W
E A T H E R
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A T A
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E A S U R E D
& C
A L C U L A T E D
Humidity
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Wind chill
Wind chill takes into account how the speed of the wind affects our per-
ception of the air temperature. Our bodies warm the surrounding air mol-
ecules by transferring heat from the skin. If there’s no air movement, this
insulating layer of warm air molecules stays next to the body and offers
some protection from cooler air molecules. However, wind sweeps that
comfy warm air surrounding the body away. The faster the wind blows,
the faster heat is carried away and the colder you feel.
Wind chill is not stored in archive memory. Wind chill is calculated when-
ever it is displayed. If you edit temperature or wind speed values, the
wind chill will change as well.
Note:
WeatherLink versions 5.1 and later use the Osczevski (1995) equation to calcu-
late wind chill. This is the method adopted by the US National Weather Service in
September of 2001.
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Heat Index
The Heat Index uses the temperature and the relative humidity to deter-
mine how hot the air actually “feels.” When humidity is low, the apparent
temperature will be lower than the air temperature, since perspiration
evaporates rapidly to cool the body. However, when humidity is high (i.e.,
the air is saturated with water vapor) the apparent temperature “feels”
higher than the actual air temperature, because perspiration evaporates
more slowly.
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THW (Temperature - Humidity - Wind)
Finally, like Heat Index, the THW Index uses humidity and temperature to
calculate an apparent temperature, but includes the cooling and heating
effects of wind on our perception of temperature.
H u m i d it y
Humidity itself simply refers to the amount of water vapor in the air. However,
the amount of water vapor that the air can contain varies with air temperature
and pressure. Relative humidity takes into account these factors and offers a
humidity reading which reflects the amount of water vapor in the air as a
percentage of the amount the air is capable of holding. Relative humidity,
therefore, is not actually a measure of the amount of water vapor in the air, but a
ratio of the air’s water vapor content to its capacity. When we use the term
humidity in the manual and on the screen, we mean relative humidity.
It is important to realize that relative humidity changes with temperature, pres-
sure, and water vapor content. A parcel of air with a capacity for 10 g of water
vapor which contains 4 g of water vapor, the relative humidity would be 40%.
Adding 2 g more water vapor (for a total of 6 g) would change the humidity to
60%. If that same parcel of air is then warmed so that it has a capacity for 20 g of
water vapor, the relative humidity drops to 30% even though water vapor content
does not change.
Relative humidity is an important factor in determining the amount of evapora-
tion from plants and wet surfaces since warm air with low humidity has a large
capacity for extra water vapor.
