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Article: What Drives a Derecho?

What Drives a Derecho?

A derecho is a damaging, straight-line windstorm that arises from a line of thunderstorms. Over the years such events have been called "wind rushes," "blowdowns" (when they strike forests), and "plough winds" by the people who have experienced them. These monikers are apt: the derecho causes considerable destruction along a long path (400 kilometers long or more, by definition), and it races forward with train-like speed and force. The straight-line windsthe "derecho" part of the stormappear at the front of the system, plowing everything aside like a cowcatcher on an old-fashioned locomotive. Once a derecho gets moving, it can keep moving--sometimes for as long as 24 hours--before it eventually runs out of steam.

 Thunderstorms thrive on warm, moist air. As a result, many derechoes occur during the hot, humid days of summer. "They seem to be most common towards the last of June and the beginning of July," says Robert Johns, a former meteorologist with the National Weather Service's Severe Storm Prediction Center, and a leading expert on derechoes. "That's when moisture levels peak in the lower atmosphere. In fact, many derechoes, including the one in 1999, occurred on July Fourth." On the day of the Boundary Waters storm, Johns recalls, temperatures had reached into the 90s, and the dewpoint temperature, which is a more sophisticated measure of humidity, was near 80°F"which is very high. It was a nasty, sticky day. And this was true all the way across U.S. to New York City, where it got up to 96 degrees."

A derecho grows in roughly four stages, Johns explains. First comes the rainstorm. On the hot, sticky day in question, warm air near the ground begins to rise, carrying moisture with it. As it rises, the warm air cools and the moisture within starts to condense into clouds. The process of condensation releases heat, which warms the surrounding air and causes the air to rise even farther and faster. Eventually the rising moisture condenses into droplets too heavy for the cloud to carry, and rain begins to fall. 

(In general, given two equally sized packets of air, one moist and one dry, the moist air will rise farther than the dry air. Even though the two air packets start out at the same temperature, the moist air gains additional heat from its condensing moisture, so it contains more inherent energy than the dry air. Scientists sometimes describe moist air as being more "buoyant" than dry air, since it has a tendency to rise higher. The buoyancy of moist air plays an important role in the late stages of a derecho's formation.)

 The second critical phase is the formation of a "downburst." As rain falls from the storm, it pulls some upper-level air with it toward the ground, creating a downward movement of wind known as a downdraft. If conditions are rightif enough dry air is available higher up, between about 3 to 6 kilometers above groundthe downdraft can become a violent downward burst, or downburst, of wind. As the downdraft falls, it draws dry air behind it into the storm. The dry air evaporates some of the raindrops in the downdraft, which cools the downdraft. This causes the downdraft to sink quickly, transforming it into an intense downburst of wind that hits the ground and spreads out like fast-moving pancake batter. Though it lasts only a few minutes, a downburst can be 50 kilometers wide and produce winds of up to 200 kilometers per hour. A small-scale downburst is called a "microburst." These are isolated events, and are typically no more than a kilometer wide. Especially around airports, however, they pose a severe hazard: the winds from a microburst are capable of pushing an airplane from the sky during takeoff or landing. Microbursts caused several commercial airline crashes in the 1970s and early 1980s. The strongest microburst ever recorded, with winds approaching 200 kilometers per hour, occurred at Andrews Air Force Base on August 1, 1983, just six minutes after President Reagan had landed there safely. That close call was instrumental in the federal government's decision to install advanced Doppler radar equipment at major airports around the country.

Under the right conditions, solitary downbursts can combine to form a large, fast-moving "macroburst." This is the third critical phase in the formation of a derecho. Two ingredients are necessary: a moderate to strong wind pushing the storm system from behind, and a rich supply of moist air in front of the storm. If the high-elevation winds are sufficiently strong, they provide the storm with a running jump-start. Meanwhile, the winds from the downburst hit the ground and begin to spread out, most strongly in the forward direction (because the storm is being pushed from behind). If there is moist air ahead of the storm, it is pushed aloft by the winds at the storm's front edge. The moisture condenses and new rain-clouds form. This is now the front edge of the storm: the clouds produce new downbursts, which scoop yet more moisture into the storm, and the cycle continues. The downbursts have evolved into a macroburst, an atmospheric locomotive powered by what scientists call a "convective engine." Gulping down moist air, the convective storm system advances with gathering speed and force.

At a certain point, the storm systemwhich by now has made itself apparent to forecasters by the bow shape of its radar echohas moved far enough and reached sufficient strength for meteorologists to call it a derecho. "To be classified as a derecho, a bow-echo must produce wind damage for at least four hundred kilometers," Johns says. The number is somewhat arbitrary, he adds, "but this qualifies it as being a very large-scale wind event." The winds in a derecho can reach 200 kilometers per hour; the storm itself can advance as quickly as 150 kilometers per hour, many times the typical speed of an advancing tornado.

Johns has found that it takes at least three hours for a large-scale "convective system" to evolve to the point where it produces a full-blown derecho. The derecho that hit the Boundary Waters Canoe Area Wilderness at midday on July 4, 1999, actually formed at about six o'clock that morning as a group of severe thunderstorms near Fargo, North Dakota. "As they were crossing Minnesota, they became a series of bow-echoes that accelerated up to eighty or a hundred kilometers per hour. By the time they reached the Boundary Waters region they had become a full-fledged bow-echo complex and were producing a lot of devastation. It continued moving very rapidly across Ontario and Quebec during the rest of the afternoon and night-time hours, reaching Maine by the early morning hours of the next day, creating devastation along the whole route."

Once it gets moving, a derecho follows a path dictated in part by the moisture content of the air ahead of it. Moist air is more buoyant than dry air, so it is more easily pushed aloft by the advancing storm and fed into the convective engine. A derecho may travel for hours, sometimes as long as a day, providing there is sufficient moist air to fuel it. When the supply of moist air runs out, or the pushing winds weaken, the derecho dies out. The July 4, 1999 derecho was particularly long-lived, lasting for about 24 hours before it moved out to the Atlantic.