Heating Earth’s Surface andHeating Earth’s Surface and ...rcg.gvc.gu.se/jj/ · Heating Earth’s Surface andHeating Earth’s Surface and Atmosphere Chapter 2-3 September 6 2009September

Heating Earth’s Surface andHeating Earth’s Surface and Atmospherep

Chapter 2-3

September 6 2009September 6, 2009

This chapter discusses:This chapter discusses:

• Temperature and heat transferConduction convection radiation– Conduction, convection, radiation

• Solar radiation, earth energy balance• The seasons and temperature

variationsvariations





fTemperature and Heat Transfer‘ ’ ‘‘Temperature’ is the quantity that tells us ‘how hot or cold

something is relative to some set standard value’.

• Air is made up of billions of atoms and molecules, moving in all directions, spinning and bumping around. They don’t all , p g p g ymove at same speed. – Energy associated with this motion is called kinetic energy, the

energy of motionenergy of motion.– Temperature of air is the measure of its average kinetic energy

– Temperature is a measure of the average speed of the atoms and molecules, where higher temperatures correspond to faster average speeds.

– Absolute zero – at this temp the atoms and molecules would posses a minimum amount of energy and theoretically no thermal motion

Vibration, spinning, moving, bumping around

=> Kinetic energy ≈ Temperature=> Kinetic energy ≈ Temperature

Temperature Scales

• Kelvin Scale - a temp scale begins at absolute zero (“no motion”)F h h it S l i d 32• Fahrenheit Scale – assigned 32 as the number where water freezes and 212 where water boils. 180 equal divisions called degrees180 equal divisions called degrees

• Celsius Scale – Zero on this scale assigned to the temperature at which pure water freezes and 100 pto temp where pure water boils. Divided into 100 equal degrees

• C=5/9(F-32)• K=C+273

Comparison of the Kelvin, Celsius, and Fahrenheit scales

Heat• The transfer of energy into or out of an object due to temperature

differences between one object to another

• Heat flows from a region of higher temp to one of lower temp

• Three types of heat-transfer1. Conduction2 R di ti2. Radiation3. Convection

• Additionally latent heat is very important in the atmosphereAdditionally, latent heat is very important in the atmosphere

1) Conduction)

• The transfer of heat from the hot end of the metal pin to the cool end by molecular contact is called

d ticonduction.

• Molecules in the end of the pinMolecules in the end of the pin absorb energy from the flame and vibrate faster than those farther

f fl iaway from flame, energy is eventually transferred from molecule to molecule to hand.molecule to molecule to hand.

2) Convection)

• The transfer of heat by the mass ymovement of a fluid (such as water and air) is called convectionconvection.

• Convection happens naturally in pp ythe atmosphere. (i.e. Thermals –rising bubbles of air.)

• Radiator induces convection in a room.

Note: In our atmosphere, air that rises expands and Cools air that sinks is compressed and warmed!

Incoming solar energyIncoming solar energy

The development of a thermal. A thermal is a rising bubble of air that carries heat energy upward by convection

)3) RadiationC• Carrying radiant energy

• Only heat transfer that can travel through the vacuum of space (without medium)of space (without medium)

• The energy transferred from the sun to your face onThe energy transferred from the sun to your face on a warm day is called radiant energy or radiation. The sunlight travels through the air with little effect on th i it lf Th t l i th f fthe air itself. The energy travels in the form of wavesthat release energy when they are absorbed by an object. These are called electromagnetic waves j gbecause they have magnetic and electrical properties.





Electromagnetic Spectrum of Radiation

Solar radiationue

ncy

freq

u

Radiation characterized according to wavelengthaccording to wavelength. As the wavelength decreases, the energy carried per wave increases.

Energy carried per wave or

photon carried per wave increases.

micrometer is 10-6

photon

*Radiation laws• All objects continually emit radiant energy• Stefan-Boltzman law

Hotter object emits more energy– Hotter object emits more energy– The rate of radiation emitted by a body is proportional to

the fourth power of the body’s temperatureE=σT4– E=σT4

– Sun: ~6000K, E=~ 73,483,200 W/m2

– Earth: ~300K, E=~ 459 W/m2 (~0.006% of Esun)• Wien’s displacement law

– Wavelength of maximum emission is inversely proportional to temperature of a radiating body

– λmax = C/T– Sun: ~0.483 ɥm– Earth: ~9.66 ɥm

• Good absorbers are good emitters

Solar radiation and Earth radiationSolar radiation and Earth radiation

The hotter sun notonly radiates more energythan that of the cooler earththan that of the cooler earth(the area under the curve),but it also radiates themajority of its energy atmajority of its energy atmuch shorter wavelengths.

(Th d th(The area under the curvesis equal to the total energyemitted, and the scales forthe two curves differ by afactor of 100,000.)

Solar radiation is shortwave radiation. Earth radiation is longwave radiation

Balancing act absorptionBalancing act – absorption, emission, and equilibrium

If the earth and all things on it are continually radiatingenergy, why doesn't everything get progressively colder?

Absorption = Emission (radiative equilibrium)

• Absorptivity, the rate at which something radiates and absorbs energy depends strongly on its surface characteristics, such as color, texture,

i t d t tmoisture and temperature.• Blackbody – an object that is a perfect absorber (it absorbs all the

radiation that strikes it) and a perfect emitter (emits the maximum di i ibl i i ) D h b bl kradiation possible at its given temperature). Does not have to be black

in color. Earths surface is nearly 100% efficient and thus behaves like a blackbody

• Radiative equilibrium temperature – average temp at which the rate of absorption of solar radiation equals the rate of emission of infrared earth radiation.

Going to equilibrium temperatureGoing to equilibrium temperature

1. Incoming solar radiation is fixed2. Absorption warms the Earthp3. Radiation from Earth increases as Tearth

increasesincreases4. Tearth reaches a critical temperature,

eventually Absorption = Emissioneventually Absorption = Emission occures

Radiative equilibrium temperature of the EarthRadiative equilibrium temperature of the Earth

• A simplest zero-dimensional energy balance model can be formulated as belows (assuming black-body)

44/)1( eTS σα =−Incoming S energy Earth’s radiation

• In case α=0.3 (albedo), S=1370Wm-2

Te≈255K

• But observed Ts≈288K, ∆T = 33K => greenhouse effect!• In case of the planet Venus α=0 7 S=2619Wm-2 Te≈242K Ts

Jee-Hoon Jeong

In case of the planet Venus, α=0.7, S=2619Wm , Te≈242K Ts ≈730K, ∆T = 488K (!)

27 October 2008Climate Change Modelling 18

What happens to Incoming SolarWhat happens to Incoming Solar Radiation

1. Reflection and Scattering2. Absorption by Earth’s surface and

atmosphereatmosphere

3. Radiation emitted from Earth surface* Convection and latent heat Convection and latent heat

1) Reflection and scattering by atmosphereatmosphere

On the average, of all the solar energy that reaches the earth's atmosphere annually, about 30 percent (30/100) is reflected and scattered back to space, giving the earth and its atmosphere an albedo of 30 percent. Of the remaining solar energy, about 19 percent is absorbed by the atmosphere and clouds, and 51 percent is absorbed at the surface.

*Albedo in Earth’s surfaceAlbedo in Earth s surface

2) Absorption

2) Absorption- selective absorbers and the atmospheric greenhouse effectatmospheric greenhouse effect

Many selective absorbers in the environment.

O O H O absorbO3, O2, H2O absorb some solar radiation

CO i t dCO2 is not a good absorber of solar radiation, but a good absorber of Earth’s radiation

3) Radiation from Earth

Insolation

Sunlight warms the earth's surface only during the day whereas the surfaceSunlight warms the earth s surface only during the day, whereas the surface constantly emits infrared radiation upward.

Without water vapor, CO2, and other greenhouse gases, the earth's surface would f ( ) fconstantly emit infrared radiation (IR); incoming energy from the sun would be equal to

outgoing IR energy from the earth's surface. Without the greenhouse effect, the earth's average surface temperature would be -18°C.

3) Radiation from Earth – IR absorption by atmosphere

Greenhouse effect

With greenhouse gases, the earth's surface receives energy from the sun and infrared energy from its atmosphere. Incoming energy still equals outgoing energy, but the added IR energy from the greenhouse equa s outgo g e e gy, but t e added e e gy o t e g ee ousegases raises the earth's average surface temperature about 33°C, to a comfortable 15°C.

3) Radiation from Earth, IR absorption by atmosphere

Absorption of radiation by gases in the atmosphere. The shaded area p y g prepresents the percent of radiation absorbed

Water vapor and carbon dioxide (CO2) are strong absorbers of IRRadiation and poor absorbers of visible solar radiation.

Total heat budget

*Convection and latent heat release

If convection were to suddenly stop - the averageEarth temp would rise about 18F (? C?)

Air in the lower atmosphere is heated from below. Sunlight warms the ground, and the air above is warmed by conduction, convection, and radiation. Further warming occurs during condensation as latent heat is given up to the air inside a g occu s du g co de sat o as ate t eat s g e up to t e a s dethe cloud. Most absorption takes place near the surface – lower atmosphere is mainly heated from below.

Latent Heat

Change of State (Phase Change)- changesfrom gas liquid solid (ice)from gas…liquid…solid (ice)• The heat/energy required to change a substance (water),

from one state to another is called latent heat. ((Why?)Why?)

When a cloud forms from water vapor it warms atmosphereWhen a cloud forms from water vapor, it warms atmosphere through releasing latent heat. Namely, additional heat can be transferred as a form of latent heat.

Latent heat is an important source of atmospheric energy! Water vapor rising into the air cools and becomes liquid water and ice particles…thisbecomes liquid water and ice particles…this process releases heat into the environment

Every time a cloud forms, it warms the atmosphere. Inside this developing thunderstorm a vast amount of stored heat energy (latent heat) is given up to the air, as invisible water vapor becomes countless billions of water droplets and ice crystals In fact for the duration of this storm alonecountless billions of water droplets and ice crystals. In fact, for the duration of this storm alone, more heat energy is released inside this cloud than is unleashed by a small nuclear bomb.

Break?!Break?!





Controls of TemperatureControls of Temperature• Latitude (colder near the poles warmer near equator)• Latitude (colder near the poles, warmer near equator)• Geographic position plus land and water distribution • Ocean currents (warm/cold currents upwelling)• Ocean currents (warm/cold currents, upwelling)• Elevation (air cools with increased elevation)

Cl d d lb d• Cloud cover and albedo

January July

Why do we have seasons?

• The Earth has an elliptical path around the sun that takes a little over 365 days - revolutiontakes a little over 365 days revolution

• One spin on its own axis in 24 hours - rotation• Average distance from earth to sun is ~150 million e age d sta ce o ea t to su s 50 o

kilometers, varies from 147.3 to 152.1

• Elliptical path takes us closer to sun in January than it does in July – Say what?Seasons are mostly regulated by sun angle and the• Seasons are mostly regulated by sun angle and the number of daylight hours

perihelion aphelion

The elliptical path (highly exaggerated) of the earth about the p p ( g y gg )sun brings the earth slightly closer to the sun in January than in July.

=> Minor role in producing seasonal temperature variations

Tilted Earth’s rotation axis=> Changes in angle of sun, daylight lengthg g , y g g

As the earth revolves about the sun, it is tilted on its axis by an angle of 23.5° (inclination of the axis). The earth's axis always points to the same area in space (as viewed from a distant star). Thus, in June, when the Northern Hemisphere is tipped toward the sun more direct sunlight and long hours of daylightwhen the Northern Hemisphere is tipped toward the sun, more direct sunlight and long hours of daylight cause warmer weather than in December, when the Northern Hemisphere is tipped away from the sun. (Diagram, or course, is not to scale.)

The changing position of the sun, as observed in middle latitudes in the Northern Hemisphere.

Sunlight that strikes a surface at an angle is spread over a larger area thanSunlight that strikes a surface at an angle is spread over a larger area than sunlight that strikes the surface directly. Oblique sun rays deliver less energy (are less intense) to a surface than direct sun rays.

D il T t V i tiDaily Temperature Variations

• Air temperature is a very important weather element Impacts us every day If it is warmelement. Impacts us every day. If it is warm we don’t always mind the rain, but if its cold Or if it is 40°C outsidecold…..Or if it is 40 C outside…

• A sunny day has its own cycle of warmingand cooling.

• Temperature lag – Sun directly overhead at noon but noon is not warmest point of a sunnynoon, but noon is not warmest point of a sunny day. Why?

What about precipitation cycle?

The daily variation in air temperature is controlled by incoming energy (primarily from the sun) and outgoing energy from the earth's surface Where incoming energy exceeds outgoingand outgoing energy from the earth s surface. Where incoming energy exceeds outgoing energy (orange shade), the air temperature rises. Where outgoing energy exceeds incoming energy (blue shade), the air temperature falls.

On a sunny, calm day, the air near the surface can be substantially warmer than the air a meter or so above the surface Sunlight warms ground and ground warms air very nearmeter or so above the surface. Sunlight warms ground and ground warms air very near the surface of the earth. [EX: Higher Field temperatures than air temperatures. Higher track temperatures than air temperatures.]

Nighttime Cooling

• As sun lowers, its energy is spread over a larger area, which reduces the heat available to warm the ground.g

• Sometime in the late afternoon or early evening, the earth’s surface and air above begin to lose more energy th th i th t t t lthan they receive, they start to cool.

• Radiational Cooling – ground and air above cool by radiating infrared energy The ground being a muchradiating infrared energy. The ground, being a much better radiator than air, is able to cool more quickly. Shortly after sunset the earth’s surface is slightly cooler th th i di tl b it ( h t i thi ?)than the air directly above it (what is this?).

• By late night or early morning the coldest air is next to the ground with slightly warmer air abovethe ground with slightly warmer air above.

On a clear, calm night, the air near the surface can be much colder than the air above. The increase in air temperature with increasing height above the surface is called a radiation temperature inversion

Radiation InversionStrong radiation inversion occurs when theair near the ground is much colder than theair higher up.

Ideal conditions:• Calm conditions – no mixing• Calm conditions – no mixing• Long nights – more time for radiative cooling• Dry air cloud free clear skies allow maximum• Dry air, cloud-free – clear skies allow maximum

cooling at the surface

Cold air

On cold, clear nights, the settling of cold air into valleys makes them colder than di hill id Th i l th id f th hill h th i t tsurrounding hillsides. The region along the side of the hill where the air temperature

is above freezing is known as a thermal belt

Orchard heaters circulate the air by setting up convection currents.

Wind machines mix cooler surface air with warmer air above.

The daily range of temperature decreases as we climb away from the earth's surface. Hence, there is less day-to-night variation in air temperature near the top of a high-rise apartment complex than at the ground level.

Air Temperature andAir Temperature and Human Comfort

Ai t t f l diff t diff t iAir temperatures can feel different on different occasions.15°C in March can feel warm after a long winter, but can feel cool on a summer afternoon.feel cool on a summer afternoon.

Sensible temperature – temperature we perceive

*Humidity, evaporation – we feel much warmer in humid 30°C than in dry 30°C30 C than in dry 30 C

*Wind – windy 0°C vs. windless 0°C

Wind Chill factor – how cold the wind makes us feel

Instrument Shelter

• Why we need this?

• To protectp– Wind– Direct solar radiationDirect solar radiation– Radiation from sfc– Precipitation (water)Precipitation (water)

Dif of Daily Monthly and YearlyDif. of Daily, Monthly and Yearly Temperatures

• Daily diurnal range of temperature – difference between the daily maximum and daily minimum

• Mean daily temperature – average of the highest and l f 24 h i dlowest temperature for a 24 hour period.

• Annual range of temperature – difference between th t t f th t d ld tthe average temperature of the warmest and coldest months

• Mean annual temperature average temperature of• Mean annual temperature – average temperature of a station for the entire year.

Temperature data for San Francisco, California (37°N), and Richmond, Virginia (37°N) - two cities with the same mean annual temperature.

Thanks!Thanks!

Aurora – caused by charged particles from the sun interacting with the atmosphereAurora – caused by charged particles from the sun interacting with the atmosphere.Solar wind collides with atmospheric gases. Gases get “excited” and emit lightaurora borealis in NH and aurora australis in SH

When an excited atom, ion, or molecule de-excites, it can emit visible light. The electron in its normal orbit and jumps into a higher energy levelcan emit visible light. The electron in its normal orbit becomes excited by a charged particle...

...and jumps into a higher energy level.

When the electron returns to its normal orbit, it emits a photon of light.

Aurora Belt

The aurora belt (solid red line) represents the region where you would most likely e e you ou d os e yobserve the aurora on a clear night. (The numbers represent the averagerepresent the average number of nights per year on which you might see an aurora if the sky were clear )aurora if the sky were clear.) The flag MN denotes the magnetic north pole, whereas the flag NPwhereas the flag NP denotes the geographic north pole.

*Photons

• Think of radiation as streams of particles, or photons, that are discrete packets of energy. UV p , p gyphotons carry more energy than a photon of visible light. UV photons produce sunburns, penetrate kiskin, can cause cancer.

Light has characteristic of a particle as well as a waveas a wave.

‘photon’ represents the ‘particle’ side of light.

‘ h t ’ ‘l ’ t th‘short wave’ or ‘long wave’ represent the wave side of light.

Heating Earth’s Surface andHeating Earth’s Surface and ...rcg.gvc.gu.se/jj/ · Heating Earth’s Surface andHeating Earth’s Surface and Atmosphere Chapter 2-3 September 6 2009September

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