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CLOUDS
A cloud is a visible mass of condensed droplets or frozen crystals floating in the atmosphere above
the surface of the Earth. They are made of water vapor which forms small droplets or ice crystals.
Billions of these droplets or crystals collected together become visible as clouds. Most clouds form
in the troposphere which is the lowermost part of our atmosphere located 10 to 15 kilometers above
the Earth. Dense clouds have high reflectivity which makes them appear white when seen from
above. Clouds may appear grey and dark from below when they have a high concentration of water
droplets building up within them which refract sunlight, hence why rain clouds are always much
darker than normal clouds at the bottom. The effect that clouds have on the Earth depends upon
their type and the time of day. Low thick clouds at night reflect very little radiation from above
but take in thermal radiation from the earth which can have a warming effect on the air below.
Areas with heavy cloud cover experience more moderate drops in temperature. Areas with no cloud
cover demonstrate more temperature extremes and the temperature goes much higher than it does
when there is a cloud cover. The effect of clouds on surface temperature results from their ability to
reflect sunlight from their top side and their ability to absorb and reradiate downward the thermal
radiation of the Earth's surface.
Surface Temperature Under Clouds
Modeled surface temperature over time for nighttime (top) and daytime (bottom) cases, and for
various wind speeds. The time series incorporates transitions from cloudy to clear after 24 hours,
then to cloudy again at hour 48.
Effects of Cloud Cover on forecasted temperatures.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.
During the day, the earth is heated by the sun. If skies are clear, more heat reaches the earth's
surface (as in the diagram below). This leads to warmer temperatures.
However, if skies are cloudy, some of the sun's rays are reflected off the cloud droplets back into
space. Therefore, less of the sun's energy is able to reach the earth's surface, which causes the earth
to heat up more slowly. This leads to cooler temperatures.
Forecast Tip: When forecasting daytime temperatures, if cloudy skies are expected, forecast lower
temperatures than you would predict if clear skies were expected.
At night cloud cover has the opposite effect. If skies are clear, heat emitted from the earth's surface
freely escapes into space, resulting in colder temperatures.
However, if clouds are present, some of the heat emitted from the earth's surface is trapped by the
clouds and reemitted back towards the earth. As a result, temperatures decrease more slowly than if
the skies were clear.
Forecast Tip: When forecasting nighttime temperatures, if cloudy skies are expected, forecast
warmer temperatures than you would predict if clear skies were expected.
Sources:
I. A Primer on Our Claim that Clouds Cause Temperature September 3rd, 2011 by Roy W. Spencer,
Ph. D.http://www.fundanomics.org/
II. Clouds,Cloudiness, Surface Temperature,the Greenhouse Effect and Global Climate Change by
ThayerWatkins,http://www.sjsu.edu/faculty/watkins/cloudiness.htm
III. Global Clouds 2009. NASA Earth Observatory image by Kevin Ward, based on data provided
by the NASA Earth Observations (NEO) Project. Instrument: Terra - MODIS Image via
Wikipediahttp://wattsupwiththat.com/2011/10/06/high-level-cloud-and-surface-temperature/
IV. High level clouds and surface temperature, October 6, 2011 by erl
happhttp://wattsupwiththat.com/2011/10/06/high-level-cloud-and-surface-temperature/
HeySteve@WeatherSavvy.com, http://weathersavvy.com/Q-Temperature_Clouds.html
V. Surface Temperature Under Clouds, http://stratus.ssec.wisc.edu/projects/tscloud/tscloud.html
Effects of Cloud Cover on forecasted temperatures,
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/tmps/cld.rxml