axis being tilted at a constant angle to the plane of its rotation around the Sun.

The Earth’s axis of rotation is tilted (inclined) at 23.5 degrees to its plane of revolution around the Sun,

and constantly points to one direction in space.

in December, the southern hemisphere of the Earth has longer days than the northern hemisphere. In

June, the reverse occurs, with the days being shorter in the southern hemisphere. The Earth’s axis is

still tilted at 23.5 degrees in September and March, so both the northern and southern hemispheres

have the same length of day. the angle of the Sun above the horizon at a given time of day will also

vary throughout the year.

LOCATING THE POSTION OF SUN

The position of sun on the sky hemisphere can be specified by two angles

Solar altitude angle (γ) and solar azimuth angle (α)

Given below is the solar chart for Chennai.

Angle of incidence

From these two angles the sun's position in relation to any wall surface of any orientation (thus the

angle of incidence) can be established.

The horizontal component of angle of incidence (δ) will be the difference between the solar

azimuth(SA) and the wall azimuth(WA).

The vertical component is same as the solar altitude angle itself (γ)

The angle of incidence (β) ,i.e. the angle between a line perpendicular to the wall and the sun's

direction ,can be found by the spherical cosine equation

Cos β = cos δ x Cos γ

Angle of incidence will be required both for selecting the appropriate solar gain factor in heat gain

calculations through windows and for calculating the incident radiation on an opaque surface.

SHADOW ANGLES

Shadow angles express the sun's position in relation to a building face of given orientation and can be

used either to describe the performance of (i.e. the shadow produced by) a given device or to specify

a device.

Horizontal shadow angle (δ)

Horizontal shadow angle (δ) is the difference between solar azimuth and wall azimuth I.e the same as

horizontal component of angle of incidence.

By convention, this is positive when the sun is clockwise from the orientation (when SA > WA) and

negative when the sun is anticlockwise (when SA < WA).

When the HSA is between +/- 90o and 270o, then the sun is behind the facade, the facade is in shade,

there is no HSA.

The horizontal shadow angle describes the performance of a vertical shading device

Vertical shadow angle (ε)

The vertical shadow angle is measured on a plane perpendicular to the building face. Vertical shadow

angle can exist only when the Horizontal shadow angle is between -90o and +90o, i.e. when the sun

reaches the building face considered.

Vertical shadow angle characterises a horizontal shading device

When the sun is directly opposite, i.e. when SA = WA (HSA = 0o), the vertical shadow angle is the same

as the solar altitude angle (ε = γ).

When the sun is sideways, its altitude angle will be projected, parallel with the building face, onto the

perpendicular plane and the Vertical shadow angle will be larger than the Altitude angle .

Alternatively, vertical shadow angle can be considered as the angle between two planes meeting

along a horizontal line on the building face and which contains the point considered, ie. between the

horizontal plane and a tilted plane which contains the sun or the edge of the a shading device

Vertical devices consist of louvre blades projecting fins in a vertical position. The horizontal shadow

angle measures their performance. Narrow blades with close spacing may give the same shadow

angle as broader blades with wider spacing.

Using shadow angle protractor,the shading mask of a given device can be established.FOr vertical

devices this is the characteristic sector shape.

These devices are most effective when the sun is to one side of the elevation,such as eastern or

western elevation.

A vertical device to be effective when sun is opposite the wall considered would have to almost

completely cover the whole window.

Horizontal devices

Horizontal device can be canopies,horizontal louvre blades or externally applied venetian blinds. Their

performance will be measured by a vertical shadow angle.

The shading mask is of segmental shape.These will be most effective when the sun is opposite to the

building face considered and at a high angle, such as north and south facing walls.

To cover low angles the device would have to cover the window completely permitting only downward

view.

Egg crate devices

Egg crate devices are a combination of horizontal and vertical shading devices. The many types of

grille-blocks and decorative screens fall in to this category. These can be effective for any orientation

depending on detail dimensions

CLIMATE GRAPH FOR WARM HUMID CLIMATE

CLIMATE GRAPH FOR HOT-DRY DESERT CLIMATE

CLIMATE GRAPH FOR COMPOSITE CLIMATE