Derecho
Picture to yourself black clouds racing across the sky, only a little above the treetops, churning and boiling as if they are heated from within by some hidden furnace. Torrential rain gushes down, silver and blue cracks of lightning split the space between cloud and earth. Thunder crashes deafeningly, and the morning light is nearly blotted out by a hideous greenish-black gloom.
Amid this hellish scene, the wind rises more and more, first shrieking, then roaring and bellowing over the landscape with incredible force. Huge clumps of leaves are torn from the summer trees to go spinning through the air, soon to be followed by branches, boards, roof tiles, and anything else the wind can pick up. Roofs slide to the ground in ruin, a building under construction partially collapses, and trees are ripped up by their roots. After the storm has passed through Memphis, Tennessee, seven people are dead, 300,000 are without power for weeks, and wreckage is strewn everywhere.
Reading this account, you would probably think that it is a description of one of America's more destructive tornadoes, but it is not. There was no rotating funnel, no 'twister,' no tornadic rotation on radar allowing the National Weather Service to issue precisely-pinpointed warnings and tracking information.
Instead, this storm of July 22nd, 2003 -- nicknamed "Hurricane Elvis" by the inhabitants of Memphis -- was a derecho, a violent windstorm that can contain straight-line winds averaging around 100 miles per hour and sometimes perhaps reaching speeds of 130 miles per hour. These winds are as violent as those found in a tornado or a hurricane, but they do not rotate, and they occur across a broad front, possibly causing even more damage than a weak tornado would. These menacing weather phenomena occur in the summer with specific types of thunderstorms, and can happen in areas of the world that normally aren't subject to tornadoes, such as continental Europe or India.
The characteristics of a derecho
A derecho is a wide, powerful windstorm that is caused by a potent line of thunderstorms. There are several different kinds of derechos, but to qualify as a derecho, the storm front must be at least 240 miles wide and the winds must be a minimum of 57 miles per hour along most of the storm's length. The storm lasts for a minimum of at least 6 hours. These figures give some idea of the power and scale of these weather phenomena, which are much less well known than tornadoes but can be just as dangerous and destructive to those in their path.
Derechos are mostly produced by "bow echo" storms, which are storms that have a bowed or curved shape when viewed on Doppler radar. The bow echo storm results from the powerful winds reaching the ground and the pressure on the Earth's surface 'feeding back' and distorting the entire storm. Nice first aid kits online for Canada. The storm is generally rapidly moving as well, another precondition for creating the massive straight line winds that make up a derecho.
Derechos sometimes include tornadoes or form near them, but the presence of a tornado is often hard to verify because the derecho itself is so destructive. Damage from the derecho tends to obscure tornado tracks and the Doppler effects of the derecho and the bow echo may conceal the tornadic activity from weather observers.
A derecho is an impressive sight for those who have the opportunity -- and perhaps the misfortune -- to witness one. Beneath a roof of dark stormclouds, a huge, jutting "shelf cloud" projects close to the ground, with darkness underneath it. This huge mass of clouds tumbles and seethes across the sky at incredible speed, sometimes advancing as fast as 70 miles per hour, accompanied by an intense wall of wind. The approach of a derecho resembles nothing so much as the end of the world -- its speed, darkness, hugeness, and violence is a memorable event for anyone who has lived through one of these rare but spectacular storms.
When and where derechos occur
Derechos, like many of the most violent weather patterns, are warm weather occurrences. Like tornadoes, they can actually happen at any time of year, but are extremely rare outside the late spring, early summer, and mid summer seasons. The conditions that form them -- the collision of vigorously unstable, moist air masses from the tropics with dry, cold air masses from the arctic -- are most common in these months. Unlike tornadoes, which primarily form in isolated mesocyclones, derechos are usually the product of squall lines and other cold front storm lines, which explains their physical length and accounts for their straight-line winds as well.
In the United States, derecho territory encompasses the Mississippi Valley and the southern Plains states. These are the areas where bow echo storms are most likely to occur, both because of the air flow patterns and because of the low-lying or flat topography of much of this region. There are no obstacles to break up a row of thunderstorms as they rush across the landscape, and their winds are likewise unhindered by tall hills or mountains. These winds can therefore spread freely and potentially become a derecho.
Derechos occur in other parts of the world, too. Violent derechos have sprung up in Germany, and some vernal storms in India and Bangladesh seem to show many of the features of derechos as well, although this has not been reliably confirmed. A derecho can probably be born anywhere in the world where moist air masses, heat, and a large area of fairly level terrain are to be found. Since the oceans are among the most level terrain types on Earth, there are probably many derechos at sea, and some have been recorded over the sea near Florida.
The varieties of derecho
Although it might seem that a derecho is straightforward enough for there to be only one kind of this type of storm, there are in fact three different varieties -- serial, progressive, and hybrid. Someone who is in an area struck by a derecho will probably be unlikely be able to identify the type, since the effects are much the same, but the type affects how wide the derecho is and how deep the area affected by its winds are.
- Serial derechos are created by extremely powerful low pressure systems, with a leading edge of thunderstorms which extends for many hundreds of miles. In these systems, multiple bow echo storms develop along the storm front, and the winds from these separate bow echo storms blend together into a truly colossal derecho that can span much of the width of a state and be many miles deep. These derechos affect a very large area of landscape and can cause damage to dozens of locations separated by many miles.
- Progressive derechos are generated by a single bow echo storm, often produced by a weak low pressure system that doesnt have as much 'punch to generate a long frontal storm line, and thus tend to be narrower than serial derechos, at least during the early part of their existence. As the derecho advances, however, it broadens more and more until it can be almost as large as a serial derecho.
- Hybrid derechos are spawned from strong low pressure systems, like those that produce serial derechos, but resemble progressive derechos more than the serial type. They can include winds of great force -- one hybrid derecho that affected Minnesota, Wisconsin, and Michigan in 1998 had measured straight-line winds as high as 130 miles per hour.
Dangers of the derecho
Since derecho winds are straight-line and exhibit no rotation, a derecho cant suck up people and animals like a tornado can. However, a derecho can be just as lethal as a tornado in many other ways. Derecho winds fling debris and objects with great force, and being struck by a plank or large tree branch flying at over a hundred miles per hour can kill or mangle a human being with ease. People inside cars and buildings generally have protection from such missiles, but high-profile vehicles can be flipped over if the winds strike them side-on, and weak buildings might collapse under the shock and pressure of the moving air. Falling trees cause many deaths and injuries, both among people on foot and those in cars.
People in boats and mobile homes are especially at risk. Derechos flip over boats with ease, and since they also churn the water into violent waves, people tossed out of their boats can drown in a matter of moments, finding themselves helpless in masses of churning, roiling waves. Mobile homes also tumble over easily unless they are tied down (which most are not), and the inhabitants can be hurled against walls and ceilings with enough force to break necks or backs, or rupture internal organs.
Derechos are nearly as dangerous as tornadoes and should be treated with the same caution. They occur in much the same season and under similar conditions, but provide an interesting view of how the flow of air masses can produce massive straight-line winds instead of rotating ones. They are also a dramatic and visually impressive event, and anyone who has lived through one is unlikely to forget it.