Thunderstorms and Tornadoes

-Formation of Thunderstorms
-Squall line
-Formation of Tornadoes
-Thunderstorms Making Hail

Thunderstorms are convective storms. They need unstable air, a temperature profile with warm air near the ground and cold air aloft. Thunderstorms are more likely in the spring and summer than in the fall and winter. In spring and summer the sun warms the ground, which warms the air near the ground. Air near the ground is also warmed in the fall, but there is an important difference. In the spring the air aloft retains its winter cold; the air will be more unstable than in the fall when the air aloft retains its summer warmth.

The life cycle of the thunderstorm is as follows:

1. Warm, humid air-called updrafts-rises from the ground.

2. As the air cools to its dew point, condensation begins forming a cloud.

3. When ice crystals or water drops grow big enough to overcome the updraft, they begin falling, dragging down the air.

4. In summer, ice melts on the way down.

5. The falling precipitation and air being dragged down from downdrafts.

6. Updrafts continue feeding warm, humid air into the storm. Existence of both updrafts and downdrafts make this the storm's most violent stage.

7. Downdrafts grow, choking off updrafts.

8. With its supply of humid air cut, the storm begins dying and rain tapers off.

Squall line

A squall line is made of several thunderstorms. It can be more than 100 miles long. Squall lines are often 50 to 150 miles ahead of an advancing cold front.

First, water vapour in the air forms precipitation. The precipitation then evaporates into dry air, cooling the air more. After that, cooled, heavier air plunges downward, blasting the ground with high winds. Thus a squall line is formed.

Squall lines can form along cold fronts, but the strongest and most common lines form as much as 100 or more miles ahead of an advancing cold front in the warm sector of an extratropical cyclone. Such lines are known as 'prefrontal squall lines'.

In addition to the thunderstorms, squall lines often trail a large are of flat, stratus clouds that produce steady rain for hours after the thunderstorms and their heavier rain have moved on.

Formation of Tornadoes

While tornadoes come in several varieties, we will classify them into two general kinds:

The first kind of tornado forms in the outflow of air from a thunderstorm. These storms are usually weak. They have a shallow, localized vortex, usually last less than five minutes and their winds rarely exceed 150 miles an hour, F-2 on the Fujita Scale. You might think of them like the swirls of water you sometimes see in a fast-moving stream as it flows past obstacles.

The second kind of tornado is the strongest. These storms are found where air is flowing into a thunderstorm, in the updraft are. Doppler radar measurements show that such tornadoes begin at the storm's middle levels and then grow both up into the storm and down toward the ground.

To the question of how tornadoes get their twist, scientists are still working hard to find the answer. They have rejected old ideas that it is created by the Coriolis Effect acting on inflowing winds. In theory, a tornado could get spin in this way, but it would take more time than some tornadoes seem to have. Also, if the Coriolis Effect were the only source of tornado rotation, all Northern Hemisphere tornadoes would spin counterclockwise. While most Northern Hemisphere tornadoes spin in that direction, clockwise twisters have been photographed.

Thunderstorms Making Hail

Hailstones are balls of ice that grow as they are held up by thunderstorm updrafts while supercooled water drops hit and freeze onto them. the faster the updraft, the bigger the stones can grow.

The speeds of a thunderstorm's updrafts are among the factors that determine hailstone size.

Here are approximate upward wind speeds needed to make various size stones:

- 1/2"-22mph

- 3/4"-37mph

-1 3/4"-56mph

- 3" -100mph

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