Meteorological hazards: Storms and tidal waves

Definition and characteristics

A winter storm emerges from a extra-tropical cyclone, a synoptic scale low pressure system that occurs in the middle latitudes of the Earth and is connected to fronts and horizontal gradients in temperature and dew point. A winter storm comes along with high wind speeds, gusts, thunderstorms, rain and often storm surges.

A severe storm or thunderstorm is the result of convection and condensation in the lower atmosphere and the accompanying formation of a cumulonimbus cloud. A severe storm usually comes along with high winds, heavy precipitation (rain, sleet, hail), thunder and lightning.

A hail storm is a type of storm that is characterised by hail as the dominant part of its precipitation. The size of the hailstones can vary between pea size (6mm) and softball size (112mm) and therefore cause considerable damage.

Lightning is an atmospheric discharge of electricity, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms.

Tornadoes are rotating columns of air (vortex) that emerge out of the base of a cumulonimbus cloud and have contact to the Earth's surface. Typically a tornado forms during a severe convective storm in so-called supercells and is often visible as a funnel-shaped cloud. Tornadoes are usually short-lived, lasting on average no more than 10 minutes. They can generate wind speeds above 400 km/h and are considered the most destructive weather phenomenon. The intensity of tornadoes is assessed using the Enhanced Fujita Scale. Other names for this weather phenomenon are twister, waterspout.

Local windstorm refers to strong winds caused by regional atmospheric phenomena which are typical for a certain area. These can be katabatic winds, foehn winds etc.

A sandstorm/dust storm typically occurs in arid or semi-arid regions if high wind speeds cause the transportation of small particles like sand or fine clastic sediment by saltation and/or suspension.

A snowstorm refers to a storm, usually in the winter season, where large amounts of snow fall. If it's a severe snowstorm that meets certain criteria, such as strong winds, blowing snow and low or falling temperatures, it's called blizzard.

A tidal wave/storm surge is the rise of the water level in the sea, an estuary or lake as result of strong wind driving the seawater towards the coast. This so-called wind setup is superimposed on the normal astronomical tide. The mean high water level can be exceeded by five and more metres. The areas threatened by storm surges are coastal lowlands.

A glacier lake outburst flood (Jökulhlaup) occurs when a lake - dammed by a glacier or a terminal moraine - fails. The outburst can be triggered by erosion, a critical water pressure, a mass movement, an earthquake or cryoseism. A jökulhlaup is a special type of a glacier lake outburst flood related to the outburst of an ice-dammed lake during a volcanic eruption.

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