Weather Definitions

*Reprinted and edited from the Davis Instruments Logo WeatherLink 5.8 help files.*

Air Density

Air Density (the weight of 1 cubic foot or 1 cubic meter of air ) is a valuable tool for racing enthusiasts, because it helps determine the optimal jetting under current weather conditions.

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. 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 in pressure at different elevations, and to facilitate comparison between locations with different altitudes, meteorologists adjust atmospheric pressure so that it reflects what the pressure would be if measured at sea-level. This adjusted pressure is known as barometric pressure.

Barometric pressure changes with local weather conditions, making barometric pressure an 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, 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.

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's temperature into account. High dew-point indicates high vapor content; low dew-point indicates low vapor content. In addition a 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 ³ 50%, the afternoon'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.


Moisture content in wood affects both the size and strength of lumber. If you know the EMC of the storage or manufacturing area (which is derived using temperature and humidity readings), you can also determine the moisture content of the wood stored there.

Heating and Cooling Degree-Days

Although degree-days are most commonly used in agriculture, they are also useful in building design and construction, and in fuel use evaluation. The construction industry uses heating degree-days to calculate the amount of heat necessary to keep a building, be it a house or a skyscraper, comfortable for occupation. Likewise, cooling degree-days are used to estimate the amount of heat that must be removed (through air-conditioning) to keep a structure comfortable. Just like growing degree-days, heating and cooling degree-days are based on departures from a base temperature. 18ºC is almost always used as this base.

One heating degree-day is the amount of heat required to keep a structure at 18°C when the outside temperature remains one degree below the 18°C threshold for 24 hours. One heating degree-day is also the amount of heat required to keep that structure at 18°C when the temperature remains 14°C below that 18° threshold for 1 hour.

Likewise, one cooling degree-day is the amount of cooling required to keep a structure at 18°C when the outside temperature remains one degree above the 18°C threshold for 24 hours. One cooling degree-day is also the amount of cooling required to keep that structure at 18°C when the temperature remains 14°C above that 18º threshold for 1 hour.

Temperature Humidity Wind (THW) Index

The THW Index uses humidity, temperature and wind to calculate an apparent temperature that incorporates the cooling effects of wind on our perception of temperature.

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 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 affects our perception of air temperature. Your body warms the surrounding air molecules by transferring heat from your skin. If there's no air movement, this insulating layer of warm air molecules stays next to your body and offers some protection from cooler air molecules. Wind disperses this layer of warm air, causing the air temperature to "feel" colder. The faster the wind blows, the quicker the layer of warm air is dispersed, and the colder you feel. Above 76.7ºF (24.8ºC), wind movement has no effect on the apparent temperature.

High Rain Rate

The rain rate is calculated by measuring the time interval between each rainfall increment. When there is rainfall within the archive period, the highest measured value is reported. When no rainfall occurs, the rain rate will slowly decay based on the elapse time since the last measured rainfall.

UV Index & UV Dose

Energy from the sun reaches the earth as visible, infrared, and ultraviolet (UV) rays. Exposure to UV rays can cause numerous health problems, such as sun burn, skin cancer, skin aging, and cataracts, and can suppress the immune system.

UV Index

The UV Index measures the intensity of UV. It was first defined by Environment Canada and since has been adopted by the World Meteorological Organization. UV Index uses a scale of 0 to 16 to rate the current intensity of UV. The UV value logged by WeatherLink is the average UV measured during the archive interval.

The US EPA categorizes the UV Index values as shown below in Table 1.

Table 1 – UV Index and Exposure Category

Index Values

Exposure Category










Very High

Hi UV Index

This is the peak UV measured during the archive interval.

Note:  Vantage Pro consoles with Rev A firmware will only show the daily high UV value.


MED stands for Minimum Erythemal Dose, defined as the amount of sunlight exposure necessary to induce a barely perceptible redness of the skin within 24 hours after sun exposure. In other words, exposure to 1 MED will result in a reddening of the skin. Because different skin types burn at different rates, 1 MED for persons with very dark skin is different from 1 MED for persons with very light skin.

Both the U.S. Environmental Protection Agency (EPA) and Environment Canada have developed skin type categories correlating characteristics of skin with rates of sunburn. Tables 2 and 3 below list these skin types.


Table 2 – EPA Skin Phototypes

Skin Photo type

Skin Color

Tanning & Sunburn History

1- Never tans, always burns

Pale or milky white; alabaster

Develops red sunburn; painful swelling, skin peels

2 – Sometimes tans, usually burns

Very light brown; sometimes freckles

Usually burns, pinkish or red coloring appears; can gradually develop light brown tan

3 – Usually tans, sometimes burns

Light tan; brown, or olive; distinctly pigmented

Rarely burns; shows moderately rapid tanning response

4 – Always tans; rarely burns

Brown, dark brown, or black

Rarely burns; shows very rapid tanning response

Table 3 – Environment Canada Skin Types and Reaction to the Sun

Skin Type


Skin Color

History of Tanning & Sun burning



Always burns easily, never tans



Always burns easily, tans minimally


Light Brown

Burns moderately, tans gradually


Moderate Brown

Burns minimally, tans well


Dark Brown

Burns rarely, tans profusely



Rarely burns; shows very rapid tanning response