Humidity refers to the amount of moisture vapour in a specific volume of warm air. For any particular dry-bulb temperature there is only a certain amount of moisture vapour that can be absorbed in the air before it becomes saturated and precipitation occurs. The actual amount of moisture in the air is referred to as the absolute humidity and is measured in g/kg. The saturation point or dew point refers to the maximum amount of moisture that the air can hold at a given temperature. Relative humidity (RH) is therefore the ratio between the absolute humidity of the air in its current state compared to this maximum amount, expressed as a percentage.
Relative humidity is important for human comfort mostly for its effect on the evaporation of sweat. In order to evaporate, there needs to be a source of high temperature from which the latent heat of vaporisation can be drawn, and sufficient vapour pressure to allow vaporisation. In hot dry climates sweat is readily evaporated. In humid or tropical environments there is abundant heat, but very low vapour pressure as the air is already almost at saturation point, so sweating is much less effective.
At relative humidities above 80%, sweat is produced but most of it cannot evaporate as the air immediately surrounding the body quickly becomes saturated. Humidities less than 20% result in large amounts of evaporation or both sweat and other bodily fluids, drying out the eyes and mucous membranes which can greatly increase susceptibility to infection.
Select Display: :: Relative Humidity Lines :: Animation
Measuring Humidity
The instruments used to measure humidity are called hygrometers. The simplest mechanical hygrometer uses a single human hair to determine humidity. The hair is attached to a spring and a dial. When the relative humidity increases, the cells in the hair swell and contract, pulling the spring and the dial hand. At a lower relative humidity, the hair cells relax and it lengthens, releasing the tension on the spring and allowing the dial hand to move in the opposite direction.
A more accurate measurement is achieved with an electronic hygrometer. It measures the change in the electrical resistance of a thin layer of lithium chloride, or of a semiconductor device, as the relative humidity changes. Other hygrometers sense changes in weight, volume, or the transparency of various substances that respond to relative humidity.
The dew-point hygrometer is a polished metal mirror that is cooled until moisture just begins to condense onto it. This occurs when the surrounding air reaches its dew point. The temperature of the metal is then the same as the dew point temperature. Knowing the atmospheric temperature and the dew-point temperature, the relative humidity can be determined by means of a table.
Another type of hygrometer is called a psychrometer. It consists of two thermometers mounted on the same frame that can be whirled in the air by hand. This is described in detail in the wet bulb temperature topic.
Related Links
- INNOVA Thermal Comfort booklet
- http://www.innova.dk/books/thermal/thermal.htm
- David D. Houghton, "Hygrometer,"
Discovery Channel School, original content provided by World Book Online, http://www.discoveryschool.com/homeworkhelp/worldbook/atozscience/h/269640.html