Humidity

Humidity refers to the amount of water vapor (H2OV) present in the atmosphere.  Water vapor is the gaseous state of water and is invisible. Humidity indicates the likelihood of precipitation, dew, or fog.

Higher humidity reduces the effectiveness of sweating in cooling the body by reducing the rate of evaporation of moisture from the skin. This effect is calculated in a heat index table or humidex, used during summer weather.

Measurements of Humidity

• There are five different methods to express humidity namely : absolute humidity, mixing ratio, vapor pressure, relative humidity and dew point

Relative Humidity

• Relative humidity, expressed as a percent, measures the current absolute humidity relative to the maximum for that temperature. Specific humidity is a ratio of the water vapor content of the mixture to the total air content on a mass basis.

Absolute Humidity

• Absolute humidity is the mass of water vapor in a given volume of air (usually grams/meter3) Since, temperatures and pressures change constantly and they change from place to place, meteorologists usually prefer mixing ratio.

Vapor pressure

• Vapor pressure constitutes part of the total atmospheric pressure attributable to water vapor.  In chemistry we would call this the partial pressure.

Mixing Ratio

• Another measure to express humidity is  mixing ratio which  is mass of water vapor in a unit of air compared to dry air  or in other words, mass of water vapor (in grams)/mass of dry air (kilograms).  

Dew Point

• Dew Point is also a popular with the meteorologists to measure the humidity or atmospheric pressure; it refers to temperature to which the air would have to be cooled, without changing pressure or moisture content, to reach saturation point. The drier the air, the more is moisture be cooled to reach saturation.

Dew points are directly related to the amount of moisture in the air, so is a much better indicator of humidity. High dew points indicate a large amount of moisture in the air. Low dew points indicate small amounts of moisture in the air.