Excessive rainfall and flash flooding

A flash flood is not necessarily a wall of water roaring down a canyon. Definition: a flash flood is any significant flooding which occurs within a few hours of the rain event; it is flooding whose onset is sudden and rapid. Contrast that to general flooding which can take days or even weeks to develop.

In areas with relatively flat terrain (such as across metropolitan Chicago), flash flooding usually takes the form of ponding of water on low-lying streets and in low-lying areas.

Rule: Never drive into water covering a road or highway. Most flash-flood deaths are automobile related.

Hail

Although not a significant threat to human life (there have been only a few hail-caused deaths in the United States since 1916), hail is massively damaging. In recent years, the annual loss from hail has been $1,900,000,000 - nearly two billion dollars - just in this country. Hail damage to automobiles, roofs, windows and farm crops is staggering. Large hail is also a threat to small mammals and it kills many birds.

A thunderstorm is considered to be severe when it produces hail 3/4 inch in diameter or larger. When the National Weather Service issues a severe thunderstorm warning based on the hail criterion, that means the storm is expected to produce hail of at least 3/4 inch somewhere in the warned area.

Lightning

No other act of nature can match lightning for its swiftness, beauty, or frequency. Lightning strikes the earth 14,000,000 times per year in the United States. Nationally, lightning claims more lives than any other weather event. In addition, lightning and lightning-caused fires result in 3.3 billion dollars damage annually.

An enduring myth: "It was just heat lightning." It is widely but incorrectly believed that heat lightning is somehow different from ordinary lightning. It is not: heat lightning is always ordinary lightning, except that it is too far away for its thunder to be heard or for the thunderhead cloud which is producing it to be seen.

Thunder: Sound waves move through the atmosphere at a speed of 720 mph, or one mile in five seconds. To obtain the distance in miles of a lightning bolt, divide by 5 the number of seconds between flash and thunder.

Straight-line winds

The straight-line winds of thunderstorms (different from the rapidly shifting winds of tornadoes) are also known as gust-front winds, plow winds, and downbursts.

A thunderstorm is considered to be severe when it produces straight-line winds of 58 mph (50 knots) or higher. When the National Weather Service issues a severe thunderstorm warning based on the wind criterion, that means the storm is expected to produce straight-line winds of at least 58 mph somewhere in the warn area.

Because of frictional drag and evaporative cooling, the rain area of a thunderstorm is usually associated with descending air - a downdraft. As descending air nears the ground, it rushes out ahead of the storm, thus producing the characteristically gusty, cooling winds that we experience at the onset of a thunderstorm.

Tornadoes

No other atmospheric event can match the sheer energy and fury of tornadoes. With top speeds of about 320 mph, they produce nature's highest winds.

Tornadoes develop in the rising-air environment of thunderstorms, and within tornadoes themselves the air spirals upward in corkscrew fashion. The stronger a thunderstorm's updraft, the more likely it is the thunderstorm will spawn one or more tornadoes. Because it takes a powerful updraft to keep hailstones aloft long enough for them to grow to large size (golf-ball or larger), the occurrence of large hail is the single best indicator of a tornadic thunderstorm. Not all thunderstorms that bring large hail will necessarily produce a tornado, but most tornadoes occur in thunderstorms that also produce large (golf-ball or larger) hail.

Three-quarters of all the world's tornadoes occur right here in the United States, and in the United States most tornadoes occur in "tornado alley" - the area between the Rockies on the west and the Appalachians on the east. Although the number of tornadoes is probably not increasing, the number of tornadoes being reported is increasing because of higher population and better reporting procdures.

Rule: When a tornado approaches, always abandon a vehicle for more substantial shelter; even lying flat in a deep ditch is safer than attempting to survive a tornado in your car.

Fact: Half of all people caught in a tornado in their automobiles DIE.

When a tornado threatens, the safest place in a house is in the basement under the stairwell or under a strong bench, or on the ground floor in a small room (like bathroom) toward the interior of the structure.

A point to consider:

Oftentimes, the cause of disaster is one's unfamiliarity with a life-threatening situation. When severe weather strikes - perhaps suddenly and with little warning - it will be necessary to act quickly and to act correctly. Knowing what to do before severe weather strikes can mean the difference between life and death.