The amount of water vapour in the air varies. The percentage of water vapour in the air compared to what the air can hold at that certain temperature is called humidity. People often confuse the terms humid and heat when talking about what they are feeling. Heat, as discussed in the section on temperature, relates to the amount of solar energy contained in the air. Humidity relates to the amount of water vapour within the air and is recognised by the discomfort through stickiness.
Humidity is measured using instruments called hygrometers .
The standard hygrometer measures the difference in temperature of the air and that of a thermometer attached to a wet wick known as a wet and dry bulb thermometer . As evaporation occurs, it represents a cooling process and hence the temperature on the thermometer containing the wick will give a lower reading of temperature. A conversion scale is used to convert the difference in temperature of both thermometers to the relative humidity reading at that dry air temperature. Other hygrometers have scales that indicate humidity directly. The humidity scale is measured on a scale 0 to 100 since it is a percentage.