Ground Cover

We have mentioned that different surfaces absorb and emit radiation at different intensities and wavelengths. The atmosphere and earth's surface are composed of countless substances, each with its own energy properties. In this section, we discuss how some of the more important factors and substances affect energy balance in the atmosphere. These areas include ground cover, cloud cover, greenhouse gases, and pollution. This page briefly discusses ground cover.

Sunlight (shortwave radiation) hits the surface of the earth, and is converted into heat. However, the earth's surface is made up of highly varied areas each with their own radiation absorption and emission characteristics. For instance, a forest does not absorb or release heat as quickly as asphalt on a highway. The heated air on the highway will rise more quickly than air in the forest. This may seem obvious to some, but the details are very important when looking at the specifics of turbulence in the atmosphere. In general, darker surfaces absorb and emit energy more quickly than lighter surfaces. Lighter surfaces tend to reflect more energy and absorb less of it.

Water is another surface of great importance. You may have noticed that air temperatures near major bodies of water seem warmer in the winter and cooler in the summer. That is because water absorbs and emits radiation much slower than land. Water also has a much greater heat capacity than the land. The heat capacity of a substance is the ratio of the amount of heat absorbed by the substance to the amount of temperature rise. The specific heat of a substance is the amount of energy required to raise the temperature of one unit of mass of the substance one degree. Pure water, with an initial temperature of 15 C, has a specific heat of one cal/g for a change of one degree Celsius. The specific heat of sandy clay is only 0.33 cal/g x C. Though a body of water may receive intense summer sunlight, it will warm much slower than the land near it. If the winds are blowing onshore, the cooler air above the water can be mixed with the warmer air over land, allowing the surrounding air to cool. The opposite is true in the winter. In the same way the water warms more slowly than the land, the water also cools more slowly. The warmer air from above the water mixes with the surrounding air to warm the land. This often occurs on a daily basis, forming what we call the sea breeze and land breeze (discussed in other readings). Also, water goes through evapotranspiration processes that impact the local energy balance near the surface. Water (moisture) also has an impact on the energy balance in the atmosphere, especially when it is visible, in the form of clouds.