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Humidity is the amount of water vapour (the gaseous form of water) in the air. A distinction is made between absolute and relative humidity.


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Swiss federal authoritiesSwiss federal authorities

The proportion of water vapour contained in the air is not constant, but ranges from 0.1 to 5 percent. This proportion is strongly affected by the air temperature. Like all gases, water vapour is not visible. However, when the air is saturated with water vapour, this condenses, forming droplets. These droplets then form clouds, which precipitate in the form of rain, snow or hail.

There are two ways of quantifying humidity, or the amount of water vapour in the air: absolute humidity and relative humidity.

Absolute humidity

Absolute humidity refers to the amount of water vapor contained in the air, expressed in grams per cubic meter of air. This amount does not vary with temperature, although the maximum possible water vapour content in the air increases with temperature.

Relative humidity

Relative humidity indicates the amount of water vapour contained in the air, expressed as a percentage. Theoretically, the lowest value is zero percent, but such a low moisture content is never reached. Conversely, the maximum value of 100 percent is reached when the air is fully saturated with water vapour. At that point, the water vapour condenses, and either fog or clouds form, or dew precipitates.

Relative humidity varies with air temperature: the warmer the air, the more water vapour it can contain. Consequently, an airmass with constant absolute humidity does not have the same relative humidity; this changes with temperature. Relative humidity is therefore higher at night when temperatures are low than during the day, when temperatures are higher. The fact that warm air can hold more water vapour also explains why precipitation is heavier in summer than in winter, such as during thunderstorms. There is consequently more water available for precipitation.

Which type of humidity is monitored?

Two parameters recorded at the stations in the MeteoSwiss automatic monitoring network provide information about air humidity, namely the relative air humidity and the dew point temperature. Relative humidity is given as a percentage. The dew point temperature indicates the temperature to which the air must have cooled (at a constant absolute humidity) for water vapour to condense. In other words, when relative humidity of 100 percent is reached.