Anemometer An instrument used to measure wind speed.

An instrument used to measure atmospheric pressure.

The mercury barometer was the barometer of choice in Isaac Cline's day. It is still in wide use today. It consists of an inverted glass tube resting in a pool of mercury. Pressure forces the mercury in the pool downward, which causes mercury inside the tube to rise and fall in response to changes in the pressure outside of the tube.

Coriolus Effect The deflective effect of Earth's rotation on all free-moving objects, including the atmosphere and oceans. Deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

For example, imagine you are inside a rocket traveling in a straight line from the North Pole to the Equator. During your flight, the earth would rotate under you, so that you would land at a different longitude than where you started. Because the earth spins on its axis, winds (which naturally travel in a straight line) are deflected into a curved path, even when they are travelling parallel to the Equator. This is what causes the high-velocity winds in a hurricane system to form into spiral "arms" spinning around the center. Deflection is very weak near the Equator itself, which is why hurricanes cannot form within five degrees latitude of the Equator.

Eye A roughly circular area of relatively light winds and fair weather at the center of a hurricane.

Hurricane A tropical cyclonic storm having minimum winds of 74 miles per hour; also known as typhoon (western Pacific) and cyclone (Indian Ocean).

Latent Heat The energy absorbed or released during a change of state.

Saffir-Simpson Scale A scale, from 1 to 5, used to rank the relative intensities of hurricanes.

Storm Surge The abnormal rise of the sea along a shore as a result of strong winds.

Tropical Cyclone An area of low atmospheric pressure characterized by rotating and converging winds and ascending air, where the central core is warmer than the surrounding atmosphere.

Tropical Depression By international agreement, a tropical cyclone with maximum winds that do not exceed 39 miles per hour.

Tropical Disturbance A term used by the U.S. National Weather Service for a cyclonic wind system in the tropics that is in its formative stages.

Tropical Storm By international agreement, a tropical cyclone with maximum winds between 39 and 74 miles per hour.



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