METEOROLOGY
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Transcript of METEOROLOGY
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METEOROLOGY
GEL-1370
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Chapter TwoChapter Two
Warming the Earth and the Warming the Earth and the AtmosphereAtmosphere
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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
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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
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Various Temp. Scales
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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
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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
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Heat Energy absorbed and Released
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Cloud Formation leads to warming of atmosphere; development of thunderstorm releases large amount of heat
energy to the air
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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
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Heat Conduction – transfer of heat from hot end to cold end of the metal pin
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Development of rising bubble of air that carries heat energy upward by convection-Thermal
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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
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Rising air expands & cools; sinking air is compressed & warms
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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
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Radiation characterized according to wavelength
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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’
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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
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Sun’s electromagnetic spectrum -
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Radiation distribution of sun’s energy
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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)
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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
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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
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Absorption of radiation by N2O and CH4
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Absorption by water vapor and O3
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Absorption by CO2
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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
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With and Without Greenhouse effect
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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)
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Warming of Air
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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%
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Brilliant sunset produced by scattering
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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%
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Distribution of incoming solar radiation
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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
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Earth-Atmosphere Energy Balance
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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
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Sun closer to the earth in Jan than in July
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Angle effect
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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
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Tilting Effect
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Northern hemisphere summer – oblique effect
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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]
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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
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Energy balance
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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