Humidity - water vapour in the atmosphere

The amount of water vapour in the atmosphere is known as its humidity. The ease with which our bodies lose water vapour depends on the humidity of the air, that is, how much water vapour is already present in it. Water vapour will condense from the air when it is saturated, and since saturation varies with temperature cooling down a sample of air will often result in condensation occurring. These effects govern the formation of clouds and rain. If the humidity is high it means that the atmosphere contains a lot of water vapour. It is therefore difficult for water vapour to evaporate from our bodies and so we sweat more freely.

The formation of a cloud in the cloud chamber is caused by cooling the air containing meths vapour (by expansion or by the use of 'dry ice') to a point where it becomes saturated and liquid is forced to condense.

The humidity of the atmosphere is measured by an instrument known as a hygrometer, the most common types of this being the wet and dry bulb hygrometer and the hair hygrometer.

A simplified diagram of a wet and dry bulb hygrometer is shown in figure 1.

The wet and dry bulb hygrometer consists of two thermometers, one of which has its bulb wrapped in muslin. The muslin is kept wet by its lower part being immersed in a small can of water. Evaporation occurs from the muslin and so the temperature recorded by the 'wet bulb' is lower than that recorded by the dry bulb. Since the rate of evaporation will depend on the humidity of the atmosphere the difference in temperature between the two thermometers can be used to find the relative humidity.









Knowing the wet and dry bulb temperatures the relative humidity can be determined using set of tables to be found at the National Physical Laboratory website: Humidity tables

If you wear glasses you will know that if you put them on when lying in a hot bath the glasses steam up. The glasses are cold compared with the air around them and so water vapour condenses on them. They will only clear if you keep wiping them or warm them up.

Dew forms on the grass during a cold night following a relatively warm day. The cold air at night cannot hold as much moisture as it could during the day and so some must condense out as dew. If the night is realty cold this dew freezes and you get frost.

The dew point is the temperature to which the air must be cooled in order to deposit dew on a surface in contact with it. Therefore it follows that at the dew point there is just enough water vapour in the air to saturate it.

The dew point can be found using Regnault's dew point hygrometer.

Relative humidity

The relative humidity of the atmosphere may be defined as:


At the dew point the pressure of water vapour in the air is its saturated vapour pressure (S.V.P) at that temperature and so:


An amateur astronomer brings her 15 cm mirror into the kitchen after an hour or so of observing outside on a winter evening. Explain why a layer of water condenses on it.