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Name: Dirk K.
Status: other
Age:  30s
Location: N/A
Country: N/A
Date: Tuesday, April 30, 2002

Many times, I hear meteorologists say something like "the dew point is 'x' degrees, so the humidity is 'y' percent", suggesting that the [measurement of] humidity is dependent on the dew point. Shouldn't the relationship between dew point and humidity be the other way around (ie, the dew point is dependent on the humidity - at a given temperature)? Also, is there a difference between humidity and relative humidity?

Dirk -

The first thing you need to realize is that warm air can hold more water than cold air. Then dew point makes sense.

Dew point and relative humidity are related. Dew point is measured in degrees of temperature. It is the temperature to which the present air would have to be lowered to become saturated... to be holding all the water it can hold. The greater the temperature/dewpoint spread, the less the amount of water in the air compared to how much it could hold.

The temperature/dew point spread measures the same thing as relative humidity, but relative humidity is given as a percent. At 75% relative humidity the air has 3/4 of the water vapor it can hold.


The other type of humidity of importance in weather is absolute humidity. It is a measure of the mass of the water vapor in the air per unit of mass of air. This is important in predicting the amount of precipitation that might be produced by a weather system.

Larry Krengel


Relative humidity is the normal term we use when discussing atmospheric moisture...it is a relative figure, that is, it tells the current humidity relative to the amount of humidity the atmosphere can support at a given temperature. If the relative humidity is 50%, it means the air is holding, at that temperature, have the amount of moisture it is capable of holding. Any additional moisture would condense from the air as dew.

The dew point is the temperature at which dew will begin to drop out of the air, in effect, as an air mass cools it is able to hold less and less water. When the first moisture begins to drop out as dew, that temperature is the dew point. it is indeed related to the relative humidity of the air mass. In effect, you can plot for a given evening, if, say the relative humidity were 50% and the temperature began dropping, the relative humidity would begin to increase, even without adding additional moisture. This is simply because the atmosphere can hold less moisture at the lower temperature so that what is was and now is holding represents a greater (higher) percentage of what it is capable of holding. I would agree your way of looking at the relationship is a more easily understood interpretation. In any discussion you should include dew point, relative humidity and temperature. Obviously they are all interrelated.

Thanks for using NEWTON!

Ric Rupnik

Dear Dirk-

Dew point and humidity are directly related. And the dew point is used to compute the RELATIVE humidity. Some definitions are in order here for a clear understanding of the measurement of moisture (water vapor) in air. RELATIVE humidity is a measure of the actual amount of water vapor in the air compared to the total amount of vapor that can exist in the air at its current temperature, and is expressed as a percentage. Other ways to express the moisture in air are by its SPECIFIC humidity and its ABSOLUTE humidity. These last two measurements are usually only of interest to scientists.

Here is a link to a short but EXCELLENT discussion of water vapor and humidity and the different ways it can be measured.

http://www.crh.noaa.gov/lmk/soo/docu/humidity.htm For more information about humidity, go to http://google.com, and type in "relative humidity" in the search box.

Wendell Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO

Dirk, This is a wonderful question! Thank you for asking it.

When we speak of the "dewpoint", we are actually talking about the dewpoint temperature, which is in degrees. The dewpoint temperature is the temperature to which the air would have to be cooled at constant pressure and constant water vapor content for the air to become saturated (in other words, relative humidity 100%).

The dewpoint temperature reflects the "absolute humidity" of the air, which is the actual mass of water vapor (in grams) in a cubic centimeter of air; this is also called "vapor density".

Relative humidity is temperature dependent and is a measure of how near saturation the air is. The term "humidity", as you used it in your question, is commonly used in place of the term "relative humidity" (not to be confused with "absolute humidity", as defined above). As the temperature decreases, the relative humidity increases, indicating that the air is closer to being saturated.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory

"Dew point" and "relative humidity" (same as humidity) are different ways of expressing the amount of water vapor present in air. The "dew point" is the temperature at which the amount of water vapor present in the air reaches the maximum amount possible (or saturation), i.e. the temperature at which the "relative humidity" would equal 100% for that amount of water vapor present. The "relative humidity", expressed as a percent is the volume percent of the maximum amount of water vapor that the air, under the conditions of temperature and barometric pressure that exist. They are different ways of looking at the same thing -- the amount of water vapor present in the air.

Vince Calder

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