A wet bulb thermometer is a thermometer wrapped in a wet cloth. The evaporation of the water, which uses heat as it volatilizes, has a cooling effect on the thermometer. Wet bulb thermometers are used in conjunction with a dry bulb thermometer to measure relative humidity and dew point, which are the amount of water there is in the air, and the temperature at which the air reaches 100 percent relative humidity, when precipitation will occur.
Dry Bulb Temperature
Dry bulb temperature is typically called “air temperature.” The temperature is read off a normal thermometer, and the water content of the air does not affect its reading. It is simply the ambient temperature of the surrounding atmosphere. Temperature is typically measured in the shade, to avoid heating caused by direct sunlight upon the thermometer.
Wet Bulb Temperature
The wet bulb temperature is read from a thermometer with its bulb wrapped in saturated muslin. The wet bulb temperature is always less than the dry bulb temperature, as the evaporation of water exerts a cooling effect. (Heat is dissipated by evaporating the water, just like when a human perspires.) If the air is completely saturated with water vapor, or at 100 percent relative humidity, then the wet bulb temperature and the dry bulb temperature will be equal. The air already is saturated with water, and the wet bulb does not provide any cooling because its water does not evaporate.
The dew point temperature is the temperature that causes water vapor to condense out of the air. Any temperature above the dew point will cause evaporation from the wet bulb. The higher the relative humidity of the air, the closer the dew point temperature will be to the air temperature.
Relative humidity expresses ambient air's saturation with water vapor. Relative humidity of 100 percent means the air is incapable of holding any more water vapor without precipitating, or without an increase in temperature. Relative humidity (RH) can be obtained via the following equation:
RH = E/Es x 100
Where E is the vapor pressure of the air and Es is the saturated vapor pressure of the air. Typically, a psychometric chart is used to compute relative humidity when wet and dry bulb temperatures are known.
When relative humidity is high, sweat will not evaporate: there is already too much water in the air. This means that perspiration will collect on the skin and drench the clothes. This creates those uncomfortable summers, compared with a dry heat. In a dry heat the sweat evaporates and a cooling effect is realized. This is why it is important to drink a lot in arid regions, and why higher temperatures in ambient air can actually feel more comfortable than lower temperatures with a higher humidity.