NW3 Weather

Detailed Humidity Information

NB: Valid humidity records began in February 2009

NB: 98% is the physical limit of the hygrometer (measuring RH); in reality this tends to means 100% saturation of the air.

A note on the different measures of Humidity: The Dew Point (or frost point if the temperature is <0°C) is the saturation temperature of a parcel of air, i.e. the point at which water condenses out. It is the temperature at which an object would need to be for dew to form on it (dew forms because certain objects - like cars - cool more rapidly than the air, enabling them to reach the dew point). It is directly proportional to the specific humidity, and very well correlated to the absolute humidity. In everyday terms, high dew points are more uncomfortable as sweating is less effective, and low dew points can make it feel raw. When people say "it's humid", it's most likely because the dew point is high. Relative Humidity, on the other hand, is rather more abstract and has a very technical definition (the ratio of the partial pressure of water in the air to the saturated vapour pressure of that water). The saturated vapour pressure is proportional to the air temperature, and the partial pressure is related to how much water is an air mass, so at a given temperature, the RH is entirely dependent on the specific humidity (or dew point). The RH is therefore useful in determining the extent to which the air is water-saturated. For example: When it rains the relative humidity will increase, but the dew point may not, as the temperature will usually fall as well. If the dew point is the same when the RH increases, the air has the same amount of water but it is cooler, so it is more saturated (less water vapour can exist in cooler air).
Simply (and concisely!): Dew point is a rough measure of how much water vapour is physically in the air; relative humidity is just a measure of the degree to which the air is full of water vapour (i.e. its saturation). One is absolute, the other is relative.