Earth’s Energy Balance The hydrologic cycle is fueled by energy from the sun. Planetary geometry creates areas of energy surpluses and deficits which drive all active meteorological processes. Earth and the atmosphere are the media through which the energy transport occurs Water transport and phase changes [i.e. liquid (oceans)  vapor (humidity)  liquid (precipitation)] play a major role energy transport

Earth’s Energy Balance reflected from particulates in air, clouds and the earth’s surface - 30 % longwave radiation emanated - 70 % longwave radiation from clouds, vapor, etc. absorbed by atmosphere (water vapor, dust, clouds) - 19 % back radiation from earth - 20% absorbed by earth - 51% earth heat entering atmosphere - 0.002 % (geothermal)

Earth’s Energy Balance Short-wave energy from the sun moves through the atmosphere to the earth more easily than longwave energy can move from earth through the atmosphere. This keeps the planet warm Planetary geometry creates areas of energy surpluses and deficits. Incoming solar radiation is uneven because the earth is a sphere which rotates on a tilted axis. Outgoing radiation is more uniform because the temperature of earth’s atmosphere does not vary all that much from the equator to the poles (~ 30 C). Energy gradients drive global energy transport processes such as wind and ocean currents.

Earth’s Energy Balance Net radiation balance is positive for latitudes below 35 (receive more radiation than is emitted), and negative for latitudes above 35. Therefore there is a net poleward transport of energy to maintain a balance (2/3 of this transport occurs in atmosphere and 1/3 in the oceans). Radiation (both short and longwave) is the energy source leading to evaporation. Large quantities of energy are carried by water vapor. (This is the energy absorbed by molecules during phase change from liquid to vapor)

Radiation Physics All matter at a temperature above absolute zero radiates energy in the form of electromagnetic waves that travel at the speed of light (lf=c ). The rate at which this energy is emitted is given by the Stefan - Boltzmann law: The value of E ranges from 0 to 1 depending on the material and texture of the surface. E = 1  Blackbody. Reflects no radiation. Absorbs and re-emits radiation in proportion to surface area. E  1  Grey body. Radiates a fixed proportion (less) of blackbody radiation at all wavelengths for a given temperature. Stefan-Boltzmann constant = 5.67 x 10-8 Watts/(m2K4) = 1.38 x 10-12 cal/(cm2K4sec) = 8.28 X 10-11 cal/(cm2 K4 min)

Radiation Physics Blackbody radiation intensity is distributed over various wavelengths. Spectrum of radiation of a black body: Blackbody radiation spectrum follows this curve at all temperatures. Wiens Displacement Law -T peak always at T = 3000mK area under curve is s Radiation wavelength T temperature

Radiation Physics Sun radiates energy approximately as a black body at 6000 K  high temperature/short wavelengths. Not all this energy reaches the earths surface. Some is absorbed by atmospheric gases (i.e. O2 and O3 absorb UV radiation which can be harmful to biota). Depletion of O3 will increase UV incidence at earth’s surface  concern about ozone hole.

Radiation Physics Earth radiates energy approximately as a black body at 290 K lower temperatures/ longer wavelengths. Some of this radiation is absorbed by atmospheric gases (i.e. H2O and CO2 absorb infra-red (IR) radiation  greenhouse effect). Without H2O and CO2, the earth’s surface would have a temperature of ~ -18C  Concern that fossil fuel combustion increases the CO2 levels which increases the temperature of the earth  global warming.

Radiation Physics Based on the sun’s temperature and the Stefan - Boltzmann law, the total energy emitted by the sun is: Because of the earth’s distance from the sun, only a small fraction of this total energy is received at the outer edge of the earth’s atmosphere. Intensity of solar radiation at a plane or the upper atmosphere  to incoming solar radiation is called the solar constant:

Solar Radiation Because the earth is a sphere which rotates on a tilted axis the intensity of solar radiation on a plane perpendicular to the earth’s atmosphere varies in space and time. Solar radiation (wo) spread over larger surface area on the earth’s surface. Thus less radiation/(area time)  lower temperatures   =  - latitude  - solar altitude - angle of incoming radiation with plane tangent to earth-atmosphere surface  - declination of the sun - latitude at which sun is directly overhead - ranges from 23.17S to 23.17N

Solar Radiation Rs=insolation = effective radiation intensity incident at outer edge of atmosphere Rs = wosin If earth’s axis were perpendicular to plane of revolution,  would be a function of latitude only (  90 - ). However, because of the angle of revolution,  varies with latitude, declination (time of year), and longitude.

Solar Radiation Equation for total daily insolation is: Tsunset = Number of hours after solar noon that sunset occurs (Note: sunrise and sunset occur at equal times before and after solar noon)

Solar Radiation This equation gives radiation at outer edge of atmosphere. This solar radiation is further reduced as it moves through the atmosphere by scattering by molecules and particulates and absorption and scattering by clouds. The net radiation received at the earth's surface is further reduced by absorption by vegetation and reflection by earth materials. albedo - A - Reflectance of solar radiation by earth materials. Earth’s average albedo for shortwave radiation, As = 0.32. It ranges from 0.08 for black moist soil to 0.4 - 0.8 for snow. Longwave albedo is essentially zero for all earths surfaces except water. For water, Al = 0.03

Net radiation received at earth surface: