|HOW CAN TEMPERATURES WARM|
ON A CLOUDY DAY?
METEOROLOGIST JEFF HABY
clouds act as a "heat shield". Much of the incoming solar radiation from the sun is reflected
or absorbed by the clouds. If clouds prevent incoming solar radiation from reaching the surface, then how do
temperatures rise during the day on a cloudy day?
1. Clouds do not prevent all of the incoming solar radiation from reaching the surface. Some radiation
still manages to penetrate all the way through the clouds and make it to the surface. If no radiation
made it to the surface then it would look like it was night outside during the day. This diffuse radiation that
manages to make its way to the surface can warm temperatures a little as the day progresses, especially
if the sun angle is high and days are long. In winter when sun angles are lower and days shorter, solar
energy will not be able to warm the surface much on a cloudy day.
2. The cloud height and thickness are important. High clouds tend to have a lower moisture content
than low clouds. This occurs because high clouds are located where temperatures are much colder.
Cold air has a low capacity to contain moisture. Thus, more of the sun's radiation is able to penetrate
high clouds and make it to the surface as compared to low clouds. Thin clouds such as cirrus will let
more radiation pass through as compared to thick clouds such as nimbostratus.
3. Temperature can warm on a cloudy day due to
thermal advection. Warm air advection can transport
warmer air into the region. Warm air advection often occurs when the wind direction is coming from
the tropical latitudes such as in the warm sector of a mid-latitude cyclone.
4. If precipitation is falling from the clouds on a cloudy day, temperatures will not warm as much as
when there is no precipitation.
Evaporative cooling from the precipitation will partially offset
any warming during the day. When some warming does occur, the
dewpoint will tend to rise since rain
continues to try to saturate the air.