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Tropical Storms
Energy from the sun, ocean water of 80 degrees Fahrenheit or warmer and a developing low-pressure system -- a tropical depression -- are the ingredients for the formation of a tropical storm. Solar power drives the evaporation of surface waters in warm air. As this heated, moist air begins rising, some of that latent energy is released as the water vapor condenses. Winds streaming toward the low-pressure center spiral due to the effect of the planet's rotation, forming the swirling vortex of a cyclone. By meteorological convention, a tropical storm is a cyclone with wind speeds of between 39 and 74 miles per hour.
Hurricanes
A particularly powerful tropical storm whose winds blow at 74 miles per hour or more -- fueled by plunging atmospheric pressure in the heart of the system -- is considered a full-blown hurricane (also called, depending on the region, a typhoon or cyclone). These are the largest organized storms on Earth: their cloud walls may tower close to 10 miles above the surface of the ocean, and they may sprawl more than 600 miles across. Driven by prevailing trade winds, hurricanes may last for days, eventually expiring as they pass over colder waters or large landmasses, which remove their essential ingredients of heat and moisture.
Tornadoes
Hurricanes may be the most mammoth of the world's storms, but the whirling vortices called tornadoes are the most ferocious. They typically form in the clashing of warm and cold air in the violent chaos of a big thunderstorm. While they have been recorded in many spots worldwide, by far the greatest concentration of tornadoes form in the U.S., particularly in a broad swath of the central part of the country called "Tornado Alley." A tornado's specific birth is still only partly understood. Many develop from so-called mesocyclones in supercell thunderstorms, which develop when horizontally spiraling air within the storm is tilted vertically by updrafts -- but there are non-supercell tornadoes, as well. A funnel cloud descends toward the ground, made visible by condensing water vapor and by dust and other material whipped up below. Most tornadoes last only 15 minutes or so before downdrafts destroy their funnel, but some may snake over land for hours. Winds in the biggest tornadoes may exceed 300 miles per hour.
Influence
Tropical cyclones and tornadoes are, from a human perspective, hugely destructive atmospheric phenomena. Every year they kill people and destroy millions of dollars in property. Nonetheless, they also perform ecological functions: for example, a hurricane in a tropical hardwood hammock in southern Florida or a tornado in a mixed deciduous forest in the Midwest fells trees, opening up the canopy and promoting the establishment of a new, opportunistic plant community in the storm-ravaged tracks.