Air humidity is the amount of water vapour the air contains. It accounts for clouds, fog, dew, the clammy feel of a thundery evening and the dryness of air in a hard frost. It is most often given as a percentage, the relative humidity, but that percentage hides a subtlety: it depends as much on temperature as on the amount of water present.
Relative humidity: a percentage of capacity
Relative humidity compares the amount of water vapour present with the maximum the air can hold at the same temperature. At 100% the air is saturated: any excess condenses into droplets, that is into cloud, fog or dew. At 50% the air holds half of what it could.
But the air's capacity varies enormously with temperature. As the curve and the table below show, a cubic metre of saturated air holds about 4.8 g of water at 0 °C and about 17 g at 20 °C: at 20 °C the air can hold around 3.5 times more vapour than at 0 °C. The same amount of water therefore gives a high relative humidity when it is cold and a low one when it is warm.
The dew point: an absolute measure
The dew point is the temperature to which the air would have to be cooled, without changing its water content, to become saturated. In the chart's example, air at 20 °C and 50% relative humidity holds about 8.6 g of vapour per cubic metre: it saturates at 9.3 °C, its dew point. Unlike relative humidity, the dew point does not change when the temperature rises or falls: it really measures how much water is in the air.
The smaller the gap between temperature and dew point, the closer the air is to saturation. When the two meet, dew settles on surfaces, fog forms, or a cloud base appears. A low dew point, below 10 °C, gives air that feels dry and pleasant; above 20 °C the air feels muggy and sweat evaporates poorly.
How humidity changes through the day
At dawn the temperature is lowest, so relative humidity is at its highest, often close to 100%, with dew or mist. In the afternoon the temperature climbs while the amount of water changes little: relative humidity falls, sometimes by half. The same mechanism works in a house: outdoor air, once heated to 20 °C, becomes drier, which is why airing a home in winter dries it out instead of making it damp.
For comfort, a range of 40 to 60% indoors is often suggested. Above 60% for long periods, dust mites and mould thrive; below 30%, the airways dry out. Humidity also affects how hot or cold it feels: damp air conducts cold better, and humid heat stops sweat cooling the body.
How it is measured
A hygrometer measures relative humidity, today usually with a capacitive sensor whose electrical capacity changes with the water absorbed. The older psychrometer compares two thermometers, one dry and one wrapped in a wet cloth: evaporation cools the second, the more so the drier the air. Forecasts derive humidity from temperature and dew point using the Magnus formula, as this site does.
| Temperature (°C) | Maximum vapour (g/m³) |
|---|---|
| −10 | 2.4 |
| 0 | 4.8 |
| 10 | 9.4 |
| 20 | 17.2 |
| 30 | 30.3 |
| 40 | 51.0 |
A worked example: Toronto
For Toronto, using the forecast of 2026-11-02 08:13: the temperature is 8.2 °C and the dew point 6.1 °C, a gap of 2.1 °C. Relative humidity is 87%. At this temperature saturated air holds 8.4 g of vapour per cubic metre; the air actually present holds 7.2 g.
Frequently asked questions
What is the difference between relative humidity and dew point?
Relative humidity is a percentage of the air's capacity, which changes with temperature. The dew point is the saturation temperature: it measures how much water is in the air and stays steady as the air warms.
Why does relative humidity fall in the afternoon?
Because the air warms: its capacity increases while the amount of water stays about the same. The percentage drops without the air holding any less water.
What humidity is ideal in a home?
A range of 40 to 60% is often recommended. Above it, mould and dust mites thrive; below it, the air is too dry for the airways.
Sources
- Alduchov, O. A. and Eskridge, R. E., Improved Magnus form approximation of saturation vapor pressure, Journal of Applied Meteorology, 35, 1996: constants used for saturation vapour pressure and dew point.
- World Meteorological Organization (WMO), Guide to Instruments and Methods of Observation, WMO-No. 8: definitions of relative humidity and dew point.
- Today's example: temperature and dew point from the Open-Meteo forecast for the current hour; vapour mass from the ideal gas law.