Wind- & Pressure-related

Wind is air motion relative to the Earth’s surface. Unless otherwise specified, only the horizontal component is considered (WMO, 1992). 

A depression or cyclone is a region of the atmosphere in which the pressure is lower than that of the surrounding region at the same level (WMO, 1992).

An extra-tropical cyclone is a low-pressure system, which develops in latitudes outside the tropics (WMO, 1992). 

A subtropical cyclone is a non-frontal, low-pressure system that has characteristics of both tropical and extra-tropical cyclones. Like tropical cyclones, they are non-frontal, synoptic-scale cyclones that originate over tropical or subtropical waters and have a closed surface wind circulation about a well-defined centre (WMO, 2024 a). 

Derechos are fast-moving bands of thunderstorms with destructive winds. The winds can be as strong as those found in hurricanes or even tornadoes. Unlike hurricanes and tornadoes, these winds follow straight lines (NOAA, 2019).

A gale is wind with a speed of between 34 and 40 knots (62–74 km/h, 32–38 mph). Also known as Beaufort scale wind force 8 (WMO, 1992). 

A squall is an atmospheric phenomenon characterised by a very large variation of wind speed: it begins suddenly, has a duration of the order of minutes and decreases suddenly in speed. It is often accompanied by a shower or thunderstorm (WMO, 2018). 

A tropical cyclone is a cyclone of tropical origin of small diameter (some hundreds of kilometres) with a minimum surface pressure in some cases of less than 900 hPa, very violent winds and torrential rain; sometimes accompanied by thunderstorms. It usually contains a central region, known as the 'eye' of the storm, with a diameter of the order of some tens of kilometres, and with light winds and a more or less lightly clouded sky (WMO, 2023).

A tornado is a rotating column of air extending from the base of a cumuliform cloud and often visible as a condensation funnel in contact with the ground, and/or attendant circulating dust or debris cloud at the ground (WMO, 2017).