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Weather Glossary

Need to know what heat index is or what is THW Index or Humidity well below explains all browse this page carefully. 

Data: Heat Index

The Heat Index uses the temperature and the relative humidity to deter mine 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.

Note: WeatherLink uses the Steadman (1979 & 1998) formula to calculate Heat Index, which is more accurate than the method used by the Vantage Pro/Vantage Pro2 consoles and is calculated for all temperatures.

The Heat Index is not stored in archive memory or in the database, rather it is calculated as necessary (for example, when plotting or displaying database information). When Heat Index data is needed, the software calculates an average for each archive interval based on the temperature and humidity readings for the archive period.

Data: 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.

Data: Temperature Humidity Sun Wind (THSW) Index

The THWS Index uses humidity, temperature, the cooling effects of wind and the heating effects of direct solar radiation to calculate an apparent temperature.

Data: 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, 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 thunder storms. 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.

Dew point is not stored in archive memory or the database. Dew point is calculated whenever it is displayed. If you edit the temperature or humidity value, the dew point will change as well.

Data: 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.

Internal combustion engines operate at their peak efficiency (producing the most power or using the least fuel) when the correct ratio of oxygen and fuel are introduced into the combustion chamber. The carburetor jets (or fuel injector setting) control the ratio of how much fuel is introduced into a given airflow. Carburetors do not automatically compensate for changes in the amount of oxygen in that airflow and therefore changes in oxygen concentrations can result in inefficient operation. Changes in the oxygen content of air result from changes in the weather (barometric pressure, temperature, and humidity).

Measuring oxygen concentration in air is difficult; other measures are commonly used to estimate oxygen content. For example, determining changes in the density of the air (i.e., how much a given volume of air weighs) can produce a reasonable estimate of changes in oxygen concentration.

Data: 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.

Data: Solar Radiation & Solar Energy

The solar radiation sensor is only available for Vantage Pro and Vantage Pro2 stations.

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 of solar radiation hitting the solar radiation sensor at any given time, expressed in Watts per square meter (W/m2). The value logged by WeatherLink is the average solar radiation measured over the archive interval.

Solar Energy

The amount of accumulated solar radiation energy over a period of time is measured in Langleys.

1 Langley  = 11.622 Watt-hours per square meter

 = 3.687 BTUs per square foot

 = 41.84 kilojoules per square meter

The console displays UV readings in two scales: UV Index and UV MEDs.

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

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.

UV MEDs

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

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

UV Dose and Sunburn – Use this plot to estimate the MED dose leading to sunburn. A person with Type II (Environment Canada) skin type might choose 0.75 MED as the maximum for the day; in contrast, a person with Type V (Environment Canada) skin type might consider 2.5 MEDs a reasonable dose for the day.

Note:  The Vantage Pro/Vantage Pro2 consoles

Data: Wind Run

Wind run is measurement of the "amount" of wind passing the station during a given period of time, expressed in either "miles of wind" or "kilometers of wind". WeatherLink calculates wind run by multiplying the average wind speed for each archive record by the archive interval.

For Example:

Average Wind Speed = 5 mph

Archive Interval = 30 minutes (0.5 hours)

Wind Run = 5 mph x 0.5 hours = 2.5 miles of wind

• If you are using mph to measure wind speed, wind run will be reported in miles.

• If you are using knots to measure wind speed, wind run will be reported in nautical miles.

• If you are using km/h to measure wind speed, wind run will be reported in kilometers.

• If you are using m/s to measure wind speed, wind run will be reported in kilometers.

Data: Bar Trend

The Bar Trend shows the direction of change (higher, lower, steady) of the barometric pressure over the last three hours.

• The Bar Trend is updated every 15 minutes.

• Rapid rise is indicated if the pressure increases >= 0.06"

• Slow rise is indicated if the pressure increases >= 0.02" and < 0.06"

• Steady is indicated if the pressure changes < 0.02"

• Slow fall is indicated if the pressure decreases >= 0.02" and < 0.06"

• Rapid fall is indicated when the pressure decreases >= 0.06"