Storm Watcher (21 page)

Luke's

Weather

Notebook

(
Important facts about all kinds of weather)

F
i
v
e Horsemen

of Thunderstorms

The most dang
erous clouds are cumulonimbus clouds. These thunderstorm clouds are called “weather factories.” They cause wind, hail, lightning, tornadoes, and even flooding. These five weather events are called the five horsemen of thunderstorms.

Below is a look at how each of these horsemen ride across the sky to create storms.

Wind

A thunderstorm is like a giant vacuum cleaner. As the storm grows, it sucks up the warm moist air near the ground. The upward-moving air is called an updraft. Once this column of air reaches 20,000 to 40,000 feet in the sky, it cools and descends. Then it’s called a downdraft.

The leading edge, or front, of the downdraft is called the forward-flank downdraft. This is what shakes the leaves on the trees as a thunderstorm approaches. The rear-flank downdraft is the back edge of the storm.

Downdrafts can be intense. They may slam to the ground with gusts that are stronger than hurricanes (75+ miles per hour). These are called downbursts. Tiny ones are called microbursts. Some microburst winds have been clocked above 120 miles per hour (mph). The highest microburst ever recorded was measured at 149.5 mph at Andrews Air Force base in Washington, DC, in 1983.
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Hail
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Hail is a ball of ice that forms inside a thunderstorm. As the warm moist air rises, the air cools and condenses. This creates droplets of water that freeze. These frozen raindrops are called graupel. They get caught in the updrafts and downdrafts. Each time they ride an updraft, they gain another layer of ice. Over time, the hail gets too heavy to stay in the air. So it falls to the ground.

Powerful thunderstorms with strong updrafts can “hold” hail up longer than weaker storms. Strong storms also produce large hailstones. To make a 1” hailstone, the updraft must be at least 50 mph. A 2” hailstone needs close to 70 mph updrafts.

Hailstone sizes in Inches
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0.25 pea
0.50 marble
0.75 penny/large marble/dime
0.88 mothball/nickel
1.00 quarter
1.25 half-dollar
1.50 walnut
1.75 golf ball
2.00 hen egg
2.50 tennis ball
2.75 baseball
3.00 tea cup
4.00 grapefruit
4.50 softball

Lightning (and Thunder)

The largest hailstone recorded landed in Vivian, South Dakota, on July 23, 2010.This hailstone was about the size of a volleyball. It weighed 1.94 pounds and was 18.6” around.
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Strange things have been found inside hail. Twigs, leaves, pebbles, nuts, and even insects have been trapped inside.

Lightning is called
thunderstorm electrification
in weather language.

Lightning lives in the center of a thunderstorm where the air moves fast. Frozen raindrops ride the air currents like a roller coaster. Some of the drops are going up, rising on updrafts. Others are falling, riding downdrafts. With so much movement, the drops sometimes collide. When they do, the raindrop that was going up becomes positively charged. The one going down takes on a negative charge.

A thunderstorm is like a battery: The top end is positive, and the bottom end is negative. 

When a thunderstorm is directly overhead, the large negative charge at the base of the storm repels the negative charge on the ground. That causes the ground and any people (or objects) on the ground to become positively charged. These strong opposite charges close together create the electrical discharge (spark) that is lightning. It’s like what happens when you shuffle across a rug in your socks. If you then touch a pet or a friend or a metal doorknob, you may get “shocked” by a spark. That spark and shock are tiny compared to the ones lightning produces.
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The most common forms of lightning are:

• Cloud-to-cloud:
  the strike occurs between clouds
  
• Cloud-to-ground:
  lightning hits the ground
  
• In-cloud:
  lightning happens inside the cloud; it often looks like a sheet
  
• Cloud-to-air:
  a positively charged cloud hits negatively charged air
  

The brightness of lightning is equal to about 100 million light bulbs going on and off. The temperature of lightning is about 50,000° F. That is around five times hotter than the surface of the sun.
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When lightning flickers, it’s not just one bolt hitting the ground. It’s a series of multiple strikes. The bolts all hit the same spot. Then like a yo-yo, they return to the cloud. Each stroke, or flash, moves at a speed of about 200,000 mph. That’s the fastest yo-yo ever. And although it may look thick, a bolt of lightning is only about as wide as a pencil.
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Thunder comes from lightning cutting through the air. The bolt is like a yo-yo. When it travels down, it heats up and expands the air. Then it zings right back to the cloud. It cools and contracts the air as it goes. So thunder comes from air coming and going at terrific speed. Hot and cold, up and down. Everything collides and BOOM! The sky sounds like bowling balls smacking into pins, or like a growling animal.

Tornadoes

Light moves faster than sound, so the flash happens right away. But thunder takes five seconds to travel one mile. Counting to five lets you know how far away the storm is. If you count to five three times, the lightning is three miles away.

Tornadoes form only in severe (supercell) thunder-storms. The National Weather Services says a severe thunderstorm has hail at least 1” in diameter, and/or wind gusts of 58 mph, and/or a tornado.
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Not all supercells cause tornadoes. The storm must have strong winds in order to produce a tornado. These winds need to blow at different speeds and in different directions at various heights from the ground. This may happen when warm, moist air meets cool, dry air. If the rotating winds are caught in an updraft, they spin faster and tighter. This cone of air becomes a funnel cloud. Once it hits the ground, it becomes a tornado.