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Note: Vantage Pro2 measures Heat Index only when the air temperature is above 57° F
(14° C), because it’s insignificant at lower temperatures. (Below 57°, Heat Index =
the air temperature.) The Heat Index is not calculated above 135° F (52° C).
Temperature/Humidity/Sun/Wind (THSW) Index
The THSW Index uses humidity and temperature like the Head Index, but
also includes the heating effects of sunshine and the cooling effects of
wind (like wind chill) to calculate an apparent temperature of what it
“feels” like out in the sun. The THSW Index requires a solar radiation sen-
sor.
Humidity
Humidity itself simply refers to the amount of water vapor in the air. How-
ever, the amount of water vapor that the air can contain varies with air tem-
perature 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 rela-
tive humidity.
It is important to realize that relative humidity changes with temperature,
pressure, 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
evaporation from plants and wet surfaces since warm air with low humid-
ity has a large capacity to absorb extra water vapor.
Dew Point
Dew point is the temperature to which air must be cooled for saturation
(100% relative humidity) to occur, providing there is no change in water
vapor content. The dew point is an important measurement used to predict
the formation of dew, frost, and fog. If dew point and temperature are close
together in the late afternoon when the air begins to turn colder, fog is
likely during the night. Dew point is also a good indicator of the air’s
actual water vapor content, unlike relative humidity, which takes the air’s
temperature into account. High dew point indicates high water vapor con-
tent; low dew point indicates low water vapor content. In addition a high
dew point indicates a better chance of rain, severe thunderstorms, and tor-
nados.