In terms of the area it directly affects, a tornado is the most dangerous and destructive of all natural weather phenomena. When a tornado is indicated on the Special Weather tables, this means that conditions suitable for the development of tornadoes will arise during the day in question. The temperature will rise to at least two steps above the normal temperature for the area in question (if the result of 12 on the Day-To-Day Change Table did not increase it to this level already), and for 1D3 hours before the tornadoes pass through the area will be very strong and heavy precipitation will occur. Then the rain will stop abruptly, tapering to nothing or a trace amount within minutes, and in the area immediately around a tornado’s path (one-half mile on either side) the wind will drop off to slight or very slight velocity. The tornado will come through the area 3D6 minutes after the start of this cessation of activity. During this time and for about 30 minutes after the tornado passes, the temperature will drop abruptly by 15 degrees, or three steps (whichever is greater). After this ‘cooling off’ period, the temperature will begin to rise until it reaches an appropriate level according to what was predetermined for the day. For the rest of the day after a tornado comes through an area, the weather will be calm and non-threatening.
The path of a tornado when it touches the ground is typically one-quarter mile wide and 15-20 miles long, and most tornadoes travel along the ground at about 40 mph. Thew whirling vortex of air is distinguishable from the surrounding sky, especially near the ground where the cloud contains dirt and debris that it has picked up in its rampage across the countryside. Within and around the whirlwind, the velocity of the spiraling air can reach 300 mph or more. Because of the centrifugal force pushing the air outward from the center, the air pressure in the center of the vortex is considerably lower than the surrounding vicinity. Structures that are able to stand up to the high-velocity winds may still be destroyed – virtually exploded – when this area of extremely low pressure passes over their locvation.