Humidity and temperature

Humidity can be defined as the amount of water vapour contained in the air, or simply the wetness of the atmosphere.

• The amount of water vapour air can hold depends on temperature. When temperature rises air can hold more vapour.
• When temperature falls it cannot hold as much water vapour.
• When relative humidity is at 100% the air cannot contain excess water vapour.
• Any further addition will cause condensation to occur.
• Condensation may take the form of; Minute droplets of clouds, Rainy, mist, Dew or fog NOTE: The critical temperature at which the air is fully saturated is called the dew point. Below this point condensation will take place.

Measurement of humidity

• An instrument called a Hygrometer is used to measure Humidity.
• This consists of a wet bulb and dry bulb thermometers.

The hygrometer

The wet and dry thermometers, (also called hygrometers) allow the calculation of the humidity of the air

• When air is not saturated water evaporate from the container and the muslin become wet.
• This cools the wet bulb and caused the mercury to contract.
• The dry bulb is not affected and so the two thermometers show different readings.
• The difference between the readings of the two thermometers is an indication of humidity in the air.

Thermometer reading

• Large difference
• Small difference
• No difference Atmospheric air pressure
• Air exerts weight on the earth’s surface.
• This weight is called atmospheric pressure.
• A column of air 1sq cm in cross sectional areas extending from the sea level to the top of the atmosphere weights 1. 034 kg.
• Hence at sea level the atmospheric pressure is 1.034 kg per sq cm or 14.7 ibs per sq in.
• Pressure is made up of a number of mixed gases and has weight.
• It therefore exerts a pressure on the earth’s surface which varies from place to place and from time to time.
• On the maps places of equal pressure are joined by lines called isobars.
• Atmospheric pressure varies with both temperature and altitude.