GLOSSARY


Heat Index
The Heat Index uses the 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 the humidity is high ( i.e., the air is saturated with water vapor) the apparent temperature "feels" higher than the actual air temperature because perspiration evaporates more slowly.  

Wind Chill
Wind Chill takes into account how the speed of the wind effects 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 comfortable warm air surrounding the body.  The faster the wind blows, the faster the heat is carried away and the colder you feel.  Above 91F, wind movement has no effect on apparent temperature, so wind chill is the same as outside temperature.

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 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 temperature into account.  High dew point indicated high water vapor content; low dew point indicates low vapor content.  In addition, high dew point indicates 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% or more, the afternoon 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.

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 percentage of the amount the air is capable of holding.  Relative humidity, therefore, is nor 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 the term "humidity" is used in these documents it means relative humidity.

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.  Generally, the more air above an area, the higher the atmospheric pressure, which means the atmospheric pressure changes with altitude.  For example, atmospheric pressure is greater at sea level than on a mountain top.  To compensate for this difference and facilitate comparison between locations with different altitudes, atmospheric pressure is generally adjusted to the equivalent sea level pressure.  The adjusted pressure is known as barometric pressure.  In reality the weather station measures atmospheric pressure and then translates it to barometric pressure given the altitude of the station's location.

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 weather conditions.

Solar Radiation  
What we call "current solar radiation" is technically known as Global Solar Radiation, a measure of the intensity of the sun's radiation reaching a horizontal surface.  This irradiance includes both the direct component from the sun and the reflected component from the rest of the sky.  The solar radiation reading gives a measure of the amount solar radiation hitting the solar radiation sensor in the weather station at any given time, expressed in Watts/sq. meter.

Ultra Violet Radiation (UV)
Energy from the sun reaches the earth as visible, infrared and ultra violet (UV) rays.  Exposure to UV rays can cause numerous health problems, such as sunburn, skin cancer, skin aging and cataracts, and can supress the immune system.  The weather station's UV sensor can help analyze the changing levels of UV radiation and can advise of situations where exposure is particularly unacceptable.  

The weather station displays UV radiation as the UV index, an intensity measurement first defined by Environmental Canada and since adopted by the World Meteorological Organization.  UV Index assigns a number between 0 and 16 to the current UV intensity.  The US EPA categorizes the Index values as shown below.  The lower the number, the lower the danger of sunburn. The Index value published by the U.S. National Weather Service is a forecast of the next day's noontime UV intensity.  The UV value displayed by the weather station is the result of a real time measurement.

UV Index and Exposure Category
Index Values
Exposure Categories
0-2
Minimal
3-4
Low
5-6
Moderate
7-9
High
10+
Very High


EvapoTranspiration (ET)
EvapoTranspiration (ET) is a measurement of the amount of water vapor returned to the air in a given area.  It combines the amount of water vapor returned through evaporation (from wet vegetation surfaces and the stoma of leaves) with the amount of water vapor returned through transpiration (exhaling of moisture through plant skin) to arrive at a total.  Effectively, ET is the opposite of rainfall, and is expressed in the same units of measure (inches).

The weather station uses air temperature, relative humidity, average wind speed, and solar radiation data to estimate ET.  (ET is calculated once an hour on the hour.)