Departamento De Compras

A derecho ( /dəˈreɪtʃoʊ/, də-ray-choh, from Spanish: derecho, IPA: [de̞ˈɾe̞tʃo̞], "straight") is a widespread, long-lived, straight-line windstorm that is associated with a fast-moving band of severe thunderstorms. Generally, derechos are convection-induced and take on a bow echo form of squall line, forming in an area of wind divergence in the upper levels of the troposphere, within a region oflow-level warm air advection and rich low-level moisture. They travel quickly in the direction of movement of their associated storms, similar to an outflow boundary (gust front), except that the wind is sustained and increases in strength behind the front, generally exceeding hurricane-force. A warm-weather phenomenon, derechos occur mostly in summer, especially during June and July in theNorthern Hemisphere, within areas of moderately strong instability and moderately strong vertical wind shear. They may occur at any time of the year and occur as frequently at night as during the daylight hours.
Contents [hide]
1 Etymology
2 Development
3 Classification and criteria
4 Characteristics
5 Location
6 Damage risk
7 See also
8 References
9 Further reading
10 External links[edit]Etymology

Derecho comes from the Spanish word for "straight" (cf. "direct") in contrast with a tornado which is a "twisted" wind.[1] The word was first used in the American Meteorological Journal in 1888 by Gustavus Detlef Hinrichs in a paper describing the phenomenon and based on a significant derecho event that crossed Iowa on 31 July 1877.[2]
[edit]Development



Development ofderechos
Organized areas of thunderstorm activity reinforce pre-existing frontal zones, and can outrun cold fronts. The resultant mesoscale convective system (MCS) forms at the point of the best upper level divergence in the wind pattern in the area of best low level inflow.[3] The convection then moves east and toward the equator into the warm sector, parallel to low-level thickness lines with the meantropospheric flow. When the convection is strong linear or curved, the MCS is called a squall line, with the feature placed at the leading edge of the significant wind shift and pressure rise.[4]
Derechos are squall lines that are bow- or spearhead-shaped on radar and, thus, also are called bow echoes or spearhead radar echoes. Squall lines typically bow out due to the formation of a mesoscalehigh pressure system which forms within the stratiform rain area behind the initial line. This high pressure area is formed due to strong descending motion behind the squall line, and could come in the form of a downburst.[5] The size of the bow may vary, and the storms associated with the bow may die and redevelop.
During the cold season within the Northern Hemisphere, derechos generally developwithin a pattern of southwesterly winds at mid levels of the troposphere in an environment of low to moderate atmospheric instability (caused by heat and moisture near ground level or cooler air moving in aloft and measured by convective available potential energy) and high values of vertical wind shear (20 m/s (45 mph) within the lowest 5 kilometres (16,000 ft) of the atmosphere). Warm seasonderechos in the Northern Hemisphere form in west to northwesterly flow at mid levels of the troposphere with moderate to high levels of instability. Derechos form within environments of low-level warm air advection and significant low-level moisture.[6]
[edit]Classification and criteria



Composite radar image of the June 2012 North American derecho (a progressive derecho) as it moved fromIndiana to Virginia
Its common definition is a thunderstorm complex that produces a damaging wind swath of at least 250 miles (400 km),[7] featuring a concentrated area of convectively-induced wind gusts exceeding 50 knots (58 mph; 93 km/h).[8] According to the National Weather Service criterion, a derecho is classified as a band of storms that have winds of at least 50 knots (93 km/h; 58 mph)...