- Wet-bulb temperature
Wet-bulb temperature is measured
using a standard mercury-in-glass thermometer,
with the thermometer bulb wrapped in muslin, which is kept wet. The
evaporation of water from the thermometer has a cooling effect, so the
temperature indicated by the wet bulb thermometer is less than the
temperature indicated by a dry-bulb (normal, unmodified) thermometer.
The rate of evaporation from the wet-bulb thermometer depends on the
humidity of the air -
evaporation is slower when the air is already
full of water vapour.
For this reason, the difference in the temperatures
indicated by the two thermometers gives a measure of atmospheric humidity.