Describing moisture content

The quantity of water vapour present in the atmosphere varies over distance and time. The highest surface values occur in the tropics near the sea surface while the lowest occur in the high interior of the Antarctic continent where the air is very cold.

The amount of moisture in the air also tends to decrease with height. At and above the tropopause the atmosphere is almost devoid of moisture. However, an increase in water vapour with height may sometimes occur.

Relative humidity

Relative humidity (RH) is a moisture measure. It is the ratio of the mass of water vapour actually present in a unit volume of the air to the amount of water vapour required to saturate it, without a change in temperature. The relative humidity is usually expressed as a percentage.

It is important to note that warmer air requires more water vapour to saturate it compared to colder air. (Warmer and colder being relative terms). This means a warmer air mass with an RH of say, 80%, contains more water vapour than a colder airmass with an RH of 80%. More water vapour means more stored energy in the form of latent heat.

Dewpoint temperature

The dewpoint temperature is the temperature to which a sample of moist air has to be cooled at constant pressure to become saturated. If the temperature of the air is cooled below the dewpoint, condensation will occur. An everyday occurrence of condensation is the water droplets that form on the outer surface of a cold glass of your favourite beverage because the temperature of the glass is below the dewpoint of the air in contact with it.

If the surface temperature and dewpoint temperature are known, it is possible to use a rule of thumb to determine the cloud base of clouds formed by convection from the surface (rising buoyant air parcels). As will be discussed in later topics, a rising parcel of unsaturated air will cool at a rate of 3°C per 1000 feet. This is known as the Dry Adiabatic Lapse Rate (DALR). The dewpoint lapse rate is 0.6°C per 1000 feet (the dewpoint lapse rate appear as mixing ratio lines on an aerological diagram more about this later). As the difference between the DALR and the dewpoint lapse rate is 2.4°C per 1000 feet or 1°C per 400 feet, it can roughly be determined at what height rising air parcels will saturate and form into clouds using the following formula:

Convective cloud base = (Air temperature – Dewpoint temperature) x 400

For example:

  • for a surface dewpoint of 10°C and a surface temperature of 19°C, the cloud base should be near 3600 feet (9 X 400)
  • for a surface dewpoint of 15°C and a surface temperature of 30°C, the cloud base should be about 6000 feet (15 X 400)

This is a useful approximation, but like all rules of thumb, it should be used with care.

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