|FORECAST BUST: CLOUD COVER
METEOROLOGIST JEFF HABY
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
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
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.