GEL-1370
C h a p t t e r r T w o
W a r r m i i n g t t h e E a r r t t h a n d t t h e e A t t m o s s p h e r r e e
Introduction:
– Sun’s energy is not distributed evenly over the earth – highest in the trophics and lowest in the polar regions
–
1000 x molecular weight = mean free path, for any molecule
– Kinetic Energy: Associated with motion (KE = ½ mv 2 ); Temperature is a measure of the
KE (thermal KE = ½ KT where K is the Boltzman’s constant)
–
Temperature is a measure of the average speed of the atoms and molecules
–
What happens when we warm a parcel of air – Molecules move faster and move apart from each other; become less dense --- If we cool air parcel, air becomes more dense
Temperature and Heat Transfer – contd.
•
Absolute Zero: Where molecular/atomic motions freeze
•
Temperature Scales:
– Kelvin: Absolute Scale; starts with 0°K
– Fahrenheit: Temp. at which water freezes: 32 °F & Water boils at 212 °F; Zero represents lowest temperature obtained with a mixture of ice, water, and salt
– Celsius: Temp. at which water freezes: 0 °C & Water boils at 100 °C
– °C = 5/9 (°F – 32) °K = °C + 273
•
The heat energy required from one phase to another – Latent Heat (Latent heat of Steam, ice, water)
•
Evaporation of water droplet is a cooling process – Faster moving molecules escape most easily --- average motion of molecules left behind decrease --- Slower motion means lower temperature --- COOLING
•
To change from liquid to vapor, needed energy may come from the water or air; the energy lost by liquid can be ‘locked up’ within the water molecule – the energy is ‘stored; or ‘hidden’ condition – Latent Heat
•
Condensation is a warming process
•
600 calories needed to evaporate a gram of water
•
Evaporation, melting & sublimation (ice to vapor) all cool the environment
•
Freezing, condensation & deposition (vapor to ice) warm the surroundings

•
Water vapor changes into liquid and ice cloud particles at high altitudes and tremendous amount of heat energy is released into the environment
•
Evaporation-Transportation-Condensation mechanism of transport of atmospheric heat energy
Cloud Formation leads to warming of atmosphere; development of thunderstorm releases large amount of heat energy to the air
•
Conduction: Transfer of heat from molecule to molecule within a substance – warmer to colder regions; air is a poor conductor of heat
•
Still Air
•
Wood
•
Snow
•
Ice
0.023 (watts m
0.08
0.63
2.1 Iron: 80
-1 /°C) Dry soil: 0.25
Water: 0.60 (@20°C)
Wet soil: 2.1
Silver: 427
•
Convection: Transfer of heat by mass movement of a fluid (air or liquid); heated air becomes less dense than the surrounding cool air --- expanded air is buoyed up and transfer heat upward; Vertical exchange of heat is called convection
Heat Conduction – transfer of heat from hot end to cold end of the metal pin
Development of rising bubble of air that carries heat energy upward by convection-
Thermal
•
Any air parcel that rises will expand, and cool; air that sinks is compressed & warms
•
Transfer of heat by horizontally moving air is called advection
•
Radiation: Energy transferred in the form of waves that release energy & reaching an object;
EM waves do not need medium for travel; speed: 300,000 km/s;
•
Wavelength (of a wave): Distance from one crest to another;

of visible light ~ 1/100 of the hair ~ 0.000005 meter;
•
UV Radiation carries more energy (E=hc/

) than red or green color lines
Rising air expands & cools; sinking air is compressed & warms
•
Photons: Parcel of energy
•
Certain UV photons have enough energy to produce sunburns & penetrate skin tissue, causing skin cancer
•
Basic Concepts on radiation:
–
Every matter radiates energy
– Wavelength of the radiation emitted depends on the object’s temperature
 max
T = Constant (2897
 m K) ---- higher temperature, shorter the wavelength of emitted radiation
Radiation characterized according to wavelength
•
Radiation emitted by a surface =

T
4
–
E: Maximum rate of radiation emitted by 1m 2 of surface of an object; T: object’s temperature in Kelvin &

is the Stefan’s constant (Stefan-Boltzmann law)

–
Objects at high temperatures, emit short- wavelength radiation; objects glow red; objects cooler than this radiate long wavelengths that too long for us to see!
– Sun is hot (6000° K) & radiates majority of its energy at relatively short wavelengths –
Solar radiation is called ‘Shortwave radiation’ & earth’s as ‘Longwave (or terrestrial) radiation’
Wavelength (
 m)
0.20-0.29
 m (UV-C) Harmful to living t hing & cells
(cause chromosome mutation & damage cornea of the eye) – Absorbed by ozone in stratosphere
0.29-0.32
 m (UV-B) Sunburns & penetrate skin tissues, sometimes causes skin cancer; 90% of skin cancer linked to sun exposure & UV-B radiation
0.32-0.40
 m (UV-A) Can cause skin redness
Some cells exposed to UV radiation, produce a dark pigment (melanin) that begins to absorb some UV rays
Radiation distribution of sun’s energy
•
All objects absorb and radiate radiation – absorbs more than emission--- warming
•
Absorption/Emission depends on surface characteristics (color, texture, moisture, & temp.) –
Black body is a good absorber of radiation
•
Blackbody: A perfect absorber and a perfect emitter – not necessarily black in color – Earth’s surface & Sun absorb and radiate ~100% efficiency – black bodies
•
Radiative Equilibrium temperature: Temperature at which radiative equilibrium (Rate of absorption of solar radiation equals the rate of emission of infrared earth radiation) is achieved
(255°K or -18 °C or 0 °F)
Radiation Balance-contd.
• Observed average Earth surface temp. ~ 288 °K (15 °C, 59 °F) – Difference is due to atmospheric absorption & emission of IR radiation (transparent to other radiation)
•
Selective absorbers: Good absorber of one wavelength, but transparent in other regions
[Snow: good absorber of IR, but poor absorber of sunlight; good emitter of IR; H
2
O vapor and
CO
2
: strong absorbers of IR but poor absorber of visible solar radiation; N
2
O, CH
4
and O
3
are all absorb IR, but not selective absorbers
• Most of the IR energy emitted from earth’s surface keeps the earth’s lower atmosphere warm; water vapor and CO2 absorb and radiate IR energy and serves as an insulating layer around the earth. If selectively absorbing gas were not present, earth would be colder
Radiation Balance – contd.
•
Greenhouse Effect: Behavior of water vapor and CO
2
in the atmosphere – Entrapment of IR
& inability to mix and circulate with outside air
•
Atmospheric window: 8-11
 m IR wavelength emitted by the earth is not readily absorbed by water vapor and CO
2
– energy pass upward through the atmosphere into space – clouds absorb this wavelength enhancing greenhouse effect

•
Clouds keep night time temperatures higher and day time temp. lower – day time, sun’s radiation is reflected back; night time, base of clouds radiate back to the earth’s surface making it warmer
•
Greenhouse Effect is essential to life on earth
Absorption of radiation by N
2
O and CH
4
Absorption by water vapor and O
3
2
Man-induced Greenhouse Effect
• In the past ~100 yrs, mean global surface air temp. increased by 0.6°C – GC Models that simulate the physical processes of the atmosphere and oceans predict continued increase in temp leading to shift in the wind’s pattern – steer the rain-producing storms
•
Trace gases (CH
4
, N
2
O, and CFCs) have an effect ~ CO
2
; Water vapor ~60% atmos.
Greenhouse effect; CO
2
for 26% and remaining greenhouse gases ~ 14%
• CFC-12 absorbs in the region of atmospheric window (8-11
 m) & hence in terms of absorption impact, one CFC-12 molecule is equivalent to 10,000 molecules of CO
2
With and Without Greenhouse effect
•
Positive Feedback: Initial change in a process will tend to reinforce the process – Increase in CO
2
level in the atmosphere leads to increased temp --- increase in evaporation --- more water vapor --- more greenhouse effect --- more warming (present 370 ppm to 500 ppm by end of this century --- 2.5°C increase)
•
Role of Oceans and Cloud cover on the Feedback mechanism is not well known
•
Negative Feedback: Initial change in a process will tend to decrease the process – higher temp --- more cloud --- clouds will cool the earth (reflect sunlight)
•
Solar Constant: Amount of solar energy received on a surface perpendicular to the sun’s rays is constant (2 cal cm -2 min -1 or 1367 W m -2 )
•
Scattering: Sunlight is scattered by air molecules and dust particles (effective scattering of shorter waves than longer waves, sizes of dust & air molecules are much smaller the
 vis
)
•
Albedo: Percent of radiation returning from a surface to the amount initially striking that surface
Fresh Snow 75-95% Clouds (thick): 60-90%
Ice 30-40% Sand: 15-45% Cloud (thin): 30-50%
Water: 10% Earth & Atmosphere: 30% Moon: 7%
Brilliant sunset produced by scattering
•
Earth returns same amount of energy it receives from sun

•
19% absorbed by atmosphere and clouds
•
51% absorbed at surface
•
30% reflected and scattered due to Earth’s albedo
– Earth’s Surface: 4%
–
Clouds: 20%
–
Atmosphere: 6%
radiation
•
Earth surface receives 147 units from sun + atmosphere
•
It radiates 117 units (147-117 = 30 units surplus)
•
Atmosphere receives 130 units (sun: 19 + earth: 111)
•
It losses 160 units (130-160 = 30 units deficit)
–
This 30 units goes to the warming of the atmosphere – Conduction and Convection (7 units) & release of latent heat (23 units)
EARTH AND THE ATMOSPHERE ABSORB ENERGY FROM THE SUN, AS WELL AS
FROM EACH OTHER – A DELICATE BALANCE IS MAINTAINED
•
Distance from the sun to the earth varies in a year (147 x 10 6 km in January & 156 x 10 6 km in
July)
•
CLOSER TO THE SUN MEANS WARMER – WHY NOT???
•
1) Angle at which sunlight strikes the surface; & 2) How long the sun shines
•
Striking the earth at an angle spreads out and must heat a larger region than sunlight impinging directly on the earth – more slanted, more atmosphere thickness to penetrate --- more scattered and more absorbed
•
Longer daylight hours --- more energy is available
• Earth’s axis points to the same direction in space all year long
Sun closer to the earth in Jan than in July
•
Northern hemisphere is tilted toward the sun in summer (June) and away from the sun in winter (December)
•
If there is not tilting, 12 hours of day and 12 hours of night at every latitude, every day of the year
•
Above the Arctic circle (66.5 °N), daylight lasts for 24 hours; in North Pole, sun rises above the horizon on March 20 and sets on September 22
•
In far Northern latitudes, sun is never very high above the horizon --- radiant energy passes through thick portion of the atmosphere – less radiation is received and does not effectively heat the surface

Northern hemisphere summer – oblique effect
•
Winter Solstice: On December 21, daylight decreases from 12 hrs at the equator to 0 hr at latitudes above 66.5°N – shortest day of the year
•
Vernal Equinox: March 20, days and nights throughout the world are of equal length
•
Why hot summer is not in June? Cold winter in December??
–
Lag in seasonal temperature (Incoming energy from sun is the highest, it exceeds outgoing energy from the earth; when incoming = outgoing, highest temp. is attained) [In Winter, outgoing energy is higher than incoming energy --- lag time, Jan and Feb cold]
•
High latitudes tend to lose more energy to space than they receive from sun
•
Low latitudes tend to gain more energy than they loose
• At mid latitudes near 37°, Amount received = Amount Lost
•
Winds in the atmosphere and Ocean currents circulate warm air and water toward the poles and cold air and water toward the equator – prevents low latitudes getting warmer and high latitudes getting colder steadily
•
Difference in distance between the earth and sun in July & December ~3%; Energy that strikes the top of earth’s atmosphere ~7% higher in January – summer should be warmer in Southern hemisphere – Why??
–
81% of surface in southern hemisphere is water
–
61% of surface in Northern hemisphere is water – due to higher specific heat of water, average summer (Jan) of southern hemisphere is cooler than the summer (July) temp. in the Northern hemisphere