Basics of Environmental Science

Earth Sciences / 65

(which states that in an isolated system, such as a parcel of air, the total internal energy of the
molecules can be changed only by work done by them or on them). The concept of total internal
energy explains why cold air high above us does not simply sink to the surface: were it to do so it
would warm adiabatically to a temperature as high as, or higher than, that of air near sea level;
although it is cold, its ‘potential temperature’ is high (HIDORE AND OLIVER, 1993, p. 112).

The rate at which air temperature decreases with height in the troposphere is called the ‘lapse rate’.
The average sea-level temperature is 15°C, the average tropopause temperature -59°C, and the average
height of the troposphere 11 km, so the ‘standard’ lapse rate is about 6.7°C km-1. The actual lapse
rate, called the ‘environmental’ lapse rate, varies from the standard according to local conditions,
and if air is cooling (or warming) adiabatically its rate of temperature change depends on the amount
of moisture it contains. For dry air, the dry adiabatic lapse rate is 10°C km-1, but if the cooling
triggers the condensation of water vapour the cooling air will be warmed by the latent heat of
condensation, so the saturated adiabatic lapse rate is lower than the dry adiabatic lapse rate. Its value
varies with the amount of condensation, but averages about 6°C km-1.

Adiabatic cooling and warming

When air rises it expands, because there is less weight of air above it in the

column reaching to the top of the atmosphere and, therefore, atmospheric

pressure decreases with increasing altitude. As a gas expands, its molecules

move further apart. In doing so they expend energy and, since the molecules

have less energy, the temperature of the expanded air decreases.

It follows that a rising ‘parcel’ of air will cool without exchanging energy with

the surrounding air. This is ‘adiabatic’ cooling (from the Greek adiabatos,

‘impassable’).

A descending ‘parcel’ of air warms by the same adiabatic process. As it enters

a region of higher pressure it is compressed and gains energy, which heats it.

Water vapour is a gas and the amount of it in a particular body of air is expressed as the ‘humidity’
of the air. Humidity can be measured in several ways. Absolute humidity is the mass of water vapour
in a given volume of air, and specific humidity is the mass of water vapour in a given mass of air
(including the water vapour). Relative humidity, which is the measure most widely used, is the
proportion of water vapour in relation to the amount required to saturate the air, and is given as a
percentage. Warm air will hold more water vapour than cool air, so relative humidity varies with
temperature as well as actual water-vapour content. The converse of this is that if air is cooled, a
temperature will be reached at which its water vapour condenses. This is the ‘dewpoint’ temperature.

When water changes phase, between solid and liquid, liquid and gas, or directly (by sublimation)
between solid and gas, energy is either absorbed or released, as latent heat. Latent heat warms or
cools the surrounding air. It is why the air feels warmer when snow starts to fall and cooler when ice
thaws, and it also governs the dynamics of storm clouds, hurricanes, and tornadoes. The amount of
heat is considerable. When 1 gram of water evaporates, 2500 joules of energy is absorbed and the
same amount is released when water vapour condenses; the change between liquid and solid requires
the absorption or release of 334.7 J of energy; and sublimation between solid and vapour requires the
absorption or release of the sum of these, 2834.7 J for every gram.