kookaburra

Tornado (Latin, tonare, “to thunder”), in meteorology, violent whirling wind, characteristically accompanied by a funnel-shaped cloud extending down from a cumulonimbus cloud. Commonly known as a cyclone or twister, a tornado can be a few metres to about a kilometre wide where it touches the ground, with an average width of a few hundred metres. It can move over land for distances ranging from short hops to many kilometres, causing great damage wherever it descends. The funnel is made visible by the dust that is sucked up and by condensation of water droplets in the centre of the funnel. The same condensation process makes visible the generally weaker sea-going tornadoes, called waterspouts, which occur most frequently in tropical waters. Most tornadoes spin anticlockwise in the Northern hemisphere and clockwise in the Southern, but occasionally that pattern is reversed.

CYCLONE

Cyclone, in meteorology, an area of low atmospheric pressure surrounded by a wind system blowing (in the northern hemisphere) in an anticlockwise direction. A wind system blowing round a corresponding high-pressure area with clockwise winds is known as an anticyclone. In the southern hemisphere these wind directions are reversed. Cyclones are commonly called lows and anticyclones highs. The term cyclone has often been more loosely applied to a storm and disturbance attending such pressure systems, particularly violent tropical hurricanes and typhoons, which centre on areas of unusually low pressure.

EARTHQUAKE

Earthquake, vibrations produced in the Earth's crust when rocks in which elastic strain has been building up suddenly rupture, and then rebound. The vibrations can range from barely noticeable to catastrophically destructive. Earthquakes can release energy thousands of times greater than the world's first atomic bomb.

HURRICANE

Hurricane, name applied to migratory tropical cyclones that originate over oceans in certain regions near the equator, and particularly to those arising in the West Indian region, including the Caribbean Sea and the Gulf of Mexico. Hurricane-type cyclones in the western Pacific are known as typhoons.

AVALANCHE

Avalanche, sudden flow of a large mass of snow or ice down a slope or cliff, sometimes at speeds exceeding 160 km/hr (100 mph). Such flows can be destructive of life and property. Avalanches are most common on slopes exceeding 30°, frequently when a deep snow falls suddenly and does not have a chance to cohere, or when a thaw undercuts a blanket of older snow.

TSUNAMI

Tsunami, Japanese word meaning “harbour wave” and used as the scientific term for a seismic sea wave, a large ocean wave generated by an undersea earthquake. A tsunami is thought to be triggered when the ocean floor is tilted or offset during the quake, creating a set of waves similar to the concentric waves generated by an object dropped into water. Another possible cause is an undersea landslide or volcanic eruption. Most tsunamis originate along the so-called Ring of Fire, a zone of volcanoes and seismic activity, 32,500 km (24,000 mi) long, that encircles the Pacific Ocean.