Tropical cyclones include tropical depressions, tropical storms, hurricanes and typhoons. Mature tropical cyclones are the most feared weather conditions in the world, according to the American Meteorological Society. Its winds have been measured at 175 knots (198.6 miles per hour) and its rains are torrential. Disturbances such as easterly waves and monsoon troughs can start a cyclone. Once one is started, it pulls latent heat from the ocean. It generally moves west and slightly poleward when it first forms, and then it may move into the westerlies and back toward the east. Cyclones die over land.
A tornado is similar to a cyclone but occurs over land. These form beneath cumuliform clouds and are often visible as a funnel cloud. The vortex of a tornado is usually a few hundred feet in diameter; wind speeds have been measured at 300 miles per hour but can be as low as 40 miles per hour. Wind speeds are sometimes measured according to the damage they cause using the Fujita scale (F0 through F6). Tornadoes are found all over the world but are most common in the United States.
Indian summer is a weather condition that occurs in mid-fall to late autumn when the temperature is abnormally warm during the day and cool at night. The skies are generally clear but hazy. A lengthy period of cool weather must precede the warm weather in New England for this to be called an Indian summer; there should also be at least one killing frost. The term Indian summer dates back to 1778; previous terms include "old wives' summer" and "halcyon days."
Precipitation can come in many forms, from torrential rain to drizzle. Snow, hail and fog are other forms of precipitation. Drizzle drops are less than 0.5 millimeters in diameter, but because they are thicker in number, they can reduce visibility. A light rain is easier to travel in than a drizzle. Precipitation is also measured according to inches per hour. A rain is light that stays under 0.10 inches per hour; moderate rain is 0.11 to 0.30 inches per hour, and heavy rain is anything more than 0.30 inches per hour or more than 0.03 in six minutes.
