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Cyclone: The name given to a hurricane when it forms over the Bay of Bengal or the northern Indian Ocean

Doppler radar: Weather radar that reads the direction and speed of moving objects (such as drops of precipitation) by determining whether atmospheric motion is horizontally toward or away from the radar

Downdraft: Rain-cooled air that falls to the ground in a thunderstorm

Earthquake: The sudden motion or shaking of the ground produced by abrupt displacement of rock masses

Epicenter: The point on the Earth's crust directly above the hypocenter of an earthquake

Eye: The center of a hurricane, characterized by a roughly circular area of light winds and rain-free skies

Eye wall: The ring of thunderstorms that surrounds a hurricane's eye

Fault: A crack in the Earth's crust where there is displacement of one side relative to the other

Front: The boundary between air masses of different densities

Funnel cloud: A funnel-shaped cloud that does not touch the ground

Hurricane: A severe tropical cyclone with wind speeds in excess of 74 miles (119 kilometers) an hour

Hurricane warning: A warning that sustained winds of 74 miles (119 kilometers) an hour or higher associated with a hurricane are expected in a specified coastal area in 24 hours or less

Hurricane watch: A warning that hurricane conditions are possible within 36 hours

Hypocenter: The point of an earthquake's origin deep within the Earth

Jet stream: A "river" of strong winds that flows over North America from west to east

Lava: Molten rock (magma) that extrudes through a volcano

Magma: Molten rock beneath the Earth's surface

Mesocyclone: A rotating shaft of rapidly rising air within a storm. Under certain conditions, a mesocyclone can generate a tornado.

Plates: Rigid slabs that make up the Earth's crust; see "plate tectonics"

Plate tectonics: A theory that the Earth's surface is made up of large plates that are continuously moving and that interactions among the plates at their borders cause most major geologic activity

Pressure or atmospheric pressure: The force exerted on an area by the weight of the atmosphere overhead.

Rift: A break in the Earth's crust created by geological stress

Seismograph: An instrument that detects and records vibrations caused by earthquake shock waves

Spreading center: An area where two plates are pulling away from each other

Storm surge: An abnormal rise in sea level accompanying a hurricane or other intense storm

Subduction zone: An area where two plates come together and one slides under another

Supercell: A violent thunderstorm that has a persistent, rotating updraft and is capable of spawning tornadoes

Temblor: An earthquake

Tornado: A violently rotating column of air that forms at the bottom of a cloud and touches the ground

Tornado Alley: An area stretching from Texas though Iowa, where many U.S. tornadoes touch down

Tornado watch: A news bulletin announcing that atmospheric conditions are favorable for producing tornadoes

Tornado warning: A news bulletin stating that a tornado has already been spotted or indicated by radar

Tropical depression: A tropical storm with maximum sustained winds near the surface of less than 39 miles (63 kilometers) an hour

Typhoon: The name given to a hurricane that forms in the western Pacific Ocean

Updraft: A warm column of air that rises within a cloud

Volcano: An opening in the Earth's crust through which molten magma and gases erupt

Waterspout: A small, weak tornado that forms over water

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