Humidity

Humidity is a measurement of the amount of water vapour in the air.This article looks at the different types of humidity and how we measure it.

At its simplest, humidity is a measurement of the amount of water vapour in the air. It affects how humans feel and can also have a big influence on the types of weather conditions that will form. Air can only hold a limited amount of water vapour before it results in the formation of precipitation, clouds or fog. Water vapour can also be referred to as moisture and is itself influenced by wind, rainfall and atmospheric pressure.

Whilst humidity varies on a daily basis, some areas around the globe are generally more humid than others. Since warmer air can hold more water vapour than cooler air, countries nearer the equator tend to have a higher humidity.

Types of humidity

There are three measurements of humidity; absolute, relative and specific.  Relative humidity is the most commonly referenced measurement as it is related to how humans perceive temperature.

Absolute humidity

Absolute humidity is the actual water content of the air expressed as grams of water vapour per cubic meter volume of air (g/cm3). It is calculated by dividing the mass of water vapour by the volume occupied by the mixture. Absolute humidity doesn't take into account temperature but changes as temperature or pressure changes.

Relative humidity

This is commonly what is meant when the term 'humidity' is used. Relative humidity measures the actual water vapour content of the air relative to how much water vapour could be held at the current temperature. It is expressed as a percentage or fraction. Relative humidity is frequently used in weather forecasts as it affects how we 'feel' the weather. Human body temperature is inherently dependent on the air as it wicks moisture away from our skin. If relative humidity is high, the amount of moisture that evaporates from our skin is limited so we feel warmer and stifled. Conversely, if relative humidity is low it can feel colder than it actually is as moisture is readily removed from our skin dropping our temperature.

Specific humidity

Specific humidity measures the mass of water vapour in a unit mass of dry air and is expressed as a ratio of grams of water vapour per kg of air. As long as the moisture content of the air remains the same, the specific humidity does not change. It is not affected by temperature or pressure changes. Specific humidity is used in weather forecasting as it is a good indicator of a moving air mass.

Measuring humidity A hygrometer A hygrometer

There are various ways to measure humidity on different scales. A hygrometer is used to measure air humidity on a local scale. It consists of two thermometers with one dry and one moist bulb.

The humidity level is measured by how fast the water evaporates from the wet bulb. Relative humidity is then measured by comparing the readings from both the wet and dry bulbs; the larger the difference, the lower the relative humidity.

Also used to record relative humidity is a Stevenson screen which can also record temperature and is used by meteorologists as part of the weather forecasting process. For monitoring humidity on a global scale, satellites are used and can be important in future weather predictions.

Last updated: 20 May 2014