DAVIS Vantage Vue Wireless Console Manual - Manual (Page 49)

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Wind Chill
Wind Chill
Wind chill takes into account how the speed of the wind affects our perception of the air
temperature. Our bodies warm the surrounding air molecules by transferring heat from the
skin. If there is 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 away that
warm air surrounding the body. The faster the wind blows, the faster heat is carried away and
the colder you feel. Wind has a warming effect at higher temperatures.
Note: There is no windchill when the air temperature is at or above 93° F (-34° C).
Heat Index
Heat index uses temperature and the relative humidity to determine 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 more saturated with water vapor) the apparent temperature “feels” higher
than the actual air temperature, because perspiration evaporates more slowly.
Note: Heat index and air temperature are equal at or below 0° F (-18° C).
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 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.
You can also use dew point to predict the minimum overnight temperature. Provided no new
fronts are expected overnight and the afternoon relative humidity is greater than 50%, the
afternoon’s dew point gives you an idea of what minimum temperature to expect overnight.
The higher the humidity is, the more accurate the dew point prediction.
Note: Dew point is equal to the air temperature when the humidity is 100%.
Rain
Vantage Vue incorporates a tipping spoon rain collector in the ISS that measures 0.01'' or 0.2
mm for each tip of the spoon. Your station logs rain data in the same units it is measured in and
converts the logged totals into the selected display units (inches or millimeters) at the time it is
displayed. Converting at display time reduces possible compounded rounding errors over
time.
Four separate variables track rain totals: “rain rate,” “daily rain,” “monthly rain,” and “yearly
rain.” Rain rate calculations are based on the interval of time between each spoon tip, which is
each 0.01'' or 0.2 mm rainfall increment.