Relative Humidity, Temperature, Amount of Water
Country: United Kingdom
Date: Winter 2012
I want to know how relative humidity at low temperatures compares with the same relative humidity at high temperatures (but I cannot find a chart or table on this) -
I gather that colder air holds less moisture, meaning that air at 95%RH at 5degreesC holds less moisture than air at 95%RH at 30degreesC,
but I want to know how much drier the cold air is - is there such a thing as absolute humidity instead of just relative humidity?
Why do people sometimes sweat when it is hot outside, but rarely do so when it is cool outside?
When the air is really cold, why is the snow so light (low density)?
Why does one’s car windscreen fog up?
Why do we commonly find dew on the ground in the morning?
All of these questions are related to the concept of relative humidity.
Most weather reports indicate ‘humidity’ and express it in terms of percent – they really mean ‘relative humidity’, but I guess that adding the word ‘relative’ takes up too much space on the screen. Let us define a few terms:
‘Absolute humidity’ (AH) measures the partial pressure of water in air (it can be expressed as grams of water per cubic centimeter of air). In effect, it is a measure of how much water is in a cubic centimeter of air.
‘Vapor capacity’ (VC) is a measure of how much water air can hold at a given temperature; also known as the saturation limit, 100% saturation, or dew point.
‘Relative humidity’ (RH) measures how close a parcel of air is to becoming completely saturated. So, RH=AH/VC*100.
If you understand relative humidity then you should be able to figure out the best way to defog the windows inside a car… the answer: bring in fresh air (do not recycle air in the car), turn on the air conditioner, and direct a lot of warm air at the front windscreen.
A Google search yielded the following graph: http://philipmarshall.net/Teaching/rwuhp382/wood/psychrometric_chart_ex_1.htm that may be of some help.
From this article in Wikipedia:
Relative Humidity is the ratio of the partial pressure of water vapor in a volume of air compared to the amount of water vapor in the air at a given temperature and pressure when the air is saturated with water.
This chart shows the saturation point of water vapor in a volume of air over a range of temperatures, and the amount of water in the air at the 50% water vapor level.
As you can see colder air holds less water vapor than warmer air.
Please refer to this Wikipedia article for Absolute humidity:
from this article, please find these statements:
“Absolute humidity, on a volume basis, is the mass of water vapor, mw, per cubic meter of total moist air, Vnet:
Absolute humidity ranges from 0 grams per cubic meter in dry air to 30 grams per cubic meter (0.03 ounce per cubic foot) when the vapor is saturated at 30 °C.  (See also Absolute Humidity table)”
You should be able to find the absolute amount of water (relative humidity of 100%) held in a volume of air at various temperatures in references such as a handbook of physical constants. Perhaps this information is also available on the web on some meteorology sites. And then there are the many textbooks on weather and meteorology.
Hope this helps.
R. W. "Mr. A." Avakian
There a lot of misunderstanding about the term RELATIVE HUMIDITY. The origin is the INCORRECT NOTION that air CAN HOLD water vapor --- WRONG, WRONG, WRONG. The amount of water vapor in a given volume to a good approximation depends solely upon the temperature. It does not matter whether there is (or is not) any air present. REPEAT THIS CAREFULLY: It does not matter whether there is (or is not) any air present. The amount of water vapor present in a volume, V, at a given temperature T is given by the ideal gas law: P x V = n x R x T. Rearranging this gives P = [n / V] x R x T. The vapor pressure, P, and the temperature, T, are NOT INDEPENDENT. Tell me a “T” and I can give you back a “P” by just looking it up in a table of the vapor pressure of water as a function of the temperature. AIR does not have anything to do with that. The convention, which does not change anything, is to express the relation the following way: [P / R x T] = [n / V]. And the number of mols, n = [gm / 18.02]. Notice that the density [n/ V] is an intensive variable so it does not matter if “V” is large or small, [n/ V] the density, has the same value. Again, it does not matter whether there is any air is present or absent. AIR IS NOT “HOLDING” ANYTHING.
Now if at some temperature there is less water vapor present than the above, then [n / V] or [ gm/18.02 x V] is less than the vapor pressure of water (which depends only on the temperature). The relative humidity is the ratio of the actual vapor density [d(actual) / d(vapor pressure)] x 100 = R.H. Notice that the relative humidity is the density of the water vapor present divided by the density of the saturated water vapor. AIR IS NOT “HOLDING” ANYTHING. IT HAS NOTHING TO DO WITH ANYTHING. AIR IS NOT “DISSOLVING” ANY WATER.
If someone insists on describing relative humidity in incorrect terms of air “holding” water vapor; that person will remain hopelessly confused. Now what happens at low temperature is that the vapor pressure of water becomes very small, so the vapor pressure becomes very small because [P / R x T] becomes very small and insensitive to changes in the vapor density.
You can get more details on any number of web sites. The Hyperphysics website: http://hyperphysics.phy-astr.gsu.edu But if you see any web site that starts off with “How much water air will HOLD.” Delete it because the author does not understand the problem.
You are correct that for the same relative humidity (which is temperature
dependent), there is less water vapor in the air at a lower temperature
than at a higher temperature.
There is an absolute humidity measurement and it reflects the actual
mass of water vapor in the air.
For more details see http://en.wikipedia.org/wiki/Absolute_humidity
David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory
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Update: June 2012