This series of Haby Hints investigates problems that cause a forecast to bust. A bust occurs when a certain weather parameter is expected but one or more factors cause the forecast to be wrong. This particular Haby Hint will focus on how cloud cover (more or less cloud cover than expected) causes forecast problems.
Cloud cover has a dramatic effect on the temperature forecast. Clouds can cause the surface temperature to be more or less than expected. The amount of solar energy reaching the surface is a strong determinate of surface temperature. The troposphere is heated through solar radiation's sensible heat input over the earth's surface. Since temperature measurements are made near the surface, fluctuations in this solar energy change the surface temperature. Clouds act as a regulator to the amount of solar radiation that reaches the surface. Clouds reduce the temperature during the day depending on the cloud thickness, how widespread the clouds are and how long they block solar radiation. If more cloud cover occurs than is expected during the daylight, the surface temperature will generally be cooler than expected. If less cloud cover occurs than is expected during the daylight, the surface temperature will generally be warmer than expected.
This forecast challenge often occurs on the west coast of the U.S. in association with the marine layer. The timing of when the marine layer mixes out will determine how much solar warming can take place. Also, more cloud cover than expected in places such as the Great Plains in spring can reduce the risk of severe weather by limiting the surface warming. Instability will generally be less if solar radiation is significantly reduced from reaching the surface. As another example, a low stratus / fog deck can be difficult to forecast if and when it will dissipate. If the cloud deck does not dissipate when it is expected to, the surface temperature forecast will be significantly off. Huge temperature forecast busts will occur when a stratus deck is expected to mix out and does not.
Clouds have the opposite effect on temperature at night. More cloud cover than expected at night tends to result in temperatures that are warmer than expected. Less cloud cover than expected at night tends to result is temperatures than are cooler than expected. At night, the earth's surface gives off energy in the form of longwave radiation and does not receive any shorter wave radiation from the sun to warm the surface. Water vapor happens to be a greenhouse gas. When clouds are present, less longwave radiation energy will escape into outer space. The result is warmer temperatures. On a clear night, the maximum amount of longwave radiation will be able to escape into space and thus surface temperatures will cool at the maximum rate for the given weather conditions.
As mentioned in the Great Plains severe thunderstorm case, clouds can have a dramatic impact on the precipitation forecast. If clouds cause the boundary layer to be cooler than expected this could prevent the capping inversion from being broken in a thunderstorm situation. If the cap is not broken then no thunderstorm precipitation will occur in many cases. In the opposite case, a reduction of cloud cover can enhance instability during the day and increase the thunderstorm threat.