Most of us had never heard the term “derecho” until Friday, when we learned that’s what meteorologists call the kind of massive storm that swept through the Midwest and blitzed the Eastern seaboard, killing at least 20 people and leaving a 700-mile swath of destruction and downed power lines in its wake.
That got us thinking about other unusual or poorly understood weather phenomena that are lurking out there to catch us by surprise. So we put together a quick primer (and the National Oceanic and Atmospheric Administration has a few other examples here).
Derecho: We’ll start with the weather word of the moment. Pronounced deh-RAY-cho, it’s a large-scale, fast-moving and long-lived storm system that produces powerful “straight-line” winds that can cause considerable damage. The popular nickname is “land hurricane,” because winds can be as strong as a Category 1 hurricane — 74 miles an hour or higher. Derechos typically occur in the summer months and are spawned by high temperatures, such as the prolonged heat wave that has blanketed much of the United States in recent days.
Here’s how Jeff Masters, director of meteorology for the weather website Wunderground.com, described Friday’s derecho: “We had over 30 thunderstorms with wind gusts of 80 miles per hour. … It’s kind of the equivalent to having 30-plus weak tornadoes on the ground.”
Derecho, by the way, is the Spanish word for “straight.”
Microbursts (a.k.a. downbursts): These are sudden, localized columns of air that “fall to the ground almost like a physical object,” says Greg Carbin, a meteorologist at the NOAA’s Storm Prediction Center in Norman, Okla.
There are two main types of microbursts – wet and dry. As the names imply, wet microbursts are accompanied by moderate to heavy rainfall. They tend to occur in the Southeast. Dry microbursts are found primarily in the West and Midwest, and produce little or no rain.
A dramatic example of what can happen when a microburst hits can be seen in this video of a stage collapse at the Indiana State Fair in Indianapolis last year. Seven people died and dozens were injured.
Another microburst caused the August 1985 crash of Delta Flight 191 at Dallas-Fort Worth International Airport. The plane went down suddenly on its approach, killing 137 people.
Microbursts “have been particularly dangerous for aviation,” Carbin says, “and there have been numerous incidents and crashes due to them.”
Haboob: A downburst in a desert region that picks up loose sand, driving it forward. A 100-mile-wide haboob swept through Phoenix a year ago, knocking out power to at least 10,000 people and generally creating a huge mess.
“The dust gets into everything,” says Fred Gadomski, a meteorologist at Penn State University. “It gets into electronics; it gets into every nook and cranny.”
There were even concerns after the Arizona haboob last year that a fungus contained in dirt that causes “Valley Fever” would be spread by the dust storm.
Heat Burst: “Suppose you’ve had a warm summer day,” Gadomski says, “and then in the middle of the night, the temperature rises dramatically, say from 80 to 100 in a matter of minutes.”
That’s called a heat burst. The quick rise in temperature is accompanied by a similarly sudden decrease in moisture, as well as short-lived, high velocity winds.
Wichita, Kan., experienced an overnight heat burst in June 2011 that caused a 17 degree spike in temperatures, which hit 102 degrees — at just after midnight. Meteorologists said it was caused by collapsing thunderstorms that spawned wind gusts of 50 to 60 mph.
“Most summers, somewhere in the middle of the United States, some sort of heat burst will occur,” Gadomski says.
Chinook Winds: These strong, heated “snow-eating” winds occur exclusively in the North American Rocky Mountains and — like heat bursts — can produce dramatic rises in temperature. Chinook winds can quickly melt snow on mountain slopes.
Chinooks occur when an air mass descends a mountain face, causing the air to be compressed, and that heats it up. NOAA’s Carbin explains that the heating because of compression is the same thing that happens when you fill up a car tire.
“The valve gets warm because the air is being compressed,” he says.
One of the most dramatic temperature jumps ever was recorded at Spearfish, S.D., in January 1943. Thanks to a Chinook, the town went from -4 degrees to 45 degrees in just two minutes.