Humidity Calculator

Measuring Humidity

Figure 1
Dry-bulb (left) and
wet-bulb (right) thermometers.

Humidity can be measured indirectly with dry and wet-bulb thermometers. The dry-bulb is an ordinary mercury-in-glass thermometer and measures the air temperature (see Figure 1).

The bulb of the wet-bulb thermometer is covered by a muslin bag which is kept moist. It gives a lower reading than the dry-bulb because of the cooling effect of water evaporating from the muslin bag (see Figure 1).

The two thermometers may be placed in a Stevenson screen, or whirled together in a sling (sling psychrometer), or ventilated by drawing air past them with an electric fan (aspirating psychrometer).

The actual readings of the two thermometers and the difference between them enable us to calculate all the humidity quantities defined in the 'Humidity Definitions' given below.

The difference between the wet and dry-bulb readings is a good indication of the efficiency of evaporative-type coolers and the wet-bulb reading is a reasonable indication of human comfort.

Humidity Definitions

Humidity is essentially the number of water vapour molecules in the air, but there are many and varied ways of expressing it. All of these terms are used scientifically, but humidity figures provided for public use in newspapers are usually relative humidity.

Figure 2
Change in amount of water vapour in saturated air with temperature.
The air's capacity for water vapour increases as air temperature increases.
Air with a temperature of 30°C can hold more than three times
as much water vapour as air at 10°C.

Absolute humidity
The mass of water vapour in a unit volume of air. It is a measure of the actual water vapour content of the air.

Specific humidity
The mass of water vapour per unit mass of air (including the water vapour). It is another measure of the actual water vapour content of the air.

Mixing ratio
The mass of water vapour per unit mass of dry air (excluding the water vapour). Another measure of the actual water vapour of the air.

Vapour pressure
The partial pressure of the water vapour. Yet another measure of the actual water vapour content of the air. (Air pressure is the sum of the partial pressures of the gases comprising the air.)

Saturation
The air is saturated with water vapour when it holds as much water vapour as it can at that temperature.

Saturated vapour pressure
The water vapour pressure when the air is saturated.

Dew-point
The temperature to which air must be cooled (at constant pressure and constant water vapour content) for saturation to occur.

Frost point
When the dew-point falls below freezing it is called the frost point.

Wet-bulb temperature
In simple terms, the lowest temperature to which air can be cooled by evaporating water into it.

Relative humidity (RH)
The ratio of the actual amount of water vapour in the air to the amount it could hold when saturated expressed as a percentage OR the ratio of the actual vapour pressure to the saturation vapour pressure expressed as a percentage.

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

Figure 3
Relative humidity varies significantly when the temperature changes,
even when the actual amount of water vapour in the air remains the same.

Temperature ________
Relative Humidity _ _ _ _

Relative humidity =