Part 3:
Factors Influencing Energy Balance

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.

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 (see albedo table).

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 (these are discussed in Session 5). 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.

Cloud Cover

Clouds play an important part in energy balance. Clouds both reflect and release radiation efficiently. Shortwave radiation from the sun is efficiently reflected, and longwave radiation from the earth is efficiently absorbed and emitted. High, thin clouds radiate heat (longwave radiation) back towards the earth, whereas low, usually thicker clouds, reflect incoming sunlight, but also absorb and emit heat from the surface. That is why in the summer, very cloudy days seem cooler than clear days, and cloudy nights seem warmer than clear nights. The clouds act as trapping agents, keeping the surface heat in, but not allowing much more to enter or escape (see diagram below).

Influence of clouds on the earth's energy balance

Greenhouse Gases

As mentioned earlier, greenhouse gases trap heat and emit it very efficiently. This plays a crucial, but complicated role in energy balance. As greenhouse gases increase in concentration, the air will gradually warm. The warmer air will cause more surface water to evaporate, which may lead to the formation of more clouds. But these clouds could prevent sunlight from warming the earth. So although greenhouse gases may lead to global warming, they may also lead to more clouds, possibly negating the warming effects. This is but one interaction involved in the complex issue of energy balancing, one we do not fully understand.


Airborne pollutants also affect energy balance. Pollutants in the atmosphere react with other natural and anthropogenic chemicals present, sometimes releasing heat energy, sometimes absorbing heat energy. Some pollutants also play a role in the greenhouse effect. Atmospheric chemistry is an entire field of study, which we encourage you to learn more about by looking at our companion course in Computational Atmospheric Science on-line. Though the scope of this course will allow us to mention it only briefly here, we emphasize that chemistry plays an important role in energy balance in the atmosphere.

On to Energy Balance

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