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A tornado is an extremely violent whirling storm with a small diameter, usually a quarter of a mile or less. The length of the track of a tornado on the ground maybe from a few hundred feet to 300 miles; the average is less than 25 miles. When not touching the ground, it is termed a funnel cloud or tuba. The velocities of tornadic winds are in the general range of 125 to 250 knots. A large reduction of pressure in the center due to the spiraling of the air seems to cause buildings in the path of the storm to explode. The speed of the storm over Earth’s surface is comparatively slow— usually 22 to 34 knots.

Most of the tornadoes in the United States occur in the late spring and early summer in mid and late afternoon, and they are associated with thunderstorm activity and heavy rain. Tornadoes occur on all continents but are most common in Australia and the United States. They can occur throughout the year and at any time of day. Tornadoes have been observed with various synoptic situations but are usually associated with overrunning cold air. Statistics show that the majority of tornadoes appear about 75 to 180 miles ahead of a cold front along the prefrontal squall line. Figure 5-1-6 shows the various stages of development of a tornado.

A situation that is noticeably favorable to tor-nado activity is cold air advection aloft. When mP air moves across the United States, it becomes heated in the low levels in the western plateaus.

Figure 5-1-6.—Stages of development of a tornado.

The resulting density of the now warm mP air is then equal to or less than that of mT air moving northward over the Mississippi Valley. The mP air rides up over the mT air. The mP air still maintains low temperatures at higher altitudes causing extreme instability.

The following conditions may indicate possi-ble tornado activity:

1.Pronounced horizontal wind shear. (Wind shear is the rate of change of wind veloc-ity with distance.)

2. Rapidly moving cold front.

3. Strong convergent flow at the surface.

4. Marked convective instability.

5. Dry air mass superimposed on a moist air mass and abrupt change in moisture con-tent, usually below 10,000 feet.

6. Marked convection up to the minus 10°C isotherm.

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