Energy Balance and Solar Constant

How much radiation does the earth's surface absorb? What causes the air to warm during the day? How does the temperature change over the course of a day? We will try to answer these questions in this section.

The Solar Constant

As mentioned earlier, almost all the energy we see and feel comes from the sun. Scientists have determined a standard value for that amount of energy, which is called the solar constant. It is the average amount of energy that reaches the top of the atmosphere, on a plane perpendicular to the sun's rays, at an average distance from the sun. This value is approximately 1370 watts per square meter (W/m2). We say approximately because the sun's energy output does change slightly with time, due to sun spots and other phenomena, which we'll save for the astronomers. There are two key points, however. First, the sun's energy output is relatively constant, and there is essentially no interference (obstructions) with that radiation until it enters the atmosphere.

The Stefan-Boltzman Law gives the amount of energy emitted per second per unit area by a blackbody as a function of temperature.

P/A = 5.6703e-08 T4 W/m2
Quick Quiz: What temperature would the Earth have to radiate at in order to have an equal amount of radiation and absorption if the Earth was a perfect absorber and a perfect emitter (i.e. no atmosphere, low albedo surface)?
258 K
279 K
331 K
394 K

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