Consequently, there is an associated change in the surface winds, and some storms become stronger than normal. This oscillation is on the scale of a year or a few years, and has global implications such as widespread drought or flooding. Oceanic fishing is also disrupted. Squall Line A broken or solid line of thunderstorms that may extend across several hundred kilometres Subsidence A descending motion of air in the atmosphere, usually with the implication that the condition extends over a rather broad area. A relatively small-scale, rising air current produced when the earth's surface is heated.
Thermals are a common source of low level turbulence for aircraft.
Tornado A violently rotating column of air, usually pendant to a cumulonimbus, with circulation reaching the ground. It nearly always starts off as a funnel cloud and may be accompanied by a loud roaring noise. Tropical Cyclone A cyclone originating over tropical or subtropical waters with organized convection and a definite cyclonic surface wind circulation. Tropical cyclones are large and span areas of 1000 of kilometres. They can causes a great deal of damage when they make landfall.