A tornado is a "violently rotating column of air in contact with the ground." It is a center of very low pressure surrounding by high winds, usually rotating counterclockwise. Tornadoes usually form in the mid portion of a supercell thunderstorm up to 30 minutes before lowering to the ground. The visible part of a tornado is composed of water, dust and debris.
A tornado that has not picked up much debris may not appear to touch the ground, but if damage is seen it is a tornado, not a funnel. A "funnel" or "funnel cloud" is a violently rotating column of air that does not touch the ground or cause any ground damage. The terms "twister" and "cyclone" may refer to a tornado. A "waterspout" is a tornado over water. All of these are similar (except for very large tornadoes) in that they are usually smooth and tapered, they hold together well, and they rotate.
Ragged clouds and rain shafts, often confused with tornadoes, do not rotate. As 1 spotter team says, "If it don't spin, don't call it in." Formations
Most tornadoes develop from a low-hanging cloud beneath the rain-free base at the rear of a supercell thunderstorm. This "wall cloud" is usually 1 to 3 miles wide, often forming as much as 20 to 30 minutes before the tornado appears. Rotating wall clouds indicate a probable tornado while non-rotating wall clouds mean large hail is possible.
The rear portion of a tornadic storm is where the warm, moist air that fuels the storm enters. The front portion is the is the exhaust area from which wind, rain and hail can be expelled. Wind, rain and hail do not occur in all tornadic storms, but if they are present they usually occur in that order. The tornado usually appears first as a funnel cloud; then dips downward until it reaches the ground. During the first half of it's life, it increases in size; in the second half it shrinks to a rope-like feature and tilts.
These are 2 clues that it is entering its dying phase. Even as it dies, it can still cause injury or death. The danger is not over until the tornado and the thunderstorm that produced it have dissipated. Tornadoes generally increase in activity in March, reach a maximum in April and May, and end in early June.
They occur most frequently between 4 p.m. and 8 p.m. and move toward the northeast at 30 to 40 mph. They can quickly change directions, accelerate to 70 miles per hour, stop dead still, split into several separate tornadoes, or rejoin into 1.