Monitoring the weather

Humidity

Absolute humidity is defined as the mass of water vapour in a unit volume of air. More often we use the term relative humidity, defined as the ratio of the actual amount of water vapour in the air to the amount that would be required to saturate the air at that temperature.

The maximum amount of water vapour the air can hold increases with increasing temperature. Relative humidity therefore decreases with increasing temperature if the actual amount of water vapour remains the same.

Relative humidity forecasts don't really help people decide whether they will feel uncomfortable tomorrow. Comfort depends on both the temperature and the humidity; the wet-bulb temperature is probably its best indicator. In simple terms, this is the lowest temperature to which the air can be cooled by evaporating water into it.

Humidity can be measured indirectly with a pair of dry- and wet-bulb thermometers - the psychrometer. The bulb of the wet-bulb thermometer is covered by a sleeve of muslin kept moist by a wick dipping into a reservoir of water. It gives a lower temperature than the dry-bulb because of the cooling effect of the evaporating water. The two thermometers may be placed in a Stevenson screen, or whirled in a sling (sling psychrometer) or ventilated by drawing air past them with an electric fan (aspirating psychrometer). Various humidity quantities can be calculated from the individual readings and the difference between them.

Relative humidity can also be measured with hygrometers that use treated human or horse hair (hair hygrograph). The hair shrinks by about 2.5 per cent of its length when the relative humidity of the atmosphere changes from 100 per cent to zero. The change is not linear: the drum chart requires a non-linear scale over which the pen arm linked to the hair moves.

The hygrograph is often incorporated with a thermograph (in which the movement of the pen arm is actuated by a temperature-sensitive bi-metal strip) in one instrument - the thermohygrograph - with both temperature and humidity scales on the one chart.

See also 'Cloud formation and rainfall'