The wet bulb temperature (WBT) relates relative humidity to the ambient air or dry bulb temperature. When moisture evaporates, it absorbs heat energy from its environment in order to change phase (via latent heat of vapourisation), thus reducing the temperature slightly. The WBT will vary with relative humidity.

If the relative humidity is low and the temperature is high, moisture will evaporate very quickly so its cooling effect will be more significant than if the relative humidity was already high, in which case the evaporation rate would be much lower. The difference between the wet bulb and dry bulb temperature therefore gives a measure of atmospheric humidity.

Figure 1 - Lines of constant wet-bulb temperature on the Psychrometric Chart.

Select Display: :: Wet Bulb Lines :: Animation

When the air is at saturation point, there will be no moisture evaporation at all. Thus the wet bulb will equal the dry bulb temperature. This occurs at the curved line representing 100% relative humidity. Thus, using the Psychrometric Chart, you can find the WBT by tracing the closest WBT line back to the saturation point line, and then running straight down the graph to the bottom axis. The resulting temperature is the WBT in °C.

Measuring Wet Bulb Temperature

Wet bulb temperature is easily measured with a standard thermometer which has its sensing bulb encased in a wetted wick that is subjected to rapid air motion across its surface. Such devices, called sling or whirled psychrometers, have a frame that can be whirled in the air by hand. The bulb of one thermometer is covered with a tight-fitting muslin sack and wetted with water. This thermometer is known as the wet bulb thermometer and the other the dry bulb thermometer. The psychrometer is whirled to provide air movement across the bulbs. The dry bulb indicates the temperature of the air. The wet bulb helps determine the relative humidity.

Figure 2 - A sling psychrometer used to compare wet bulb and dry bulb temperatures.

When the sling psychrometer whirls through the air, water from the muslin evaporates. The evaporating water cools the wet bulb. The amount of cooling that occurs depends on the relative humidity. The lower the humidity, the faster the water in the muslin will evaporate, and the more the bulb will cool. High humidity will cause less evaporation, slowing the cooling process.

In air that has less than 100 per cent relative humidity, the wet bulb will record a lower temperature than the dry bulb. This difference in temperature is known as wet-bulb depression. A special chart is used to convert the wet bulb depression to relative humidity.

Related Links