Determining Cloud Base Altitude
What is the formula for determining the elevation of the base of a cloud?
A quick an dirty method is known at the Bradbury Rule -
Cloud base in feet = (temperature/dew point spread) * 400
Try it and see how well it works.
I do not know of an accurate easy formula to use, as the level at which
condensation of water vapor to cloud droplets occurs depends on
the height at which the temperature equals or is lower than the
dewpoint temperature. That height can vary greatly from one day
to the next and one season to another because of the ever changing
water vapor content of the air. The height can be experimentally
determined from a radiosonde sounding or profiles of the atmosphere
taken with aircraft. For operational purposes, the National
Weather Service uses the information from radiosondes in conjunction
with equations in a computer model to determine the "convective
condensation level" (CCL) - the height at which convective clouds like
cumulus will form from buoyant lifting of the air (caused by
parcels of air warmed by the Sun's heating of the ground), or the
"lifting condensation level" (LCL) - the height at which status clouds
will form resulting from mechanical lifting of the air.
A simple estimate of the LCL can be calculated from
the following equation (found in Wikipedia):
LCL P = 120 (T-Td),
where P is the pressure at the height of the cloud base, T is the
surface air temperature (in degrees F) and Td is the surface dewpoint
(in degrees F).
CCL P > LCL P
The CCL is usually higher than the LCL because warm parcels
of air that rise convectively are always warmer than their
surroundings and therefore do not cool to saturation until above
the height at which the temperature of the surrounding air equals the
dewpoint. In other words, convective air parcels overshoot the LCL.
The height at which the LCL P occurs could be determined with the
US or International "standard atmophere", but that will not always be
very accurate, as the structure of the atmosphere is rarely just
like the standard atmosphere, which is a reflection of average conditions.
You can find standard atmosphere calculators on the Internet.
David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory
Click here to return to the Weather Archives
Update: June 2012