Estimation of the height of the cloud base

Dew point depression

Cumuliform cloud heights (convective clouds) - Surface air temperature and dew point

Clouds are generally formed by the air being cooled in some way to the dew point temperature of the air; just below this temperature the invisible water vapour is condensed into the visible water drops forming the cloud. This cooling is achieved in most cases by the air being lifted to a higher level; the rate of cooling of the air as it ascends is almost constant. It is a simple matter to calculate how far the air must rise for cloud to form, providing the initial temperature and dew point temperature are known.

If cumuliform cloud (Cu, TCu or CB) is forming above the observing location, the approximate cloud base height (in feet) can be calculated by the following formula:

Cumuliform cloud base = (Air temperature – Dew Point temperature) x 400

This method can be used for determining the approximate height of cumuliform clouds on a day when the air at ground level is warmed until it becomes light enough to rise upwards. When it reaches a level where it has cooled to its dew-point temperature, the cloud begins to form.

Example:

The surface air temperature is 22.3°C

The dew-point temperature is 11.3°C

Cumuliform cloud base = (Air temperature – Dew Point temperature) x 400

= (22.3 – 11.3) x 400

= 11 x 400

= 4,400 feet

If the surface air temperature is 22.3°C and the dew-point temperature is 11.3°C then the difference is 11°C. When the surface air is lifted 4400 ft (11 x 400) the air temperature would have fallen to the dew-point temperature of the rising air and condensation will take place.  This would be a good approximation of the cloud base.

The height of cloud base obtained in this way is a guide only. It is most accurate for Cumulus clouds being formed inland and in dry climate zones (especially in the afternoon).  For coastal or humid tropical areas in the morning a ‘x 300’ multiplication factor may be more suitable.