Tornado Safety>>Tornado Facts>>Storm Conditions

Tornadoes are the result of great instability in the atmosphere and are associated with severe thunderstorms. The full details of the formation of tornadoes are not known. The existence of a strong updraft, such as that generated by a severe thunderstorm, and the conservation of angular (rotational) momentum, however, are fundamental considerations. The falling of rain or hail drags air from aloft, and the resulting rush of air tightens the rotational motion. In Figure 1 to the right, you can see the airflow within a typical severe thunderstorm.

The mesocyclone is an area of low pressure about 8 to 24 kilometers (5 to 15 miles) in diameter, with wind speeds of approximately 240 km/h (150 mph) or less. At the mesocyclone's center is the tornado proper, a funnel that becomes visible as it picks up surface matter or as water vapor condenses in the funnel. The funnel exhibits exceedingly high winds and low pressures. Such winds can pick up and hurl objects with terrible force and cause tremendous damage to structures insufficiently well built to resist them.

In the United States, tornadoes are associated most often with conditions in advance of cold fronts, and weather forecasts include tornado watches when these conditions arise. Tornadoes can occur, however, ahead of warm fronts or even behind cold fronts. Tornadoes also occur frequently in association with hurricanes. In Figure 2 to the right, we see the air flow and precipitation at the ground associated with a developing storm which could produce a tornado.