Dew Point and Humidity
What is the difference between dew point and humidity and what effect does it have on heat index?
Hello Mary -
The dew point is the temperature to which the air must be cooled before it
becomes saturated and water must condense out. The closer the temperature
and the dew point, the more humid the air. Most often we meet humidity as
"relative humidity" given as a percent - meaning the higher the percent, the
closer the temperature and dew point are.
What makes this all tick is that warm air can hold more water than cold air.
Your heat index is properly called the THI - temperature-humidity index.
When it is hot our body cools itself off by sweating. But in order to have a
cooling effect the sweat must evaporate. As the relative humidity approaches
100% (i.e. the dew point/temperature spread approaches zero), the sweat does
not evaporate well... therefore less cooling... therefore the body suffers
more from the heat. The THI is a formulaic way of describing this phenomena.
Humidity is a measure of moisture content of air. Relative humidity is a
percentage measure of moisture in the air compared to what the air actually
is capable of holding at a particular temperature. Dew point is the
temperature at or below which dew or liquid water will drop out of the air
because the cooling temperature means the air can hold less water and the
relative humidity has reached/exceeded 100% for that temperature and air
mass. The heat index takes the temperature and does a correction designed
to predict what a human body would interpret the temperature as, meaning, it
is a measure of a person's apparent comfort considering temperature and
humidity. Simply put, if a person normally becomes a bit uncomfortable in
dry heat at 80 degrees, they might notice they feel less comfortable at 70
degrees with certain amount of humidity in the air. As the temperature
and/or the humidity increase, the discomfort level increases.
Thanks for using NEWTON!
The dew point is a measure of absolute humidity. Generally, humidity is
reported as "relative humidity," that is, the amount or water actually
dissolved in the air divided by the amount of water that could possibly
dissolve in air at the current temperature. This depends on both the amount
of water in the air and on the temperature.
The Dew point tells the highest temperature an opbject could be to still
collect dew or frost if placed outside. It turns out that this value
depends only on the amount of water in the air. The higher the dew point,
the more water there is.
The relative humidity can be determined from the dew point and the
temperature; alternatively, the dew point can be determined from the
relative humidity and the temperature. There are a variety of ways that
humidity could be reported, but relative humidity and dew point are the most
Humidity drives up the heat index, because humidity makes us less able to
cool off by sweating. High values of the dew point or of the relative
humidity thus raise the heat index.
Richard E. Barrans Jr., Ph.D.
Assistant Director, PG Research Foundation
8205 S. Cass Avenue, Suite 111
Darien, IL 60561
The dew point is the temperature the air must be cooled to in order for
condensation to occur. The higher the humidity, the closer the dew point is
to the air temperature. When the humidity is 100 percent, the dew point and
the temperature are the same. The dew point can never be higher than the
temperature of the air at any given time.
Humidity can be measured in several different ways, but most commonly
humidity is reported as the "relative humidity." Relative humidity (RH) is
the ratio of the amount of moisture in the air compared to the amount the
air is capable of holding at a given temperature, expressed as a percentage.
So a relative humidity of 50 percent indicates the air, at the current
temperature, holds 50 percent of the moisture it is capable of holding. In
very dry climates, the RH is low...and in moist climates, it is high.
The heat index is an "apparent" temperature, which takes into account the
relative humidity. The formula to calculate the heat index is rather
complicated, but high temperatures combined with high humidities produces a
high heat index. The heat index can never be lower than the actual air
temperature, but rises as the humidity in the air increases.
Wendell Bechtold, Meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO
There are several measures of water vapor.
Dewpoint is the temperature at which the air becomes
saturated with water, or, in other words, the
temperature to which the air has to cool to reach
100% relative humidity. This is called the frost point
when the temperature is below freezing.
Relative humidity is the ratio of the vapor pressure of
the air to the vapor pressure at the dewpoint temperature,
or in simplistic terms, the percentage of water vapor that
the air holds compared to the amount of water vapor in the
air when it is saturated (called the capacity).
There are several other measures of water vapor in the air
(specific humidity, absolute humidity, mixing ratio) that
are not as important in our discussion.
Heat Index was a term, that originally (52 years ago), had
little to do with how hot it feels, but was used as to
determine differences from climatic means of temperature
and therefore how much water could be evaporated in comparison
to a normal year. Since then the term Heat Index has
come to be more commonly used to mean a measure of how hot a
person will feel. However, there is a close relationship
here because the ability of the body to evaporate sweat in
addition to the temperature is what determines the Heat Index.
A large number of heat indices have been developed by
researchers. A few take sun intensity into account,
but otherwise all are similar.
In general, when the temperature is less than about
52 degrees F, increasing relative humidity makes it feel
colder. Above 52 degrees, increasing relative humidity
makes it feel warmer. High relative humidity retards the
evaporation of sweat, causing the body to work harder to
stay cool. This puts a large strain on the body. At high
enough relative humidities and temperatures the body cannot
cool itself adequately by sweating and the body temperature
starts to rise.
You can find a chart of the heat index used by the National
Weather Service and a lot more information about heat-related
affects on the body at weather.noaa.gov/weather/hwave.html
meteorologist at Argonne National Laboratory
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Update: June 2012