Glossary of Weather Terms

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The weather terms listed below are used within the Trust's weather pages.   They are listed in alphabetical order along with a brief explanation.
Some of the explanations are taken from information provided by Davis Instruments, the manufacturer of the Trust's weather station. 

Davis Weather Stations

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10 Minute Average Wind Speed Average wind speed (mph) during a 10 minute period prior to listed time.
Air Density Air density measures the mass of air present per unit volume of air. (lb/ft).  The average density of air is about 0.0805 lb/ft or 1.229 Kg/m.   The metric equivalent of 1lb/ft is around 15.26 Kg/m.
Atmospheric Pressure The pressure exerted by the weight of the air above the observer. (1hPa = 1hectoPascal=1mbar)
Cooling Degree Days One cooling degree-day is the amount of cooling required to keep a structure at 65F when the outside temperature remains one degree above the 65F threshold for 24 hours. One cooling  degree-day is also the amount of cooling required to keep that structure at 65F when the temperature remains 24F above that 65 threshold for 1 hour.
Dew Point Dew-point (C) is the temperature to which air must be cooled for saturation (100% relative humidity) to occur, providing there is no change in water content.
Heating Degree Days One heating degreeday is the amount of heat required to keep a structure at 65F when the outside temperature remains one degree below the 65F threshold for 24 hours. One heating degree-day is also the amount of heat required to keep that structure at 65F when the temperature remains 24F below that 65 threshold for 1 hour.
Heat Index The Heat Index (C) uses the temperature and the relative humidity to determine how hot the air actually "feels."
Humidity, Relative Relative humidity (%) is the amount of water vapour present in the air expressed as a percentage of the amount of water vapour that the air is capable of holding.
Rainfall Total rainfall (mm) during a set period of time.
Rainfall Rate Rainfall rate (mm/hr) is a measure of how much rain falls in a unit of time.
Rain Storm Storm rain is the total amount (mm) of rain falling during a period of wet weather.  This 'wet weather' period starts when at least 0.5mm of rainfall is registered by the weather station and then continues to accumulate.  The wet weather period is only ended when it is followed by a continuous dry period of 24 hours.
Temperature A measure of the average energy of molecules in the atmosphere. (C)
Temperature-Humidity-Wind-Sun (THWS) Index The Temperature-humidity-wind-wind-sun (THWS) Index uses humidity, temperature, the cooling effects of wind and the heating effects of direct solar radiation to calculate an apparent temperature.
Temperature-Humidity-Wind (THW) Index The Temperature-Humidity-Wind (THW) Index uses temperature, humidity and wind to calculate an apparent temperature that incorporates the cooling effects of wind on our perception of temperature.
Wind Chill Wind Chill (C) takes into account how the speed of the wind affects our perception of the air temperature.  The faster the wind blows, the faster heat is carried away from your body and the colder you feel.
Wind Direction The direction from which the wind is blowing (expressed as a compass direction).
Wind Run Wind run is a measurement of the "amount" of wind passing the station during a given period of time, expressed in either "miles of wind" or "kilometers ofwind". WeatherLink calculates wind run by multiplying the average wind speed for each archive record by the archive interval.
Wind Speed Wind speed (mph).

 

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