Solar Radiation, �Heat Balance and Temperature

• Solar radiation, often called the solar resource, is a general term for the electromagnetic radiation emitted by the sun. Solar radiation can be captured and turned into useful forms of energy, such as heat and electricity, using a variety of technologies. However, the technical feasibility and economical operation of these technologies at a specific location depends on the available solar resource.

Insolation

• Heated by solar energy
• Tropics heated more than poles
• Imbalance in heating redistributed
• Solar heating and movement of heat by oceans and atmosphere determines distribution of:
• Temperature
• Precipitation
• Ice
• Vegetation

Electromagnetic Spectrum

• Electromagnetic energy travels through space
• Energy heating Earth mostly short-wave radiation
• Visible light
• Some ultraviolet radiation

Incoming Solar Radiation

• Radiation at top of Earth’s atmosphere = 1368 W m^-2
• If Earth flat disk with no atmosphere, average radiation = 1368 W m^-2
• Earth 3-dimensional rotating sphere,
• Area = 4πr²
• Average solar heating = 1368 ÷ 4 = 342 W m^-2

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30% Solar Energy Reflected

• Energy reflected by clouds, dust, surface
• Ave. incoming radiation 0.7 x 342 = 240 W m^-2

Energy Budget

• Earth’s temperature constant ~15°C
• Energy loss must = incoming energy
• Earth is constantly receiving heat from Sun, therefore must lose equal amount of heat back to space
• Heat loss called back radiation
• Wavelengths in the infrared (long-wave radiation)
• Earth is a radiator of heat
• If T > 1°K, radiator of heat

Energy Budget

• Average Earth’s surface temperature ~15°C
• Reasonable assumption
• Surface of earth radiates heat with an average temperature of 15°C
• However, satellite data indicate Earth radiating heat average temperature ~-16°C
• Why the discrepancy?
• What accounts for the 31°C heating?

Energy Budget

• Greenhouse gases absorb 95% of the long-wave, back radiation emitted from Earth’s surface
• Trapped radiation reradiated down to Earth’s surface
• Accounts for the 31°C heating
• Satellites don’t detect radiation
• Muffling effect from greenhouse gases
• Heat radiated back to space from elevation of about 5 km (top of clouds) average 240 W m^-2
• Keeps Earth’s temperature in balance

Energy Balance

Greenhouse Gases

• Water vapor (H₂O_(v), 1 to 3%)
• Carbon dioxide (CO₂, 0.037%; 365 ppmv)
• Methane (CH₄, 0.00018%; 1.8 ppmv)
• Nitrous oxide (N₂O, 0.00000315%; 315 ppbv)
• Clouds also trap outgoing radiation

Variations in Heat Balance

• Incoming solar radiation
• Stronger at low latitudes
• Weaker at high latitudes
• Tropics receive more solar radiation per unit area than Poles

Variations in Heat Balance

• What else affects variation in heat balance?
• Solar radiation arrives at a low angle
• Snow and ice reflect more radiation at high latitudes
• Albedo
• Percentage of incoming solar radiation that is reflected rather than absorbed

Average Albedo

Sun Angle Affects Albedo

• All of Earth’s surfaces absorb more solar radiation from an overhead sun
• Water reflects <5% radiation from an overhead Sun

Sun Angle Affects Albedo

• Water reflects a high fraction of radiation from a low-lying Sun
• Earth average albedo = 10%

Pole-to-Equator Heat Imbalance

• Incoming solar radiation per unit area higher in Tropics than Poles
• Sun angle higher in Poles than Tropics
• Albedo higher at Poles than Tropics
• Variations in cloud cover affect heat imbalance

Seasonal Change in Solar Radiation & Albedo

• Tilt of Earth’s axis results in seasonal change in
• Solar radiation in each hemisphere
• Snow and ice cover (albedo)

Seasonal Change in Solar Radiation

• Large seasonal change in solar radiation between the hemispheres

Seasonal Change in Albedo

• Increases in N. hemisphere winter due mainly to snow cover and to lesser degree Arctic sea ice
• Increases in S. hemisphere winter due to sea ice

Albedo-Temperature Feedback

Water a Key to Earth’s Climate

• Water has high heat capacity
• Measure of ability to absorb heat
• Heat measured in calories
• 1 calorie = amount of heat required to raise temperature of one gram of water by 1°C
• Heat Capacity (cal cm^-3) = Density (g cm^-2) x Specific Heat (cal g^-1)
• Specific heat of water = 1
• Ratio of heat capacity water:ice:air:land 60:5:2:1
• Heat capacity of air linked to water vapor

Differences in Heating Land & Oceans

• Low latitude ocean major storage tank of solar heat
• Sunlight direct, albedo low, heat capacity high
• Heats surface; winds mix heat
• Contrast with land
• Albedo high, heat capacity & conductance low
• Tropical/subtropical lands become hot, but don’t store heat

Sensitivity of Land & Oceans to Solar Heating

• Change in mean seasonal surface temperature greatest over large landmasses and lowest over oceans

Thermal Response Different

• Large land masses heat and cool quickly
• Extreme seasonal temperature reached 1 month after Solstice
• Upper ocean heats and cools slowly
• Extreme seasonal temperature reached 2-3 months after Solstice

Redistribution of Heat

• Heat transfer in Earth’ atmosphere
• Sensible heat
• Heat that a person directly senses
• Sensible heat = T x specific heat
• Latent heat (hidden or concealed)
• Additional heat required to change the state of a substance
• Sensible and latent heat affected by convection

Convection

Sensible Heat

• Sensible heating greatest
• At low latitude
• Overhead Sun
• Over land
• Low heat conductance (air heats)
• Dry regions
• Low humidity
• Sensible heat lowest
• Over oceanic regions

Latent Heat

• Heat is temporarily hidden or latent in water vapor
• Powerful process transferring heat long distances
• Transfer is two step process
• Initial evaporation of water and storage of heat in vapor
• Later release of stored heat during condensation and precipitation (typically far from site of evaporation)

Latent Heat

0°C-100°C, 1 calorie of heat energy

needed to increase 1 g H₂O by 1°C

80 cal g^-1 heat required for phase transformation, ice → water

80 cal g^-1 heat released when water freezes – latent heat of melting

H₂O(l) → H₂O(g) requires 540 cal g^-1

Condensation of water releases

540 cal g^-1 – latent heat of vaporization

Latent Heat of Vaporization

• Important – evaporation occurs at any temperature between 0-100°C
• Latent heat is associated with any change of state
• Therefore, during evaporation heat is stored in water vapor in latent form for later release

Water Vapor Content of Air

• Saturation vapor density
• Warm air holds 10X more water than cold

Redistribution of Latent Heat

• Evaporation in warm equatorial region
• Stored energy carried vertically and horizontally
• Condensation and precipitation releases energy

Water Vapor Feedback

Unequal Heating of Tropics and Poles

• Latitudes <35° have excess incoming solar radiation over outgoing back radiation
• Excess heat stored in upper ocean drives general circulation of oceans and atmosphere

Atmospheric Circulation

• Atmosphere has no distinct upper boundary
• Air becomes less dense with increasing altitude
• Air is compressible and subject to greater compression at lower elevations, density of air greater at surface
• Constant composition to 80 km
• What drives atmospheric circulation?

Free Convection

• Atmospheric mixing related to buoyancy
• Localized parcel of air is heated more than nearby air
• Warm air is less dense than cold air
• Warm air is therefore more buoyant than cold air
• Warm air rises

Forced Convection

• Occurs when a fluid breaks into disorganized swirling motions as it undergoes flow
• Fluid flow can be laminar or turbulent

Laminar vs. Turbulent Flow

• Whether a fluid flow is laminar or turbulent depends on
• Velocity (rate of movement)
• Geometry (primarily depth)
• Viscosity
• Turbulent flow occurs during high velocity movement of non-viscous fluids in unconfined geometries

Forced Convection in Atmosphere

• Horizontally moving air undergoes turbulence
• Air is forced to mix vertically through eddy motions because of
• High velocity
• Depth of atmosphere
• Low viscosity

Atmospheric Circulation

• Force of gravity maintains a stable atmosphere
• Most of the mass of air near surface
• As a result of atmospheric pressure
• Dense air at surface
• Air flows from high pressure to low pressure
• Flow is turbulent
• Turbulent flow produces vertical mixing

Mixing by Sensible Heat

• Convection driven by sensible heat
• Air parcels rise if they become heated and less dense than surrounding air
• As air parcels rises,
• Air expands
• Air cools
• Air becomes less dense
• Air parcels stop rising
• Heat transferred vertically, since air forced from high to low pressure, heat also moves horizontally

Adibatic Process

• Rising and sinking air change temperature with no gain or loss of heat
• Consider sinking parcel of air
• As it sinks, it contracts
• Contraction takes work
• Work takes (mechanical) energy
• Temperature of air rises
• Conservation of energy
• 1^st law of thermodynamics

Thermodynamics of Air

• First law of thermodynamics
• Heat added + work done = rise in Temp
• But, adibatic process (no heat added)
• Heat added + work done = rise in Temp
• Second term is not zero
• Work of compression results in a rise in temperature of air parcel

Mixing by Latent Heat

• Water vapor is less dense than mixture of gases composing the atmosphere
• Evaporation adds water vapor to atmosphere and lowers its density
• Moist air rises, expands and cools until dew point reached
• When air becomes fully saturated
• Condensation begins
• Air releases latent heat
• Air heats and becomes less dense causing it to rise further
• Eventually water vapor lost, air parcel stops release of latent heat and stops rising

Which Process More Important?

• Atmospheric circulation driven by adiabatic processes (sensible heat) redistributes about 30% heat
• Atmospheric circulation driven by latent heat redistributes about 70% heat
• Greater amount of heat stored in water
• Larger distances moist air parcels move

Dry Adibatic Lapse Rate

• Rising and sinking dry air parcel cools and heats at a constant rate
• Dry adibatic lapse rate = 10°C km^-1
• Work required to lift an air parcel
• Mix of gases
• Acceleration of gravity
• Regardless of latitude, season, altitude, etc. a dry parcel of air will heat or cool at 10°C km^-1

Dew Point Lapse Rate

• Consider a rising parcel of air with constant humidity
• Dew point decreases as parcel expands
• Drop in pressure, drop in dew point
• Lapse of dew point as parcel rises
• Dew point lapse rate 2°C km^-1
• Over 1 km, air cools by 10°C
• Air temperature rapidly approaches dew point as parcel rises
• As air temperature approaches dew point, cloud forms

Wet Adibatic Lapse Rate

• As wet air rises, it cools, dew point reached and condensation begins
• Latent heat released
• Decreasing rate of cooling
• Wet adibatic lapse rate
• 4°C km^-1 minimum (rapid condensation)
• 9°C km^-1 maximum (slow condensation)
• Differences in temperature
• For same amount of cooling, warm air looses more water than cold air

Summary

• Once saturation reached latent heat released as long as parcel continues to rise
• The saturated process assumes condensation products fall out of parcel, so the parcel maintains 100% humidity
• Upon decent, the parcel warms, relatively humidity falls below 100%
• After decent the parcel is warmer because latent heat was added during ascent
• Dry adibatic process reversible
• Wet adibatic process non-reversible

Thank you

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