DAVIS and Vantage Pro, User Manual & Setup Manual

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Appendix: Weather Data
Page 50 Vantage Pro Manual
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 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 after-
noon 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 (as
opposed to relative humidity). High dew-point indicates high vapor content;
low dew-point indicates low vapor content. In addition a high dew-point indi-
cates a better chance of rain and severe thunderstorms. You can even use dew-
point to predict the minimum overnight temperature. Provided no new fronts
are expected overnight and the afternoon Relative Humidity
≥
50%, the after-
noon’s dew-point gives you an idea of what minimum temperature to expect
overnight, since the air is not likely to get colder than the dew-point anytime
during the night.
Rainfall
The Vantage Pro provides four separate registers for tracking rainfall totals:
“rain storm”, “daily rain”, “monthly rain”, and “yearly rain”. The Vantage Pro
also calculates the rate of rainfall by measuring the interval of time between
each .01” or .254 mm rainfall increment.
Barometric Pressure
The weight of the air that makes up our atmosphere exerts a pressure on the
surface of the earth. This pressure is known as atmospheric pressure. Gener-
ally, the more air above an area, the higher the atmospheric pressure, this, in
turn, means that atmospheric pressure changes with altitude. For example,
atmospheric pressure is greater at sea-level than on a mountaintop. To compen-
sate for this difference and facilitate comparison between locations with differ-
ent altitudes, atmospheric pressure is generally adjusted to the equivalent sea-
level pressure. This adjusted pressure is known as barometric pressure. In real-
ity, the Vantage Pro measures atmospheric pressure. When you enter your loca-
tion’s altitude in Setup Mode, the Vantage Pro stores the necessary offset value
to consistently translate atmospheric pressure into barometric pressure.
Barometric pressure also changes with local weather conditions, making baro-
metric pressure an extremely important and useful weather forecasting tool.
High pressure zones are generally associated with fair weather while low pres-
sure zones are generally associated with poor weather. For forecasting pur-
poses, however, the absolute barometric pressure value is generally less
important than the change in barometric pressure. In general, rising pressure
indicates improving weather conditions while falling pressure indicates deteri-
orating weather conditions.
Note: The following variables require optional sensors and / or stations. See “Optional Sensors” starting on
page 9.