Dew Point and Humidity
Name: Dirk K.
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
Also, is there a difference between humidity and relative humidity?
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.
As the relative humidity goes up, the temperature/dew point spread decreases.
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
produced by a weather system.
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
Thanks for using NEWTON!
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.
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
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.
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Update: June 2012