Air doesn't really 'hold' water vapour, and a vacuum proves it
We talk about warm air holding more moisture, but the nitrogen and oxygen barely matter. A vacuum can take up almost exactly as much water vapour as the same space filled with air, because the limit is set by water's own vapour pressure at that temperature, not by the air around it.
Humidity is simply how much invisible water vapour is present. There are three standard ways to express it. Absolute humidity is grams of vapour per cubic metre, ranging from nearly zero to about 30 grams in saturated air at 30 degrees Celsius. Specific humidity compares the vapour's mass to the whole parcel's mass and is a staple of heating and ventilation design. Relative humidity, the percentage in weather reports, compares the vapour actually present with the most that could exist at that temperature. Because absolute humidity shifts whenever air expands or compresses, British Standard BS 1339 recommends dropping the term in favour of clearer alternatives.
Temperature drives everything. Near saturation, a cubic metre of air at 8 degrees carries about 8 grams of water, while at 30 degrees it carries about 28. Cool a parcel without changing its moisture and its relative humidity climbs until it reaches 100 percent, the dew point, when vapour condenses onto any surface or particle as dew, fog or frost. Warm foggy air and the droplets evaporate. Without particles to condense on, air can even pass 100 percent and become supersaturated. Clouds and mist are not counted in relative humidity at all, though they hint that saturation is close.
Humidity changes how heat feels. Animals that cool by sweating lose heat more slowly when damp air slows evaporation from the skin. On the heat index, 26.7 degrees Celsius at 75 percent relative humidity feels like roughly 28.7. Woodworkers watch humidity too, since timber swells and shrinks with it, so floors need room for seasonal movement.
Measurement ranges from a sling psychrometer, which compares dry-bulb and wet-bulb thermometers, to chilled-mirror instruments prized for speed and accuracy. The gravimetric method is the most accurate but cumbersome, and most industrial sensors today read capacitance. From orbit, satellites track water vapour between 4 and 12 kilometres up by sensing the infrared radiation it absorbs and re-emits, helping forecasters spot thunderstorms forming.
Source: Humidity
