The Sun, the Haeting
The figure below is taken from Solar Irradiance in Wikipedia and it shows the frequency distribution of the sun’s radiation passing through a one meter by one meter area above the atmosphere in space, measured in nanometers(nm).
The yellow portion is an average of the sun’s radiation (measured by NASA) with instruments outside the earth’s atmosphere. In other words this is the radiation received by the earth and its atmosphere.
The black curve that almost coincides with the yellow is a calculation of the radiation from a hypothetical black body that is heated to 5,778 degrees Kelvin, that is 5,500 degrees centigrade. One can see that this calculated curve is very close to the measured sun’s radiation. One could also conclude that the average temperature of the sun is about 5,778 deg, K.
The visible portion of the sun’s radiation has wavelengths from 350 to 700 nanometers. Radiation at wavelengths below 350 nm are referred to as Ultraviolet (UV) and those above 700 are referred to as infra red (IR)
Some of the sun’s radiation is reflected by elements of the Earth and its atmosphere. Clouds, deserts and snow reflect radiation whereas oceans, forests, grasslands, concrete, asphalt and the like absorb much of the sunlight. The reflected radiation has been consistently measured at 30% of the sun’s received radiation and is referred to in the scientific community as Albedo. For a more detailed discussion on Albedo read the Wikipedia article with that name and see Appendix II
Now, if one were to measure the frequency spectrum of the sun’s energy received at sea level the result would be the red curve. That curve has depressions in it showing the absorption of the sun’s rays by water in the atmosphere and, at a wavelength of around 2,000 nm, there is some absorption by Carbon Dioxide.
Th red portion is absorbed by the earth. The difference between the red and yellow portions is the radiation absorbed by the atmosphere or reflected by the earth and clouds.
The area of the yellow portion is equivalent to 1,361 watts per square meter. (w/sm) That energy falls on the rotating earth, including it’s atmosphere, and the average over the whole earth, taking into account day versus night and the curvature of the earth’s surface is 340 w/sm. About 100w/sm are reflected by clouds and the earth’s surface and the remainder heats the earth
The area of the red portion is approximately 238 watts per square meter when averaged over days and the rotating earth.
So, that is the energy coming in from the sun. We will now look at the energy radiated out by the Earth. Remember that for the Earth's temperature to be constant the Earth must radiate as much energy as it receives from the sun.
Now, if one were to measure the frequency spectrum of the sun’s energy received at sea level the result would be the red curve. That curve has depressions in it showing the absorption of the sun’s rays by water in the atmosphere and, at a wavelength of around 2,000 nm, there is some absorption by Carbon Dioxide.
Th red portion is absorbed by the earth. The difference between the red and yellow portions is the radiation absorbed by the atmosphere or reflected by the earth and clouds.
The area of the yellow portion is equivalent to 1,361 watts per square meter. (w/sm) That energy falls on the rotating earth, including it’s atmosphere, and the average over the whole earth, taking into account day versus night and the curvature of the earth’s surface is 340 w/sm. About 100w/sm are reflected by clouds and the earth’s surface and the remainder heats the earth
The area of the red portion is approximately 238 watts per square meter when averaged over days and the rotating earth.
So, that is the energy coming in from the sun. We will now look at the energy radiated out by the Earth. Remember that for the Earth's temperature to be constant the Earth must radiate as much energy as it receives from the sun.