Ask A Scientist© Weather Archive

 
> > >  name       mary
> > >  age        50s
>
> > >  Question - 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.
>
>Larry Krengel
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>Mary,
>
>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!
>
>Ric
=========================================================
>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
>common.
>
>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

=========================================================

>Mary,
>
>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
>
>David Cook
>meteorologist at Argonne National Laboratory

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