Reading 2


1                                   Tornadoes are one of the most violent of all weather systems. A tornado can produce tremendous destructive power in a restricted area as it passes by, sweeping the ground clear of all movable objects. Fortunately, tornadoes are short–lived and often strike sparsely populated regions. Occasionally, however, a major tornado outbreak causes incredible devastation. A tornado’s powerful blasts of wind put all human life in jeopardy, sending debris flying and lifting buildings from their foundations.

2                                   The formation of tornadoes has been the subject of increasingly fruitful research. Nevertheless, some mystery still surrounds tornadoes, and their formation cannot be predicted with absolute accuracy, even when conditions for their occurrence seem just right. Most tornadoes are created by, and travel with, intense thunderstorm cells. Because tornadoes require moist air, they favor the warmest part of the day, when solar heating and thunderstorm development are at their maximum.

3                                   Probably the most striking characteristic of a tornado is its spinning funnel cloud, a lowering of cloud base into a column that narrows as it reaches down to the ground from a parent cloud that is part of an active thunderstorm. The funnel cloud forms in response to the steep air pressure directed from the storm’s outer edge toward its center. Humid air expands and cools as it is drawn inward toward the center of the system. The cooling of air below its dewpoint causes water vapor to condense into cloud droplets.

4                                   The actual tornadic circulation covers a much wider area than the funnel cloud suggests. The funnel may range in diameter from a few meters to 3.2 kilometers (2 miles). However, the diameter of a funnel cloud is typically only about one–tenth that of the associated tornadic circulation. Several funnels may develop in a mature tornado system, with small vortexes continually forming and then disappearing while whirling around the central core of the main tornado. The funnels may be made visible by the presence of dust and debris. A funnel can assume a variety of forms, from a thin, writhing, ropelike pendant of grayish white to a thick mass of menacing black.

5                                   The central United States is one of only a few places in the world where the spring weather conditions and flat terrain are ideal for tornado development. Although tornadoes have been reported in all 50 states and throughout southern Canada, most occur in “tornado alley,” a level corridor that stretches from eastern Texas northward through the open plains of Oklahoma, Kansas, and Nebraska.

6                                   Almost three–quarters of the tornadoes in North America occur from March to July. The months of peak tornado activity are April (13%), May (12%), and June (21%). During that time of year, weather conditions are optimal for causing the severe thunderstorms that produce tornadoes. One contributing factor is the relative instability of the lower atmosphere. During the transition from winter to summer, days lengthen, thereby warming the ground. Heat is transferred from the ground into the troposphere, but it takes time for the entire troposphere to adjust to receiving heat from below. The upper troposphere, in fact, usually retains its winter cold into the spring months. The result is an imbalance of cold air and warm ground that favors the development of rotating thunderstorms known as supercells.

7                                   In the Northern Hemisphere, tornadoes almost always spin counterclockwise. Around 87 percent of all tornadoes and their parent cells travel from southwest to northeast, but any direction is possible. Tornado trajectories are often irregular, with many tornadoes exhibiting a hopscotch pattern of destruction as they alternately touch down and lift off the ground. Some have been known to move in circles and even to describe figure eights. Tornadoes may track along paths from several meters to more than 320 kilometers (200 miles) long. Average forward speed is around 48 kilometers (30 miles) per hour, although there are reports of tornadoes racing along at speeds approaching 120 kilometers (75 miles) per hour. During part of its course, the great Tri–State Tornado on March 18, 1925, moved at the astonishing rate of 117 kilometers (73 miles) per hour, killing 695 people in Missouri, Illinois, and Indiana, and making it one of the most deadly tornadoes on record.

vortexes: spiraling masses of air or water that suck everything toward their center
troposphere: the lowest region of the atmosphere