Clouds and Temperature

The moderating effect of clouds

Clouds both block radiation and emit radiation. During the day, the blocking effect is most important, and the temperature does not rise as much as it would on a clear day. During the night, the clouds help radiate energy back down to Earth, so the temperature does not drop as much as it would on a clear night. In a nutshell, clouds have a moderating influence on temperature, making it warmer at night and cooler during the day.

Not all clouds are created equal. As a general rule, the lower the clouds, the greater their effect on temperature. Also, the thicker the clouds, the stronger their effect. You can get a good sense for the effect of clouds on temperature by how dark they make it during day. If it's real dark, you know that not much solar radiation is getting through and that therefore the surface temperature will not warm up much. Perhaps you've noticed that thunderstorms, solid low clouds, and fog tend to create the darkest conditions. On the other hand, high, thin cirrus clouds have essentially no effect on surface temperature.

Using clouds in your forecast

Exactly how important are clouds? They can be extremely important. I've seen cases of dense fog where the temperature did not change by more than one or two degrees all day. Often, an unexpected patch of clouds at night produces a five degree temperature error in the forecasts. It definitely pays to examine the latest satellite images for hints as to whether the station of interest will be clear, cloudy, or partly cloudy. If conditions will be different from what the model forecasts, you can expect that the temperatures will be different, too.

Often the best source of guidance on the effect of clouds is persistence. Suppose you have identified a patch of clouds which you expect will make it over the station tonight. Look to see where the clouds were last night. See what the temperatures were beneath the clouds compared to outside the area of the clouds. Also, look at last night's numerical model forecasts to see if the model is correctly forecasting the influence of the clouds. Usually the model will be making a compromise forecast, sort of an average of conditions beneath the clouds and conditions outside the clouds. If you can make a confident forecast of the cloud conditions, you may be able to improve significantly on the model forecasts.

Review questions:

Which of these sky conditions would produce the coldest maximum temperatures?

Which of these sky conditions would produce the coldest minimum temperatures?