Relative humidity (RH) is the ratio of the actual partial vapour pressure of water to the saturation vapour pressure at that temperature.

The saturation vapour pressure of water increases rapidly with temperature, from 610 Pascal at zero degrees to 2800 Pa at 23oC. At 100oC the vapour pressure reaches 101,000 Pa, which is about atmospheric pressure.

The partial pressure of water vapour is proportional to its concentration, so the increase in concentration is from 5 g/m3 at zero degrees to 20 g/m3 at 23 oC

For example: The actual water vapour partial pressure is 1000 Pa and the temperature is 18 oC. At this temperature the saturation partial vapour pressure is 2054 Pa, so the relative humidity is 1000/2054 = 0.49. This result is good enough for the biologist, who calls this number the Water Activity. Meteorologists and Conservators multiply the number by 100 and call it Relative Humidity, or Percent Relative Humidity.

In practice it is often the relative humidity, or water activity, which is actually detected by the sensor. The partial vapour pressure is then calculated from the RH and the temperature. Most sensors detect water absorption on a material, whose consequent change in properties is measured, as length, weight, electrical resistance, dielectric constant, or even more esoteric properties.

The water absorption is mainly controlled by water activity, which can be roughly translated as the potential of the water to do watery things, such as partake in chemical reactions, or absorb onto hydrophilic molecular groupings.

Dew point detectors, and psychrometers are the commonly used water vapour sensors that do not use absorption as the fundamental detection mechanism.

Notice that air does not enter the definition of relative humidity. We loosely refer to the relative humidity (RH) of air, when we should be talking of the RH of the air space. This sounds daft, so we use the word air. Changing definitions for conversational convenience is dangerous, as is demonstrated in the article 'Evidence from thin air'.

Water vapour disperses through an air filled space quite rapidly, compared to its diffusion rate through solid, or even through porous materials, mainly because air is usually flowing, even when we feel no draught: 0.3 m/s is a typical value for air velocity in a room with closed windows and doors.

A consequence of this rapid movement is that water vapour concentration tends to uniformity in a container even when the temperature is not uniform. This results in a non-uniform relative humidity throughout the space, because the concentration is uniform, but the temperature isn't.

One final point: the phrase 'vapour pressure' is also used loosely instead of the more correct 'partial vapour pressure'. The other gases in air also exert their own partial vapour pressures so that the total pressure is equal to atmospheric pressure. Water vapour does not usually exert a physical pressure on its container, though this easily imagined concept frequently causes architects and building engineers in particular, to talk about 'vapour pressure differences forcing water vapour through walls', usually in connection with discussions about vapour barriers. This again is a dangerous looseness of expression, because the trouble often is that air containing water vapour flows through walls because pumps generate an increased total air pressure.

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