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There are several types of phenomena that can occur with severe thunderstorms. Learn about the different types, how they develop and the damage that they can cause.

Squall Line Thunderstorms

Everybody knows what kind of damage a tornado can do, but did you know that there are other phenomena caused by thunderstorms that can be just as hazardous? Downbursts, microbursts, derechos and haboobs are some of the dangerous phenomena that you’ll be learning about in this lesson.

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While these things can occur with any type of severe thunderstorm, they most commonly occur in squall line storms.Squall line thunderstorms are a large number of thunderstorms arranged in a line. They form in this line at the edge of a cold front or as far as 100 miles ahead of a cold front. If the air ahead of the cold front is unstable, humid and warm, squall line thunderstorms can develop.These conditions cause the air to rise and form clouds and precipitation, which leads to storm development. When the storms become fully developed, rain falls from the cloud, pulling air down with it. This is known as a downdraft, and it can be responsible for some very severe weather.

Squall lines usually have heavy precipitation, strong winds, small hail and lightning. They travel because they are pushed forward by winds in the upper atmosphere.Sometimes, these winds are not the same speed. If the winds driving the squall line are stronger along one portion of it, that part of the squall line will be pushed forward more quickly. This causes what’s known as a bow echo to develop. A bow echo is an image that appears on weather radar as an archer’s bow. It reflects the shape that the squall line has taken due to different upper-level wind speed.

When a squall line takes the shape of a bow echo, the storms can be quite severe. There are a number of damaging phenomena that can occur in these cases.

Downbursts

Sometimes, the downdraft from the storm is so strong that it creates a downburst. A downburst is when the downdraft from the storm spreads out horizontally in all directions when it reaches the ground. The wind it creates can be as strong as 150 miles per hour and can cause damage similar to what a tornado can do.Downbursts may be just wind with no rain and are called dry downbursts. Sometimes the wind is accompanied by rain, and those are called wet downbursts.

Microbursts

Downbursts are also different based on their size. Smaller downbursts that are less than 2.5 miles in extent are called microbursts.

Microbursts are small, strong downbursts that are particularly violent. The winds can reach 165 miles per hour in speed before reaching the surface and spreading out horizontally. Sometimes they only last a few seconds, but they can last for minutes at a time.

The winds they create are strong enough to knock over trees, and they are particularly dangerous in the vicinity of airports as airplanes land and takeoff. They’ve even led to fatal plane crashes in the past.

Derechos

Another phenomenon associated with squall lines is a derecho. Derechos are straight-line winds associated with a downburst. They blow out in front of the squall line and are the strongest winds created by the downburst. This happens because the movement of the storms is already in that direction.

Derechos can be as large as 200 miles wide and 1,000 miles long. They can last up to 12 hours or more and are associated with very strong straight-line winds.Derechos can knock over trees and power lines and cause rain and lightning to come from all directions. This can all happen very suddenly, so it is very dangerous to be caught outside in a derecho.

Haboobs

A type of storm associated with a derecho is called a haboob. Haboobs are dust storms or sand storms created by very strong winds.

They were first noticed and named as they occurred in the Sudan in Africa, but they can occur anywhere with a dry climate and loose sand or dirt.Some of the places they are common besides Africa are the Middle East and Australia. In North America, they occur in Arizona, New Mexico and Texas. They usually develop like a derecho.The strong winds formed by the downburst blow sand or dirt up from the ground high into the air creating a wall of dust that can be up to 60 miles wide. The winds may blow 20 to 60 miles per hour.

Often times, haboobs are different from typical storms in that no rain falls. This is because they occur over hot, dry areas and the rain simply evaporates before it reaches the ground. Haboobs are very dangerous and can cause eye and lung damage. If you are ever in a haboob, you should seek shelter immediately!

Lesson Summary

Squall line thunderstorms are a line of storms that develop at the edge of a cold front or up to 100 miles ahead of a cold front. They are often associated with severe weather phenomena like downbursts, microbursts, derechos and haboobs. The storms develop when the air is warm, humid and unstable.

The air rises to create a storm cloud and precipitation forms. When the precipitation falls to the ground, it pulls air down, creating what’s known as a downdraft. If the downdraft is strong enough, it can cause severe weather phenomena.

Squall lines travel because they are being pushed forward by winds in the upper atmosphere. If these winds are not the same speed along the squall line, part of it will be pushed forward more quickly. The result is a bow echo shape, which makes the squall line look like an archer’s bow on a radar.

A bow echo means that the storms may be quite severe with strong downdrafts, so damaging weather phenomena may occur.One of these is called a downburst. A downburst occurs when the downdraft from a storm spreads out horizontally in all directions when it reaches the ground.

The wind it creates can be very strong and can cause damage like a tornado.A downburst that is small in size is called a microburst. A microburst is just a very strong downburst. The wind created by it is so strong that it can knock over trees and is a danger to airplanes.The winds created by squall lines are strongest out in front of the squall line. These winds along an entire squall line can cause a derecho. Derechos are straight-line winds associated with the downburst.

Derechos can be very large and last for a long period of time. They can cause a lot of damage and be very dangerous.Another type of storm that is associated with a derecho is called a haboob.

Haboobs are dust or sand storms cause by straight-line winds. They occur in places that are very arid with loose dirt or sand, and they develop the same way a derecho does. The strong winds pick up the loose dirt and sand creating a wall of dust.

The blowing sediment can be very dangerous to people.

Learning Outcomes

After this lesson ends, students should be able to:

  • Identify different types of thunderstorms
  • Describe the differences in each storm type
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