Think about the worst thunderstorm you've ever witnessed. Having trouble remembering the specifics? That's because in the summertime, thunderstorms are a relatively mundane occurrence, even for those of us with a flair for exaggeration. There are some storms, though, that rise to a whole new level; a derecho is one of those storms. Derechos can sweep across entire states and leave behind more damage than a tornado, yet they're relatively unknown by anyone other than weather enthusiasts.
1. DERECHOS ARE MASSIVE.
A squall line or bow echo is a group of thunderstorms that are connected to one another and take on a bow-shaped look on weather radar. A squall line that forms under the right conditions can mature into a small but powerful storm system in its own right, developing tiny high- and low-pressure centers that can help drive the thunderstorms for hundreds of miles.
A derecho is a long-lived squall line that produces a swath of wind damage more than 200 miles long. People who go through derechos sometimes compare them to hurricanes because of the strength of the winds and the amount of damage they leave behind. The term derecho, pronounced “duh-RAY-cho,” comes from the Spanish word for “straight,” a reference to the winds that make these storms so powerful.
All derechos are squall lines, but not all squall lines turn into derechos. The term derecho only applies when large amounts of land area see damage from a storm. But regular squall lines are nothing to sneeze at: A system that only affects three counties can be just as strong as one that affects three states.
2. DERECHOS GET THEIR BITE FROM COLD AIR.
What distinguishes a squall line from a normal thunderstorm is how the updraft and downdraft develop. The thunderstorms in a squall line form along the leading edge of the pool of cold air dragged to the ground by the downdraft. This dome-shaped cold pool tilts the updraft of warm, unstable air, preventing the updraft from suffocating on cool air. Air inside the cold pool beneath the storms can begin to circulate due to upper-level winds and friction with the ground, creating a dangerous feature called a rear-inflow jet. This jet of strong winds forms a few thousand feet above the ground and races toward the front of the thunderstorms, where it's shoved into the ground at the leading edge of the squall line, creating a derecho's signature winds.
3. STRAIGHT-LINE WINDS MAKE THEM DANGEROUS.
Tornadoes get all the attention during storm season, but the truth is that straight-line winds can cause just as much damage as a tornado, but over a much wider area than a tornado could ever cover. Just as the name suggests, straight-line winds hit the ground and move in the same direction, sometimes blowing for more than five minutes. The sudden impact and long duration of these winds can put serious stress on trees, buildings, and anything not tied down to the ground.
4. A DERECHO CAN CAUSE AS MUCH DAMAGE AS A TORNADO.
The aftermath of many derechos looks like what you'd see after a hurricane makes landfall. Some people even swear that they were hit by a tornado, not believing that a severe thunderstorm could make that big of a mess. The straight-line winds in a derecho can climb as high as 100 mph—strong enough to rip the roofs off of homes and completely destroy structures like barns and silos. Even a derecho that just barely qualifies as one can leave behind thousands of downed trees, long-lasting power outages, and annoying and costly cosmetic damage to homes and businesses.
5. SOME ARE MORE SERIOUS THAN OTHERS.
There are two different types of derechos. The most dangerous type is a progressive derecho. This is the kind of storm you see in the summer that speeds across entire states and leaves tornado-like damage in its wake. A serial derecho forms along a cold front. What a serial derecho lacks in focused damage it can make up for in the sheer amount of land exposed to severe wind gusts. It's possible for the length of a serial derecho to stretch from the Great Lakes to the Gulf of Mexico as it moves from west to east. Serial derechos are most common during the fall and winter months.
6. HEAT WAVES BREED DERECHOS.
A relentless summertime heat wave can trigger multiple derechos in one week if conditions are just right. A high-pressure system that causes a brutal heat wave during the summer is sometimes called a “ring of fire” by meteorologists. The northern edge of one of these hot high-pressure systems can serve as the point for a progressive derecho to form. Once the derecho has developed, it races east or southeast along the outer edge of the high-pressure system until it dissipates or reaches the Atlantic Ocean.
7. THE CLOUDS CAN LOOK TERRIFYING.
The ominous clouds ahead of a derecho can tell you pretty quickly that this is no ordinary storm headed your way. Derechos are home to the most incredible shelf clouds nature can produce. A shelf cloud is a thick cloud that juts down from the sky, like a shelf hanging beneath the bottom of the storm. These clouds form when warm air rises up and condenses over the top edge of the cold pool that drives these storms. Shelf clouds are quite common, but the vivid shelf clouds in derechos are a spectacular, albeit scary, sight to see.
8. THE WIND COMES ON SUDDENLY.
One of the reasons derechos (or any intense squall line, really) can wreak such havoc is because they come on suddenly. There usually isn't much of a buildup to the strongest winds before they hit. Conditions can go from calm to chaos in a matter of seconds. The abruptness with which the winds can hit can even snap off the tops of trees as if they were chopped down by hand.
9. THERE ARE A FEW DERECHOS EVERY YEAR.
Derechos are most common in the central United States, but they can form just about anywhere around the world that experiences severe thunderstorms. Most derechos go unnoticed by those not directly affected by the storm, but some can have such a large impact on populated areas that they make national news.
One such storm was the derecho that formed on June 29, 2012. The storms started in Indiana, grew into a monstrous squall line in Ohio, and blew across the Appalachian Mountains virtually unimpeded before continuing on to the Atlantic Ocean. The Storm Prediction Center received hundreds of reports of wind damage after the storm. Washington D.C. and its suburbs were particularly hard-hit by the storm's winds. The derecho left millions without power, some for weeks after the storm, and multiple people lost their lives as a result of falling trees.