Energy Balance
The inputs and outputs of energy within the earth-atmosphere system that determines the net energy available for surface processes is the Energy Balance
Electromagnetic Radiation Electromagnetic radiation can be: Absorbed increases the energy content of the object, like a black car in the summer Reflected bounce the energy back toward the source, like a mirror Transmitted pass through without absorption or reflection, like a pane of glass
Transmission the passage of shortwave and longwave energy either through the atmosphere or water Inputs = shortwave radiation from the sun that passes through the atmosphere Outputs = longwave radiation from the earth s surface and atmosphere that passes through the atmosphere
Several things happen to insolation after it enters the atmosphere: Not all of the energy that enters the top of the atmosphere reaches the earth s surface
Losses of incoming solar radiation Refraction: the bending of insolation as it moves through the atmos towards earth s surface as insolation moves through the atmos it passes through one medium to another The speed of insolation is reduced as it moves through air of increasing density The decrease in speed as it enters thicker layers of he atmosphere causes it to shift direction or bend (refract)
Losses of incoming solar radiation Reflection: the portion of incoming solar energy that bounces directly back into space without being absorbed within the earth-atmosphere system Decided by the albedo Albedo: is a measure of the reflective quality of a surface the % of insolation that is reflected by the surface
Albedo depends on the color and texture of a surface and the angle of incoming solar radiation. Dark colors absorb more energy than lighter surfaces: Fresh snow = 80-90% Grass = 25-30% Crops = 10-25% Asphalt = 5-10% Forest = 10-20% Angle of insolation also affects albedo
Losses of incoming solar radiation Scattering: gas molecules absorb and reemit radiation, changing it s direction but does not change it s wavelength Some radiation absorbed by the atmos is reradiated towards earth s surface while the rest of it is reradiated back to space
Diffuse Radiation the portion of insolation absorbed by the atmos and scattered towards the earth s surface Diffuse Reflection the portion of insolation absorbed by the atmos and scattered back to space
Shorter wavelengths are scattered more easily than longer wavelengths (Lord Rayleigh, 1881) Short wavelengths are more susceptible to be scattered by small molecules in the atmos For small molecules in the atmos, scattering depends on 1/λ 4
The colors we see: When we see a color we actually see the wavelength of visible light that is reflected by that object. All other wavelengths are absorbed. Black happens when all visible wavelengths are absorbed White happens when all visible wavelengths are reflected The greater the amount of atmos that light has to pass through to reach the surface the greater the amount of scattering: That s why the sky is blue!
Absorption the assimilation of radiation by molecules changing it from one form of energy to another Absorption increases temperature of object Absorbed energy is reradiated at longer wavelengths (ozone layer)
Clouds are variable and greatly affect the amount of energy near the surface Reflect insolation back to space cooling effect (cloud-albedo forcing) Absorb longwave radiation and reradiate it back to the surface warming effect (cloud- greenhouse forcing) Clouds can make it cooler during the day by reflecting large amounts of insolation but warmer at night absorbing and reradiating longwave terrestrial radiation
Top of Atmosphere Energy Balance Source Input Sun (shortwave) 100% Output Total 100%
Source Top of Atmosphere Energy Balance Input Output Sun (shortwave) 100% Reflection (shortwave) 21% + 6% + 4% Total 100% 31%
Source Top of Atmosphere Energy Balance Input Output Sun (shortwave) 100% Reflection (shortwave) 21% + 6% + 4% Longwave loss to space 63% + 6% Total 100% 100% Top of the atmosphere is balanced!
Surface Energy Balance Source Input Sun (shortwave) 24% + 24% Output Total 48%
Surface Energy Balance Source Input Sun (shortwave) 24% + 24% Longwave 97% Output Total 145%
Surface Energy Balance Source Input Output Sun (shortwave) 24% + 24% Longwave 97% Longwave 107% + 6% Total 145% 113% The surface energy budget is not balanced!
Where is the other energy to make the surface balance?
Sensible Heat Sensible Heat heat you can feel (sense) Energy exchange between objects by flow of a fluid (atmosphere) Convection Energy exchange between objects by physical contact Conduction
Convection: Energy is transferred by the vertical mixing of a liquid or gas Ex: boiling water in a pan Conduction: As molecules warm they vibrate more rapidly, bumping into nearby molecules, causing them to vibrate This transfer of kinetic energy is heat Ex: touching a hot pan or a cold surface
Surface Energy Balance Source Input Output Sun (shortwave) 24% + 24% Longwave 97% Longwave 107% + 6% Sensible heat 10% Total 145% 123%
Latent Heat Latent Heat energy stored by changing water from solid or liquid to a gas Latent no change in temperature; change in phase of water molecules Liquid to Gas Evaporation Solid to Gas Sublimation Energy is released during precipitation or condensation
Surface Energy Balance Source Input Output Sun (shortwave) 24% + 24% Longwave 97% Longwave 107% + 6% Sensible Heat 10% Latent Heat 22% Total 145% 145% The surface energy balance is now balanced!
Atmospheric Energy Balance Source Input Output SW Absorption 3% + 18% Longwave 107% Longwave 97% + 63% Sensible Heat 10% Latent Heat 22%. Total 160% 160%
Global Energy Budget All three parts (top of the atmosphere, earth s surface, and the atmosphere) are now balanced Input = Output therefore, the planet is in a state of equilibrium (constant temperature)
The Greenhouse Effect
Like the sun, the earth emits electromagnetic radiation Unlike the sun, the earth is cooler and emits LW energy within the infrared wavelengths The earth emits longwave energy in all directions away from the earth s surface Some of this energy is radiated into space Some of this energy is absorbed by the atmosphere The energy that is absorbed by the atmosphere is reradiated by the atmosphere Some is radiated back to space The rest is reradiated back to the surgace of the earth
The counterradiation of LW energy warms the troposphere similar to the way the inside of a greenhouse is warmed the greenhouse effect Greenhouse glass transparent to SW energy from sun SW energy absorbed and and reradiated as LW energy Glass is not transparent to longer IR wavelengths LW IR wavelengths are sensed as heat Thus the process traps heat within the greenhouse making it warmer than outside
The earth s atmosphere acts like panes of a greenhouse: The atmos lets in solar rad Rad is absorbed by the earth s surface SW energy is reradiated as LW along with terrestrial rad Some LW energy is blocked from entering space warming GH effect is necessary. Without it we would freeze GH is increasing due to increased CO 2 over past 200yrs
Daily Radiation
Daily Radiation radiation evaluated for a period of 24 hours Daily radiation received at a certain location varies with season and latitude It also varies with degree of cloud cover No insolation is received at night The amount of insolation peaks at solar noon (when the sun is at its highest altitude) Insolation decreases from solar noon to sunset
Daily Radiation Air temperature responds to the changing amount of insolation throughout the day Minimum daily temperature occurs just after sunrise Air temperature begins to increase soon after sunrise when insolation starts to be absorbed by the surface Air temperature peaks around 3pm after solar noon When outputs of energy exceed the inputs, temperature begins to decrease
The Urban Environment Urban environments differ from surrounding rural environments - Urban environments generally have lower albedos - Surfaces tend to be drier Therefore urban areas tend to be warmer b/c they absorb and retain more energy and lower surface moisture leads to less cooling by evaporation and transpiration
Transpiration water is taken from the soil by plants and released by their leaves Water changes state from liquid to vapor cooling the plant through the loss of latent heat of evaporation
Rural Areas: Overall higher albedos than urban More plants for transpiration Moisture is retained by the soil Solar energy absorbed by soil is used from evaporation Evaporation cools surface through the loss of latent heat of evaporation Forests intercept insolation before the ground, spreading insolation over the top of the forest canopy rather than along the ground
Urban areas: Lower albedos Geometry Hunan activities pollution