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 theres 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 has a warming effect at higher temperatures.
The 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 the apparent temperature
feels higher than the actual air temperature, because perspiration evaporates more slowly
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 airs 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, 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 humidity has a
large capacity to absorb extra water vapor.
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 airs actual water vapor content,
unlike relative humidity, which takes the airs 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 or equal to 50%, the afternoons dew point gives you
an idea of what minimum temperature to expect overnight, since the air can
never get colder than the dew point.
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. Generally,
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 compensate
for this difference and facilitate comparison between locations with
different altitudes, atmospheric pressure is generally adjusted to the equivalent
sea-level pressure. This adjusted pressure is known as barometric pressure. In
reality, the Vantage Pro2 measures atmospheric pressure. When you enter your
locations altitude in Setup Mode, the Vantage Pro2 stores the necessary offset
value to consistently translate atmospheric pressure into barometric pressure.
Barometric pressure also changes with local weather conditions, making barometric
pressure an extremely important and useful weather forecasting tool.
High pressure zones are generally associated with fair weather while low pressure
zones are generally associated with poor weather. For forecasting purposes,
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 deteriorating