METEOROLOGY

GEL-1370

Chapter TwoChapter Two

Warming the Earth and the Warming the Earth and the AtmosphereAtmosphere

Temperature and Heat TransferIntroduction:

– 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 mv2); 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

Various Temp. Scales

Latent Heat

• 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

Latent Heat – contd.• 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

Heat Energy absorbed and Released

Cloud Formation leads to warming of atmosphere; development of thunderstorm releases large amount of heat

energy to the air

Heat Transfer

• Conduction: Transfer of heat from molecule to molecule within a substance – warmer to colder regions; air is a poor conductor of heat

• Still Air 0.023 (watts m-1/°C) Dry soil: 0.25• Wood 0.08 Water: 0.60 (@20°C) • Snow 0.63 Wet soil: 2.1• Ice 2.1 Iron: 80 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

Heat Transfer – contd. • 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

Radiation – contd.

• 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 – contd.

• Radiation emitted by a surface = T4 – E: Maximum rate of radiation emitted by 1m2 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’

Harmful UV RadiationWavelength (m)0.20-0.29 m (UV-C) Harmful to living thing &

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 rednessSome cells exposed to UV radiation, produce a dark

pigment (melanin) that begins to absorb some UV rays

Sun’s electromagnetic spectrum -

Radiation distribution of sun’s energy

Radiation Balance• 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; H2O vapor and CO2: strong absorbers of IR but poor absorber of visible solar radiation; N2O, CH4 and O3 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 CO2

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 CO2 – 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 N2O and CH4

Absorption by water vapor and O3

Absorption by CO2

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 (CH4, N2O, and CFCs) have an effect ~ CO2; Water vapor ~60% atmos. Greenhouse effect; CO2 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 CO2

With and Without Greenhouse effect

Feedbacks

• Positive Feedback: Initial change in a process will tend to reinforce the process – Increase in CO2 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)

Warming of Air

Insolation• 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

Energy Balance

• 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%

Distribution of incoming solar radiation

Energy Balance – contd.

• 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

Earth-Atmosphere Energy Balance

Reason for Season• Distance from the sun to the earth varies in a year (147 x

106 km in January & 156 x 106 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

Angle effect

Earth’s tilting

• 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

Tilting Effect

Northern hemisphere summer – oblique effect

Seasonal changes• 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]

Seasonal changes – contd.

• 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

Energy balance

Energy Balance – contd.

• 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