Transcript Powerpoint

METEOROLOGY
GEL-1370
Chapter Two
Warming the Earth and the
Atmosphere
Temperature and Heat Transfer
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 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
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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; l of visible light ~ 1/100 of the hair ~
0.000005 meter;
• UV Radiation carries more energy (E=hc/l) 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
lmax T = Constant (2897 mm K) ---- higher temperature,
shorter the wavelength of emitted radiation
Radiation characterized according to
wavelength
Radiation – contd.
• Radiation emitted by a surface = sT4
– E: Maximum rate of radiation emitted by 1m2 of
surface of an object; T: object’s temperature in
Kelvin & s is the Stefan’s constant (StefanBoltzmann 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 Radiation
Wavelength (mm)
0.20-0.29 mm (UV-C)
Harmful to living thing & cells
(cause chromosome mutation & damage cornea
of the eye) – Absorbed by ozone in stratosphere
0.29-0.32 mm (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 mm (UV-A)
Can cause skin redness
Some 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 mm 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 mm) & 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 lvis)
• 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.
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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