What is a Derecho?

Damage to a tree in Maryland following a derecho storm.
Damage to a tree in Maryland following a derecho storm.

A derecho is an extensive mass of winds associated with fast-moving thunderstorms or showers. The term "derecho" originates from a Spanish word meaning “direct” or “straight.” Derecho has been used to refer to the strong winds that move in a straight line as opposed to tornadoes which are composed of “twisted” winds. Derechos mostly occur in the summer, especially in June and July, both in the day or night.

Formation of Derecho Storms

Derechos start with thunderstorms whereby frontal zones outrun cold fronts. As a result, the winds form mesoscale convective system (MCS) in the upper divergence level of the pattern of the wind. The winds head eastwards towards the equator where there is a warm sector. They move in a parallel manner with the low-level thickness lines. Squall lines are formed as a result of strong convections leading to the rise in pressure. Derechos are squall lines which may be either spearhead-shaped or bow-shaped thus they are also known as spearhead radar echoes or bow echoes.

Derechos vary during the cold and warm seasons. Cold seasons are characterized by derechos which are formed in the form of the south westerly winds in the Northern Hemisphere. They form under low or moderate atmospheric instability conditions in the middle levels of the troposphere. On the other hand, derechos formed during the warm season result from the north westerly winds in the intermediate levels of the troposphere. Derecho storms range from low to moderate instability and form with small amounts of moisture.

Types of Derechos

Derechos are classified into four categories, namely: serial derecho, progressive derecho, hybrid derecho, and low dewpoint derecho. The serial derecho is sub-divided into single-bow and multi-bow derechos. Derechos are sorted by the National Weather Service as winds ranging with a velocity of 90km/h and lasting for approximately six hours. The time between two successive winds ranges from two to three hours with their forward speeds increasing rapidly.

Areas Most Affected by Derechos

Some of the regions which experience derechos during the summer are the Midwestern US in the Ohio Valley and Oklahoma. In the mid-summer, derechos occur further north in north western Ontario as well as Manitoba. Michigan, Minnesota, and North Dakota also experience derecho storms during this period. The other parts of the world which experience derecho winds include Bangladesh and Brazil, especially the Amazon Basin among other places.

What Are the Effects of Derechos?

Derecho storms, unlike tornadoes, occur very fast as they cannot be heard approaching. As a result, they can produce adverse effects. Strong derecho storms may cut electric power lines. The derecho storm which hit Chicago in 2011 left thousands of people without electrical power. When derecho winds start, they are so strong that transport and communication are affected. It is possible for a derecho to cause human fatalities.


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