Humidity

Cold air is humid. Heat it and the same air becomes very dry.

Heating air leaves the water in it exactly where it was. The amount of water stays the same. What changes is how much the air could hold, and that rises steeply with temperature. So the same air, warmed, reads as much drier. This is the whole reason winter rooms are dry, and it is why the remedies that work are the ones that either add water continuously or shrink the space you are trying to humidify.

Humidity converter

Outdoors

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Indoors

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0 % RH

Water in the air
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Dew point
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Vapour pressure deficit
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Magnus formula, Alduchov and Eskridge 1996 coefficients. A maximum relative error of about 0.4 percent from minus 40 to plus 50 Celsius1, which is far better than any household hygrometer.

The number that surprises people

Air at minus 5 degrees and 80 percent humidity feels damp and cold outside. Bring it in and heat it to 22 degrees and it reads 12.8 percent2. Even saturated fog at minus 5, at 100 percent humidity, cannot get past 16.0 percent once warmed to 22, because saturation at minus 5 is 4.22 hPa and saturation at 22 is 26.39. That is a hard ceiling, and no amount of fog outside moves it.

Armenia is not an abstract case. Taking the 1991 to 2020 monthly normals from the national station network and warming that outdoor air to 22 degrees with nothing added, the January floor is 16 percent in Yerevan, 15 in Dilijan, 14 in Vanadzor and Armavir, 18 in Ijevan and Meghri, and 11 in Gyumri3. Dilijan's outdoor air in January is genuinely humid at 68 percent and Yerevan's reads 78 percent, and indoors in winter they are the same place. Outdoor relative humidity tells you nothing about indoor relative humidity. Only vapour pressure does. Opening a window in an Armenian winter cannot get a room to 40 percent, because the water is not in the air to begin with.

Real rooms read higher than the floor, typically in the twenties and thirties rather than the teens, because people, cooking, washing and the plants themselves add moisture continuously. That is the useful way to think about it: humidity is a flow and not a stock. Holding a 54 cubic metre room at 50 percent with minus 5 outside takes about 195 grams of water an hour, continuously, which is close to five litres a day, and a full going over with a spray bottle is about 10 grams, or roughly three minutes of that demand. Misting is not failing because you do not do it often enough. It is short by a factor of several hundred. An enclosed cabinet works because it shrinks the volume and the air exchange rather than trying to out-supply them, a correctly sized humidifier on a humidistat is the only room-scale method that reaches the target, and choosing a plant that suits a dry flat costs nothing and never fails.

  1. Alduchov, O.A. and Eskridge, R.E. (1996). Improved Magnus form approximation of saturation vapor pressure. Journal of Applied Meteorology 35(4):601 to 609. The AERK coefficients used by the converter above, and the stated maximum relative error of about 0.4 percent from minus 40 to plus 50 degrees. DOI 10.1175/1520-0450(1996)035<0601:IMFAOS>2.0.CO;2
  2. Our own arithmetic. Not a published source. Every relative humidity figure on this page is the same two line calculation: actual vapour pressure is conserved when air is heated, so the indoor value is the outdoor relative humidity multiplied by the ratio of the saturation vapour pressures at the two temperatures, using the coefficients in source 1. Worked once so it can be checked: saturation at minus 5 is 4.219 hPa, times 0.80 gives 3.375, saturation at 22 is 26.386, and the quotient is 12.8 percent. The moisture budget figures come from the steady state balance between internal moisture generation and the moisture carried out by air exchange, for a 54 cubic metre room at 22 degrees and half an air change per hour, and the spray bottle figure assumes ten pulls at about a millilitre each with every drop evaporating. calculated, repeatable, unverified by a third party
  3. Armenian hydrometeorological service, WMO Climate Normals 1991 to 2020, as submitted to the World Meteorological Organization and archived by NOAA's National Centers for Environmental Information, Region VI, Armenia. One file per station, and mean vapour pressure is reported as a measured parameter rather than reconstructed from mean temperature and mean humidity, which is what makes the indoor floors above trustworthy. Stations used: Yerevan Arabkir, Armavir, Gyumri, Vanadzor, Dilijan, Ijevan and Meghri. NCEI archive