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Lecture 6 Diurnal Cycle

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Daily Temperature CycleDaily Temperature Cycle

Daily Temperature VariationsDaily Temperature VariationsEach day is like a mini seasonal cycle– Sun rays are most intense around noon– As is the case with the seasons, the maximum

temperatures lag the peak incoming solar radiation.

An understanding of the diurnal cycle in temperature requires an understanding of the different forms of atmospheric heating and cooling:– Radiation– Conduction– Convection and Latent Heat

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Daily Temperature CyclesDaily Temperature Cycles

Incoming vs Outgoing Radiation• Temperature rises as long as

Energyin > Energyout• Tmax typically 3-5 PM for clear

summer skies• Temperature begins to fall

when Energyin < Energyout• Tmin typically occurs near or

just after dawn on clear nights.What factors could change the timing?

Atmospheric HeatingAtmospheric Heating• Sunlight warms ground• Ground warms adjacent air by conduction

– Poor thermal conductivity of air restricts heating to a few cm

• Random motion of “hot” surface air molecules upward leads to heat transfer (diffusion)

• Hot air forms rising air “bubbles” (thermals) leading to convection and often cumulus clouds.– Mechanical mixing due to wind enhances this mode of heat

transport

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Afternoon Temperature ProfilesAfternoon Temperature ProfilesJogger on calm, summer day may have 122˚ F at their feet and 90˚F at their waist!

122˚F90˚F

TTmaxmax FactorsFactors

• Tmax depends on– Cloud cover– Surface type

• Absorption characteristics– Strong absorbers enhance surface heating

• Vegetation/moisture– Available energy partially used to evaporate water

– Wind• Strong mixing by wind will mix heated air near

ground to higher altitudes

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Cooling at NightCooling at Night• Lower sun angle in evening

means Ein < Eout• During night the ground and

air radiate energy to space• Ground is better radiator

than air, so ground cools faster (Kirchoff’s law).

• Ground cools adjacent air layer by conduction

• Heat transfer from air above is slow due to poor thermal conductivity of air. Inversion often forms

• Why isn’t convection as important at night?

Radiation InversionsRadiation InversionsFeature increasing T with height above groundOccur on most clear, calm nights (nocturnal inversion)Are strongest when

Winds are calmAir is dry

Less IR absorptionLess latent heat release

from fog/dew formationSky is cloud freeNight is long

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Cold air poolingCold air poolingAs air cools, its density increases, leading to– Downslope flow– Cold air pooling in low spots/valleys prolonged

air pollution

Cold air settles into valleys

Inversions and agricultureInversions and agriculture• Surface cooling can

produce sub-freezing temperatures near ground

• Crop damage possible– Especially problematic for

orchards in “warm” climates• Mixing down of warmer air

aloft can help limit temperature drop near surface

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Factors affecting the diurnal range• Wind: turbulent mixing• Cloud: slows the radiation heating (cooling) during day (night)• Surface type: heat capacity/thermal inertia/evaporation (large

in desert, small over wet soil/forest, & tiny over ocean)

Temperature under calm condition

Radiation inversion

Diurnal range• Desert (dry and cloud-free): 30oC (35/5)• Kuwait today (9/7/06): 14oC (44/30)• Honolulu (moist but not very cloudy): 8oC (31/23)

Annual range• Honolulu: 4.8oC (27.5/22.7)• Siberia: 60oC (19/-41)

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Diurnal range decreases with height, in support of the fact thatthe atmosphere is heated by ground.

http://www.ssec.wisc.edu/data/composites.html

Measuring Temperature• Liquid-in-glass thermometers• Infrared (IR) radiometer

Surface temperatureobserved today by space-born IR radiometers

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Wind chill factor in winter

Key words

Radiation imbalance (heat transport by atmosphere and ocean currents)

Radiational cooling, radiation inversion, thermal beltSpecific heat, controls of temperatureDiurnal range of temperatureWind-chill indexRadiometer

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Lecture 6Water in the Atmosphere

Properties of Water

• Physical States– only natural substance that occurs naturally

in three states on the earth’s surface• Specific Heat

– Highest of all common solids and liquids• Latent Heat of Fusion

– Highest of all common substances

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Properties of Water• Radiative Properties

– transparent to visible wavelengths (vapor)– virtually opaque to many infrared

wavelengths– large range of albedo possible

• water 10 % (daily average) • Ice 30 to 40%• Snow 20 to 95%• Cloud 30 to 90%

Energy Associated with Changes of State

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Water Vapor Saturation

• Water molecules move between liquid and gas phases

• When the rate of water molecules entering the liquid equals the rate leaving the liquid, we have equilibrium– The air is said to be saturated with water vapor at this point– Equilibrium does not mean no exchange occurs

Why does it take so much energy to evaporate water?

Hydrogen bond. In the liquid state, adjacent water molecules attract one another: “-”charge on O attracted to “+” charge on H

• The attraction created by hydrogen bonds keeps water liquid over a wider range of temperature than is found for any other molecule its size.

• A large amount of energy is needed to convert liquid water, where the molecules are attracted through their hydrogen bonds, to water vapor, where they are not.

H2O

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Latent Heat and Storms

Water vapor pressure

• All the molecules in an air parcel contribute to pressure

• Each subset of molecules (e.g., N2, O2, H2O) exerts a partial pressure that depends on the number of molecules.

• The VAPOR PRESSURE of water is the pressure exerted only by water vapor molecules in the air– often expressed in millibars (2-30 mb common at surface)

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Saturation Vapor Pressure vs Temp.

• The saturation vapor pressure of water increases with temperature– At higher T, faster water

molecules in liquid escape more frequently causing equilibrium water vapor concentration to rise

– We sometimes say “warmer air can hold more water”

• There is also a vapor pressure of water over an ice surface– The saturation vapor pressure

above solid ice is less than above liquid water