Question

Give a geographical reason for the following:

When relative humidity is 100% the temperature reading on both wet and dry thermometers is the same.

Answer 1

When relative humidity is 100%, the air is fully saturated with water vapour and cannot hold any more moisture. In this condition, no evaporation takes place from the wet bulb of the thermometer, because evaporation only occurs when the air can absorb more moisture.

Since evaporation is the process that normally cools the wet bulb and causes its reading to be lower than the dry bulb, the absence of evaporation means both thermometers show the same temperature. This is a clear indication that the air has reached saturation, and relative humidity is at its maximum.