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The surface of the sun is so hot (temperature is 5500°C) that it gives off an amount of light equal to more than 6 million, 100 Watt light bulbs in every square meter. As this energy spreads out into space, it is reduced to slightly less than fourteen 100 Watt light bulbs on every square meter by the time it reaches the Earth.

100% emitted sunlight

Life on Earth depends on energy coming from the Sun. About half the light reaching Earth's atmosphere passes through the air and clouds to the surface, where it is absorbed and then radiated upward in the form of infrared heat. Because Earth is shaped like a ball, not all locations face the sun directly, so tropical regions receive more sunlight than polar regions. Earth's daily rotation also decreases the time for receiving sunlight. Thus, the average amount of sunlight entering the top of the atmosphere is similar to 3.4, 100 Watt light bulbs on every square meter (=100%).

Planetary Reflectivity [IMAGE]
The reflectivity of Earth is the fraction of the sunlight that is reflected back to space (31% on average); the remaining 69% of sunlight is absorbed. The colors on the map show the changes of reflectivity from the darker, less cloudy zone near the equator to the brighter, more cloudy zones near the poles.

24% reflected by clouds -- 4 % reflected by the atmosphere

Almost all of the reflected sunlight (24% out of 31%) is reflected by clouds, which cover 3/5 of Earth. In cloud-free areas the remaining 7% of the sunlight is reflected by the atmosphere (4%) and by the surface (3%). If clouds covered the whole Earth they would reflect 38% of the sunlight, more than twice what would be reflected by a completely cloud-free Earth (17%).

19% absorbed by gases

Atmospheric gases, mostly water vapor with a little help from ozone and carbon dioxide, also absorb 19% of the sunlight, providing one third of the heating of the atmosphere.

3% reflected by the surface

Because clouds and the atmosphere prevent some of the sunlight from reaching the surface and prevent some of the sunlight reflected from the surface from going back to space, the total amount of reflected sunlight going back into space from the surface is only 3%.

Surface Reflectivity [IMAGE]
The average reflectivity of the surface is 14%. The map colors show the changes of reflectivity from the dark oceans, to land covered by vegetation, to bright deserts (like North Africa), to the brighter snow and ice of Greenland and Antarctica.

50% absorbed by the surface

The surface is heated by absorbing 50% of the sunlight, almost 3/4 of the total absorbed by the whole Earth. Most of this energy is absorbed by the dark oceans, which store so much energy that, if all sunlight were removed tomorrow, the oceans would only begin to cool noticeably after a few decades. The near-equator zone is heated more because the amount of absorbed sunlight is larger than the emitted infrared radiation (by an amount equal to one 60 Watt light bulb every square meter).The poles are cooled because the infrared radiation is larger than the absorbed sunlight (by one 100 Watt light bulb every square meter). This pattern of heating and cooling drives atmospheric winds (the weather) and ocean currents.

The Green House Effect

The "greenhouse effect" is the warming of climate that results when the atmosphere traps heat radiating from Earth's surface. Certain gases in the atmosphere resemble glass in a greenhouse, allowing sunlight to pass into the "greenhouse," but blocking Earth's heat from escaping into space. Land surface temperatures respond very quickly to changes in sunlight so that nighttime is cooler than daytime. At middle latitudes, land surface temperatures are larger than ocean temperatures in summer and smaller in winter. The warm surface cools by emitting infrared ("heat") radiation.

30% latent heat -- 6 % sensible heat

Earth's surface also cools because of direct contact between the atmosphere and surface (6%, more important over land) and because of evaporating water (30%, more important over oceans).

102% emitted longwave -- 116% emitted longwave

Earth's surface absorbs 50% of the sunlight, but its temperature is so high that it emits an amount of energy (116%), greater than the total from the sun (100%).This is possible because most of the infrared radiation is absorbed by water vapor and carbon dioxide in the atmosphere, then the heated atmosphere emits infrared radiation (102% of incident sunlight) downward towards the surface, so that the total heat loss by radiation is only 14%.This is the greenhouse effect. Without it, Earth's surface temperature would be 35°C colder, well below the freezing point of water.

Surface Temperature [IMAGE]
Surface temperatures are much higher near the equator (approx. = 30°C averaged) where more sunlight is absorbed than at the poles. Antarctica has the coldest average surface temperature because the infrared radiation is larger than the absorbed sunlight (< -53°C) This pattern of heating and cooling drives atmospheric winds (the weather) and ocean currents. The colors in the map show the surface temperature average over the whole year. High mountain tops show up as the colder locations.

19% absorbed solar -- 39% emitted by clouds

Almost all of the infrared radiation emitted into space that cools Earth comes from the atmosphere, which is heated by absorbed sunlight (19%), radiation from the surface (14%), direct contact (6%), and by the condensation of evaporated water to form clouds rain and snow (30%) The larger cloudy part of the atmosphere emits 39% of the total radiation and the smaller clear part emits 30%. If Earth were completely cloud covered, the emitted radiation would be reduced to 67%; if Earth were completely cloud-free, the emitted radiation would be increased to 75%.

Planetary Temperature [IMAGE]
The amount of infrared radiation emitted from the atmosphere is as if its average temperature is -19°C. The colors in the map show that the temperature decreases from the equator to the poles, but that clouds produce a cold zone near the equator.

69% total longwave

Of the total sunlight falling on Earth (100%), 31% is reflected back to Space and 69% is absorbed. The absorbed radiation is then emitted as infrared radiation. Scientists reach these conclusions by studying data gathered by satellites and computer simulations of the working of Earth's environment.

Technical Direction: Dr. Robert Price, NASA, Goddard Space Flight Center.
Science Mentors: Dr. William Rossow, NASA, Goddard Space Flight Center; Dr.Yuanchong Zhang, Columbia University
Alison Walker, Hughes/STX
Concept/Design/Production: M. Sara Tweedie, Faculty, Corcoran School of Art.
Illustrations, Design Prototype: Macala Devis, Corcoran School of Art.
Scientific Visualization: Dave Pape, NASA, Goddard Space Flight Center; Sara Tweedie

National Aeronautics and Space Administration
Goddard Space Flight Center
Greenbelt, MD 20771
NP-1997(01)-003-GSFC