DAVIS Vantage Pro2 Console Manual page 45

with water vapor) the apparent temperature "feels" higher than the actual air
temperature, because perspiration evaporates more slowly.
Note: Vantage Pro2 calculates Heat Index only when the air temperature is above 57° F
(14° C), because it's insignificant at lower temperatures. Below 57°, 0° F (18° C) for
consoles with firmware dated May 2005 or later, the Heat Index is equal to 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 sensor.
Humidity
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 per-
centage of the amount the air is capable of holding. Relative humidity, there-
fore, 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 evapo-
ration from plants and wet surfaces since warm air with low humidity 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 con-
tent. 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 content; low dew point indicates low
water vapor content. In addition a high dew point indicates a better chance of
rain, severe thunderstorms, and tornados.
42