Damaging Winds Basics
Winds from thunderstorms
Damage from severe thunderstorm winds account for half of all severe reports in the lower 48 states and is more common than damage from tornadoes. Wind speeds can reach up to 100mph and can produce a damage path extending for hundreds of miles. These winds are often called "straight-line" winds to differentiate the damage they cause from tornado damage. Strong thunderstorm winds can come from a number of different processes. Damaging winds are classified as those exceeding 50-60mph.
Since most thunderstorms produce some straight-line winds as a result of outflow generated by the thunderstorm downdraft, anyone living in thunderstorm-prone areas of the world is at risk for experiencing this phenomenon.
Types of damaging winds
Straight-line winds – a term used to define any thunderstorm wind that is not associated with rotation, and is used mainly to differentiate from tornadic winds.
Downdrafts – A small-scale column of air that rapidly sinks toward the ground. A downburst is a result of a strong downdraft.
Downbursts – A strong downdraft with horizontal dimensions larger than 4 km (2.5 mi) resulting in an outward burst or damaging winds on or near the ground. (Imagine the way water comes out of a faucet and hits the bottom of the sink.) Downburst winds may begin as a microburst and spread out over a wider area, sometimes producing damage similar to a strong tornado. Although usually associated with thunderstorms, downbursts can occur with showers too weak to produce thunder.
Microbursts – A small concentrated downburst that produces an outward burst of damaging winds at the surface. Microbursts are generally small (less than 4km across) and short-lived, lasting only 5-10 minutes, with maximum windspeeds up to 168 mph. There are two kinds of microbursts: wet and dry. A wet microburst is accompanied by heavy precipitation at the surface. Dry microbursts, common in places like the high plains and the intermountain west, occur with little or no precipitation reaching the ground.
Gust front – A gust front is the leading edge of rain-cooled air that clashes with warmer thunderstorm inflow. Gust fronts are characterized by a wind shift, temperature drop, and gusty winds out ahead of a thunderstorm. Sometimes the winds push up air above them, forming a shelf cloud or detached roll cloud.
Derecho –Definition: Derecho is a Spanish word meaning “straight” or “direct”. In terms of weather, a derecho is a long-lived windstorm with winds in excess of 58 mph. This is equivalent to the damage that can occur during an F1 tornado. The winds are sustained winds and not simply single bursts of wind. There can be gusts of winds in excess of 100 mph. The winds are non-tornadic, although an actual tornado can accompany a derecho. The storms are over 250 miles in length and can cause extensive damage. A derecho is a widespread thunderstorm wind event caused when new thunderstorms form along the leading edge of an outflow boundary (a surface boundary formed by the horizontal spreading of thunderstorm-cooled air). The thunderstorms feed on this boundary and continue to reproduce themselves. Derechos typically occur in the summer months when complexes of thunderstorms form over the plains and northern plains states. Usually these thunderstorms produce heavy rain and severe wind reports as they rumble across several states during the night. The word "derecho" is of Spanish origin and means "straight ahead". They are particularly dangerous because the damaging winds can last a long time and can cover such a large area. Winds in a derecho can be enhanced by downburst clusters embedded inside the storm. These straight-line winds may exceed 100 mph (160 km/h) (in some cases, sustained wind) in these clusters and straight-line wind gusts of up to 200 mph (320 km/h) are possible in the most extreme cases.Bow Echo – A radar echo which is linear but bent outward in a bow shape. Damaging straight-line winds often occur near the "crest" or center of a bow echo. Bow echoes can be over 300km in length, last for several hours, and produce extensive swaths of wind damage at the ground.
Enhanced F Scale for Tornado Damage
An update to the the original F-scale by a team of meteorologists and wind engineers, to be implemented in the U.S. on 1 February 2007.
|FUJITA SCALE||DERIVED EF SCALE||OPERATIONAL EF SCALE|
|F Number||Fastest 1/4-mile (mph)||3 Second Gust (mph)||EF Number||3 Second Gust (mph)||EF Number||3 Second Gust (mph)|
*** IMPORTANT NOTE ABOUT ENHANCED F-SCALE WINDS: The Enhanced F-scale still is a set of wind estimates (not measurements) based on damage. Its uses three-second gusts estimated at the point of damage based on a judgment of 8 levels of damage to the 28 indicators listed below. These estimates vary with height and exposure. Important: The 3 second gust is not the same wind as in standard surface observations. Standard measurements are taken by weather stations in open exposures, using a directly measured, "one minute mile" speed.
Enhanced F Scale Damage Indicators
|NUMBER (Details Linked)||DAMAGE INDICATOR||ABBREVIATION|
|1||Small barns, farm outbuildings||SBO|
|2||One- or two-family residences||FR12|
|3||Single-wide mobile home (MHSW)||MHSW|
|4||Double-wide mobile home||MHDW|
|5||Apt, condo, townhouse (3 stories or less)||ACT|
|7||Masonry apt. or motel||MAM|
|8||Small retail bldg. (fast food)||SRB|
|9||Small professional (doctor office, branch bank)||SPB|
|11||Large shopping mall||LSM|
|12||Large, isolated ("big box") retail bldg.||LIRB|
|14||Automotive service building||ASB|
|15||School - 1-story elementary (interior or exterior halls)||ES|
|16||School - jr. or sr. high school||JHSH|
|17||Low-rise (1-4 story) bldg.||LRB|
|18||Mid-rise (5-20 story) bldg.||MRB|
|19||High-rise (over 20 stories)||HRB|
|20||Institutional bldg. (hospital, govt. or university)||IB|
|21||Metal building system||MBS|
|22||Service station canopy||SSC|
|23||Warehouse (tilt-up walls or heavy timber)||WHB|
|24||Transmission line tower||TLT|
|26||Free standing pole (light, flag, luminary)||FSP|
|27||Tree - hardwood||TH|
|28||Tree - softwood||TS|