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.