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Thunderstorms


by Kevin Lin

Thunderstorms:


Thunderstorms are very dangerous, because they produce lightning, and can lead to flash flooding. Lightning kills more people each year than tornadoes. They also start many fires around the world each year. Flash floods can change small creeks into raging torrents in a matter of minutes, washing away large boulders and most man made structures. They are nearly 140 fatalities due to flash flooding. Strong winds, hail, and tornadoes are also dangers associated with thunderstorms. Thunderstorms, though, are not always severe. Only 10 percent of the estimated 100,000 thunderstorms occur each year in the United States. The National Weather Service considers a thunderstorm severe if it produces hail at least ¾-inch in diameter, wind 58 miles per hour of higher, or tornadoes. The largest, strongest, and longest lasting thunderstorms are called supercells. They can produce all of the things mentioned above, and spawn most of the USA's killer tornadoes. Mesocyclones help give supercells their destructive power, and also help the storms hold together for hoursas they create a path of destruction along the ground.

What Every Thunderstorm Needs:

Every thunderstorm needs certain things to form. They are: moisture, warm air, and something that can lift the warm air up. The moisture is needed to form the clouds and rain. The warm air is needed so it can rise rapidly, and fronts, sea breezes, and mountains are needed to lift the warm air high enough to form thunderstorms. Also, the air aloft has to be cool enough so that even as the rising air cools, it stays warmer and therefore lighter than the surrounding air. In such cases, the air is called "unstable." The humidity in the rising air condenses to form clouds, ice crystals and rain and a thunderstorm is born. If an upper air disturbance is around, it gives the storm extra energy. Such disturbances are pools of cold air aloft that move along with the upper atmosphere's winds. When a disturbance moves into an area favorable for thunderstorms, they add lifting motion. This extra list can what otherwise would have been a mild thunderstorm into a deadly tiger.

Life Cycle of A Thunderstorm

Developing Stage Mature Stage Dissipating Stage
Towering cumulus clouds indicate rising air. Most likely time for hail, heavy rain, frequent lightning, strong winds, and tornadoes Rainfall decreases in intensity
Lasts about 10 minutes. Lasts an average of 10 to 20 minutes but may last much longer in some storms. Lightning remains a danger during this stage.
Occasional lightning during this stage.

Downdrafts

During spring, many thunderstorms erupt due to the clash of warm and cold air. Often, a belt of strong winds blows at about 1000-2000 feet above the surface. As you can see in the graphic below, updrafts and downdrafts form as thunderstorms develop. Downdrafts are enhanced by rain-cooled air and if they become strong enough, downdrafts can drag the belt of strong winds above the surface to the surface. This results in bursts of strong, gusty winds which can do major damage. In some cases, intense updrafts prevent the rain-cooled air from forming the downdraft. A large pool of heavy, cooled air forms until it is heavy enough to overcome the intense updraft. Once the pocket of cooled air overcomes the updraft, it races to the surface and slams into the ground, spreading out in all directions. This is known as a downburst. Downbursts can completely destroy buildings with bursts of wind that well exceed hurricane force. Small downbursts are often mistaken for tornadoes because of the severity of the damage.

Smaller Downbursts

During thunderstorms, winds can reach more than 150 mph. Because these winds cover different areas, there are different names for them. One thing is for sure though. They cause extensive damage. They damage buildings, blow down trees, as well as causing several airplane crashes. For the winds that are concentrated in an area extending 2.5 miles or less, the winds are called microbursts. If the winds cover a larger area, they are known as macrobursts. Microbursts are most common from the Rockies eastward in the United States because showers and thunderstorms are more common there. In the humid East, "wet" microbursts are more likely, and 'dry" microbursts are likely to occur more often in the West.

Heatbursts

After sunset, warm, moist air feeding a thunderstorm shuts off and the storm collapses. All the rain in the storm's top falls into cool, dry air charging into the storm from the back side. As the rain falls, it evaporates in the dry air and cools further making it very heavy. This cool, heavy air plunges toward the ground in a giant, downward-moving burst. But unlike a true downburst that typically falls from a much lower altitude and consists of cool air until it hits the ground, a heat burst warms drastically as the high altitude air gets compressed by higher air pressure closer to the earth's surface. As the air warms, it becomes lighter and more buoyant which tends to slow its downward motion. But, it can't stop in time and the rushing air slams into the ground and spreads out as a hot, dry burst of wind. A heat burst's intensity depends on the size of the collapsing storm and how much the air warms before hitting the ground.

"Dry" Thunderstorms

In the dry west, the humidity is so low that rain falling from thunderstorms often evaporate before reaching the ground. The evaporating rain is known as the virga. Even though the rain doesn't make it to the ground, flashes of lightning streak from the clouds to the ground. This lightning can start fires in dry woods with no rain to extinguish or slow the blazes. Lightning accounts for a large number of the forest fires in the West.

Examples of Deadly Thunderstorms

Thunderstorm terms:

Safety Precautions:

Before

During

After