TROPICAL CYCLONES

Made of a myriad of thunderstorms, the vast, rapidly rotating spiral systems arise over warm ocean waters. At their greatest intensity, they are called hurricanes, typhoons or cyclones, depending on the region where they occur.

HURRICANES

They arise from tropical cyclones that spiral off the west coasts of Africa and Central America in late summer and early fall. Around a hurricane's central "eye"--a calm, sunlit cylinder that may be tens of kilometers across--winds swirl at speeds that may approach 320 km/h.

TYPHOONS

These are tropical cyclones that hit coasts along the northwest Pacific Ocean. They may take place at virtually any time of the year and sometimes kill 100,000 or more people at a time. In some parts of the world, such as India and Australia, these storms are referred to as cyclones.

STORM SURGES

Associated with typhoons and hurricanes, storm surges frequently flood low-lying areas. First the pressure drop within the storm allows water in the ocean to rise. High winds then pile up the water. The surges cause temporary increases in sea level of as much as 6 m. Tsunamis--popularly called tidal waves--are triggered by offshore earthquakes rather than cylonic winds.

TORNADOES

They are spawned by severe thunderstorms and are much more compact than hurricanes. In fact, hundreds of these spinning air masses could fit into a hurricane's eye. Like their larger cousins, tornadoes are fueled by a constant infusion of warm, moist air.

WATERSPOUTS

These are usually weak tonadoes that form over open water. Occasionally, waterspouts whirl to shore, wrecking boats and damaging docks and seafront buildings.

LANDSPOUTS

Generally small-size tornadoes, they form from cloud systems that may not even spawn a thunderstorm. They appear to build themselves from the ground up by incorporating low-level swirls of rotating air.

DUST DEVILS

They form when heat rising from the desert encounters some isolated pocket of rolling air. Because they happen in mostly uninhabited places, dust devils are considered fairly harmless, but their wind speeds can exceed 100 km/h.