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Glossary of Common Terms

BarometerA barometer is a device that measures the pressure of the air pushing on it—this measurement is called the barometric pressure. We don’t actually feel the barometric pressure because the air pressure is pushing equally in every direction.
HectoPascals (hPa)This is an international standard (SI system) for measuring air pressure. It used to be referred to as milli-bar (mb) and sometimes still is. They are equivalent.
HygrometerThe instrument that measures the relative humidity of the air. This is expressed as a percentage between 0% and 100%.
Millimetres (mm)This is the unit in which the rain amount is measured. 1 mm corresponds to 1kg/m² of water.
Rain RateRain rate is a measurement of the intensity of rainfall. It gives the amount of rain that would fall over a time period if the rain rate was constant over that time period. It is usually measured in millimetres per hour (mm/h).
Relative Air PressureRelative air pressure is the absolute air pressure compensated for the altitude of the barometer. The result is what the air pressure would be at sea level. This measurement appears on the WXM-M5 screen.
Relative Humidity (RH)It is a measure of the water vapour content of air. In particular, RH is the percentage (%RH) of water vapour that is present in a sample of air to the amount needed to achieve saturation at the same temperature.
Ultra Violet (UV) IndexThe ultraviolet index or UV-Index (UVI) is an international standard measurement of the strength of sunburn-producing ultraviolet (UV) radiation at a particular place and time. The purpose of the UV Index is to help people effectively protect themselves from UV radiation. The UV Index is a linear scale, with higher values representing a greater risk of sunburn (which is correlated with other health risks) due to UV exposure. An index of 0 corresponds to zero UV radiation, as is essentially the case at night. An index of 10 corresponds roughly to midday summer sunlight with a clear sky when the UV Index was originally designed, but values above 10 are sometimes possible. Levels above 8 are considered “very high” and above 11 are considered “extreme.”
Wind DirectionWind direction is defined as the direction the wind is coming from.
Wind SpeedWind speed is the speed of the weather-related air movement from one place to the next.
Wind GustHighest measured wind speed over a 10 minute period.