The Weather Guys

A monsoon is a seasonal reversal in wind patterns over a region. The word “monsoon” comes from the Arabic word mausim, meaning “season.” The seasonal wind shift is usually accompanied by a dramatic change in precipitation.

The best known example of a monsoon occurs over the Indian subcontinent. In summer, the sun warms the land and the air above it. With cooler air lying over the oceans that surround the subcontinent, a horizontal pressure gradient is established that generates winds directed from the ocean to land.

The air flowing over the water remains over the ocean a long time, which causes it to gain moisture. This wind brings humid air inland. The solar heating over land triggers convection that produces rain. Orographic lifting of air by the Himalaya Mountains generates more rain.

The summer monsoon is a wet season over land. These rains can cause major flooding, but they are vital to agriculture and the economy.

During the winter season, the air above the land cools faster than the air over the water, establishing a pressure gradient force from land to sea. The winds are therefore reversed from the summer monsoon flow — from land to sea instead of from sea to land. Sinking air above the land suppresses cloud development and precipitation. The winter monsoon is a dry season.

Other parts of the world also have monsoons. The North American monsoon, also called the Arizona monsoon, occurs over the southwestern United States and northwestern Mexico. It results in a rainy season that begins in July and typically lasts until mid-September, and then the drier weather regime gets re-established.

Recent episodes of severe weather both here in southern Wisconsin and around the country have included reference to “derechos.” Forecasters worried a storm that passed through Wisconsin on June 11-12 could become a derecho, although it never became that severe.

This term, which is a Spanish word for straight, refers to a widespread, long-lived wind storm that is associated with a rapidly moving band of thunderstorms. Though a derecho can produce destruction similar to what is associated with tornadoes, the damage is produced by straight-line winds, not rotary winds.

Despite the fact that “derecho” seems to be a new word, it was introduced by Gustavus Hinrichs in 1888 in his description of a storm that crossed Iowa on July 31, 1877. Hinrichs was convinced not all damaging convective storms in the Great Plains were tornadoes, and he set out to prove it. Lt. John Finley, of the Army Signal Corps (which later became the National Weather Service), was a leading tornado expert at the time and was not convinced of Hinrichs’ theory.

Hinrichs proved to be right: These straight-line wind storms were early summer phenomena (late May, June and July), and could affect huge areas in a relatively short time.

He chose a Spanish word to name them, hoping this choice would provide a complementary term to “tornado.”

Around 1890, the U.S. Weather Bureau instituted a ban on use of the word “tornado” on the grounds that it incited panic within the populace. This ban was not removed until the early 1940s when “tornado” reappeared in Weather Bureau warnings and forecasts.

“Derecho” seems to have disappeared during that period and was resurrected in 1987.

Last year, on June 29, a major derecho event roared east from Chicago to Washington, D.C., leaving 5 million people without power and substantially raising the profile of derechos in the United States.