Dew Point is the temperature at which the moisture content of the air will be too great to be held as vapour, and condensation or dew will begin to form.
Air can hold a certain amount of water vapour, but if the air is cooled to a lower temperature the amount of water vapour it can hold is reduced. When the air is cooled down to the dew point temperature, condensation will begin to form as the air can no longer hold the water as water vapour, so some condenses into liquid droplets.
Dew point is often reported instead of relative humidity (which is expressed in % RH) because it is independent of the current temperature, so it better represents the actual amount of water vapour in the air.
For example, if the air temperature is hot, the air is capable of holding a lot of water vapour, so the amount present may seem low as a % relative humidity. However the environment will still feel humid because the total amount of water vapour present is high.
Dew point is often measured in site monitoring of controlled environments where condensation could be problematic, for example in storage areas or critical environments.
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