A depression or cyclone is a region of the atmosphere in which the pressures are lower than those 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 sub-tropical cyclone is a non-frontal low-pressure system that has characteristics of both tropical and extratropical 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 welldefined centre (WMO, 2017).