Humidity Humidity is the measure of water vapour content in the atmosphere. Absolute humidity is the mass of water vapour in a given volume of air, measured in grams per cubic metre (g/m3). Specific humidity is similar but is expressed in grams per kilogram of air (g/kg) Humidity depends on the temperature of air. At any given temperature, there is a limit to the amount of moisture it can hold. When the limit is reached, the air is saturated.

Humidity Cold air holds less moisture than warm air. Thus warm air gives more precipitation than cold air. Relative Humidity (RH) is the amount of water vapour in the air at a given temperature expressed as a percentage of the maximum amount of vapour that the air could hold at that temperature. If RH is 100%, the air is saturated. If it lies between 80 and 99%, it is moist, and the weather is humid or clammy. When the RH drops to 50%, the air is ‘dry’. It can be as low as 10% over hot deserts.

Humidity If unsaturated air is cooled and atmospheric pressure remains constant, a critical temperature will be reached when the air becomes saturated. This is known as the Dew Point. Any further cooling will result in condensation of excess vapour, either into water droplets where condensation nuclei are present, or into ice crystals if the air temperature is below 0 ◦ C.

Humidity If unsaturated air is cooled and atmospheric pressure remains constant, a critical temperature will be reached when the air becomes saturated. This is known as the Dew Point. Any further cooling will result in condensation of excess vapour, either into water droplets where condensation nuclei are present, or into ice crystals if the air temperature is below 0 ◦ C.

Condensation This is the process by which water vapour in the atmosphere is changed into a liquid or, if the temperature is below 0 ◦ C, a solid. It usually results from air being cooled until it is saturated. Radiation cooling: Or contact cooling, usually occurs on calm, clear evenings. The ground loses heat rapidly through terrestrial radiation and the air in contact with it is then cooled by conduction. If the air is moist, some vapour will condense to form radiation fog, dew, or if the temperature is below freezing point, hoar frost.

Condensation Advection cooling: This results from warm, moist air moving over a cooler land or sea surface. Advection fogs in California and the Atacama Desert are formed when warm air from the land drifts over cold offshore ocean currents.

Condensation Orographic and frontal uplift: Warm, moist air is forced to rise either as it crosses a mountain barrier (orographic ascent) or when it meets a colder, denser mass of air at a front

Condensation Convectional or adiabatic cooling: This is when air is warmed during the daytime and rises in pockets as thermals. As the air expands, is uses energy and so loses heat and the temperature drops. Because air is cooled by the reduction of pressure with height rather than by a loss of heat to the surrounding air, it is said to be adiabatically cooled.

Condensation Condensation does not occur readily in clean air. If air is absolutely pure, it can be cooled below its dew point to become supersaturated with an RH of over 100%. Laboratory tests show that clean, saturated air can be cooled to -40C before condensation or in this case sublimation. However, air is rarely pure and usually contains large numbers of condensation nuclei. These microscopic particles, referred to as hygroscopic nuclei because they attract water, include volcanic dust, dust from windblown soil, smoke and sulphuric acid from urban and industrial areas, and salt from sea spray.