Daily Temperature Variations

Let's look at the energy balance in terms of the daily variations in temperature. Incoming solar energy is a maximum when the sun is highest in the sky. But the surface of the earth warms continuously from the time the sun rises to the time it sets. Therefore, the earth radiates its maximum amount at sunset, and does not begin to slow down until the sun sets (no more incoming radiation). The earth is releasing the least amount of energy early in the morning, before the sun rises. The net energy at the surface is therefore the difference in the incoming radiation (shortwave) and the radiation emitted from the earth (longwave). The chart demonstrates further. Here, the energy surplus area means that more energy is entering the earth than is leaving it. The deficit area is just the opposite -- more energy is lost by the Earth than gained (due to the absence of sunlight). Remember, the total amount of net energy gained is zero -- there is an energy balance. The surplus area on the chart is showing the daily replenishment of energy into the Earth-atmosphere system necessary to maintain that balance.

One important item to notice is that the earth's maximum emission of heat is skewed to the right (toward sunset) of the maximum incoming energy . Another key observation is that the earth emits and absorbs radiation much more efficiently than the atmosphere. That is why the late afternoon is often much warmer than at noon, when the incoming shortwave radiation is a maximum. That's also why the early morning is the coolest time of day. The atmosphere warms and cools slowly, but the ground warms and cools much more quickly. The next time you're outside, feel the ground with your hand. If it is mid-day, it will feel significantly warmer than the air. If it is late night, it will feel significantly cooler. Differences in the rate of radiation absorption and release are a major factor influencing energy balance. This, and other factors, are discussed in the next section.