Environmental physics atmosphere

PHY 355

ENVIRONMENTAL PHYSICS I

LECTURE I

(PROF. FUAKYE ERIC GYABENG)

KNUST-

PHYSICS DEPARTMENT

COURSE OUTLINE

INTRODUCTION TO ATMOSPHERE

ORIGIN OF THE ATMOSPHERE

FACTS ABOUT THE ATMOSPHERE

FORMATION OF OZONE LAYER

COMPOSITION OF ATMOSPHERE

ATMOSPHERE

INTRODUCTION TO ATMOSPHERE

A. Definition

An atmosphere is a gaseous envelope surrounding a celestial body.

B. Force of attraction retained by the atmosphere

The earth atmosphere is retained by gravitational attraction and largely rotate with it.

C. Comparison of the Atmosphere and Earth

Comparing the dimension of the earth to the atmosphere, the atmosphere is very thin.

99% of it mass lies below 30Km, that is 0.005 of the earth radius.

NOTE:

The chemical and physical properties of the atmosphere together with it fields of motion, mass

and moisture constitute the subject matter of Meteorology

ORIGIN OF THE ATMOSPHERE

The origin of the atmosphere means how the atmosphere came into existence. When the earth

was to be formed, that is about years (4.5billion) ago the earth really had no

atmosphere.

So then how did the atmosphere came into being;

It is believed that the atmosphere came into existence as a result of the expulsion of

substances from it interior by volcanoes.

These ejected mainly water vapour with some carbon dioxide, nitrogen and sulphur

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ORIGIN OF THE ATMOSPHERE

FACTS ABOUT THE ATMOSPHERE

The atmosphere is only able to hold a limited amount of water vapour , so much of it is

condensed into water to form oceans.

N.B:

It is also thought that the first stage in the evolution of life about years ago

required an oxygen free environment.

In that season, it seems that primitive forms of plant life developing in the ocean began to

release overall amount of oxygen as a waste product through the photosynthesis reaction

2222 OOCHCOOH

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FORMATION OF OZONE LAYER

The build-up of oxygen in the atmosphere led to the formation of the ozone layer.

Function of Ozone layer

Ozone layer is effective in filtering out harmful ultra-violet part of the solar spectrum

N.B:

This allowed plants to move up from the dim depths of the ocean, where they have been

sheltering from the deadly ultra-violet to progressively higher levels.

Here plants had access to increasing amount of solar radiation of the harmless kind which

boosted the rate of oxygen production by photosynthesis.

CONSTITUENT SYMBOL % by weight % by volume Molecular

Weight

Nitrogen N2 75.52 78.09 28

Oxygen O2 23.15 20.95 32

Argon A 1.28 0.93 40

Carbon Dioxide CO2 0.046 0.03 44

Neon Ne 0.012 0.0018 20

Helium He 0.0007 0.0005 4

Methane CH4 0.0008 0.00015 16

Krypton Kr 0.003 0.0001 84

Ozone O3 0-0.01 Variable 48

Water Vapour H 0 0-4 Variable 18

VARIOUS CONSTITUENTS OF THE ATMOSPHERE AND THEIR VARIOUS

AMOUNTS

COMPOSITION OF THE ATMOSPHERE

Apart from the atmosphere highly variable water vapour and ozone contents and excluding solid

and liquid matter in suspension (aerosols), the atmosphere is mixed below 100km.

N.B: The solid and liquid matter in suspension is called (aerosols)

What then happens when above 100 km ?

When above 100km, this leads to the formation of Homosphere, Heterosphere and Exosphere

Above 100km mixing of the atmosphere by turbulent fluid, motions become ineffective and

molecular diffusion becomes more important.

This means the tendency for lighter gases to separate out to higher levels.

TURBOPAUSE , EXOSPHERE

Turbopause: The level of transition from turbulent mixing to molecular diffusion is known as

Turbopause.

OR

Separating the lower well mixed Homosphere from the upper diffused Heterosphere is known as

Turbopause.

Exosphere : Above 600-800km, collisions between atmospheric particles become so infrequent

that some traveling outwards escape from the atmosphere. This region is called exosphere.

N.B:

In the atmosphere ultra-violet lights dissociates oxygen , carbon dioxide and water vapour.

This process is most effective at higher levels; as you go lower in the atmosphere more and

more is absorbed by dissociation and ionization, with hardly any reaching the earth’s

surface.

At about 300 Km monoatomic oxygen (O) becomes the commonest constituent by settling

above the heavier nitrogen.

At 500 Km and above hydrogen and helium predominate.

IONOSPHERE

Ultra-violet and X-rays from the sun ionizes air molecules. (This means they strip electrons

from them). Virtually all the sun ionizing radiation is absorbed at levels above 60 Km. (this is

the base of the ionosphere).

IMPORTANT NOTE

1. The free electrons reflect radio waves allowing long distance communication beyond the

radio horizon.

2. The atmosphere may also be divided into spherical “spheres” each characterize by the way

it temperature varies in the vertical. The top of each sphere is denoted by “pause”.

TROPOSPHERE, STRATOSPHERE, MESOPHERE AND THERMOSPHERE

A. THE TROPOSPHERE

About 80% of the atmosphere lies within the troposphere. Here the average temperature

decreases with height.

Because of the unstable profile, it is prone to vertical mixing by convective and turbulent

transfer.

The vertical motions and the abundance of water vapour make it the seat of all important

weather phenomena.

Because of this and the fact that it includes the layer in which we live, it is the most

easily observed and best known part of the atmosphere

It thermal structure is primarily due to the heating of the earth’s surface by solar

radiation. Heat is transferred up through the troposphere mainly by convective and

turbulent transfer

This is in contrast to the stratosphere which receives it heat by direct absorption of solar

radiation and where vertical mixing is very limited.

NOTE:

The position and temperature of the “tropopause” can be seen as results of the balance

between the convective and turbulent heating of the troposphere and the radiative

heating of stratosphere.

Higher surface temperature in the tropics means that convective mixing takes place to a

greater height. Adiabatic cooling of air reaching these levels makes it very cold, despite

the original surface temperature. Hence a typical tropopause is cold and high whilst

that in higher latitudes is warmer and lower.

A typical equatorial tropopause may be 16km high with at temperature of -80C, one

over the polar region is around 8km high with a temperature of -40 C in summer and

-60 C in winter

Fig 2: A vertical temperature profile for U.S standard Atmosphere

B. THE STRATOSPHERE

The temperature structure of the stratosphere is dominated by the absorption of ultra-

violet radiation by ozone(O3) . This is greatest at around 50Km, which is where the

temperature maximum that defines the stratopause occurs as seen in the previous sketch

.Temperature in this section vary according to latitude and season, ranging from -30C to

+20C over the summer pole.

Transitions from the troposphere to the stratosphere is marked by an abrupt change in the

concentration of the variable trace constituents.

N.B: Water vapour decreases sharply while ozone concentrations decrease.

The strong concentration gradients are a reflection of the fact there is very little mixing

between the moist ozone-poor troposphere and the dry ozone rich stratosphere.

Despite the dryness of the stratosphere, clouds have been observed in winter over high

latitudes at altitudes of between 17Km and 30Km. They generally display tridescence

and are called nacreous clouds.

The temperature at the troposphere stops falling significantly and generally starts to rise

with increasing height into the stratosphere.

The resulting strong static stability acts as a lid on the troposphere strongly inhibiting

exchange of air between the two.

Between them the stratosphere and the troposphere account for 99.9% of the mass of the

earths atmosphere

C. THE MESOSPHERE

Temperature in the mesosphere decreases with height from the stratopause temperature

maximum to the mesopause at around 85Km

Temperature at the mesopause vary from as low as +20C at high latitudes in summer to

-50C in winter, the summer temperatures and the warm winter temperatures being a

reversal of what occurs at the stratosphere.

The reason for the above is that the temperatures are not purely radiatively determined,

that a cross equatorial are exists with rising air over the summer pole and descending air

over the winter pole. The rising and descending results results respectively in adiabatic

cooling and warming.

As in the troposphere, the unstable profile means the vertical motions are not inhibited.

During the summer there is sometimes enough lifting to produce clouds in the upper

mesosphere in high latitudes. This is where the stratopause achieves it highest temperature

due to the optimum amount of solar radiation being received. These are known as

noctilucent clouds. They are very thin.

D. THE THERMOSPHERE

Absorption by oxygen of ultra-violet radiation in the dissociation of oxygen (O2 to O)

accounts for the rapid increase in temperature above mesopause.

The thermosphere extends upward to latitudes of altitudes of several hundred kilometres

where the temperature ranges from 500K to as high as 2000K, depending on the degree of

solar activity

Day to night temperature changes amounts to hundreds of degrees.

The thermosphere defines a level of transition to a more or less isothermal profile

The height of this varies from about 200 – 500Km

Again depending on the solar activity. At levels above 500 the temperature is difficult to

define.

Molecules are also widely spaced that they moved independently and there is no reason

why their temperatures should be the same

Vertical temperature in the earth atmosphere

WATER IN THE ATMOSPHERE

Water is relatively small and variable constituent of the atmosphere, that is 0 -4% in the

troposphere.

Because it can exists in the solids, liquids and gaseous states in the range of temperatures

encountered, it helps to determine the temperature distribution over the earth through latent

heat processes

CHANGES OF STATE REQUIRING HEAT INPUT

The following are some changes which require heat input;

1. Solid to Liquid --------------------Melting point

2. Liquid to Vapour--------------------Evaporation

3. Solid to Vapour---------------------- Sublimation

CHANGES OF STATE RESULTING IN HEAT RELEASE

The following are some changes of state which results in heat release;

1. Liquid to Solid-------------------------------Freezing

2. Vapour to Liquid ----------------------------Condensation

3. Vapour to Solid -------------------------------Deposition

LATENT HEAT

Latent heat is defined as the quantity of heat absorbed or emitted, without change of temperature

during a phase change of unit mass of material.

The following are respective latent heat values for water at 0C:

1.Latent heat of fusion = 334 × 103j/kg

2.Latent heat of sublimation = 2834 × 104j/kg

3.Latent heat of vaporization= 2501 × 103j/kg

The large value for the latent heat of vaporization means that the energy available in a

volume of air depends very much on its water vapour contents as well as temperature.

The amount of heat needed to evaporate 1g of water would raise the temperature by 592K

if used only in changing it temperature

PRESSURE VARIATION WITH HEIGHT

From the hydrostatic equation;

Where is the change in pressure

is the change in height

is the acceleration due to gravity

also is the density

From the equation of state

gpdz

d

p

pd

dzgp

2---------------nRT PV

1p

gp

ddz

M

mn

where

RTM

m PV

But for one molar mass the equation becomes;

Dividing through by V gives

Also

RT m V P

3V

m P RT

4V

m p

Putting equation 4 into equation 3 gives

Making the subject gives

Substituting equation 5 into equation 1 gives

Integrating the above equation

pRT P

5P

RT

p

p

6p

p

d

g

RTdz

Hence

The expression takes away the negative sign

N.B: T and g varies with height but assuming mean values

7p

p0

0

p

p

h

h

d

g

RTdz

h

o

p

p

)pp(0 ho InIng

RTh

)pp( ho InIng

RTh

Taking (the pressure at the surface )as constant it can be seen that as height increases the

pressure decreases, the logarithm of the pressure varies linearly with height

ASSIGNMENT ONE (I)

(10mrks)

Using the hydrostatic equation show that;

Explain all the terms involved.

op

)pp( ho InIng

RTh

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