Why is the wind gust limit set at 58 mph?
In the 1950's and 1960's, there were three types of convective watches that could be issued: Tornado Watches, Public Severe Thunderstorm Watches, and Aviation Severe Thunderstorm Watches. At first, the public severe thunderstorm watch wind criterion was 75 mph, while the limit for aviation watches was 50mph. Negotiations with the Air Force raised the minimum speed required for an aviation watch to 58 mph (50 knots) in 1962. In 1970, the Aviation Severe Thunderstorm and Public Severe Thunderstorm watches were combined into a single Severe Thunderstorm Watch, with a minimum wind gust criterion of 50knots, to reduce confusion.
What are straight-line winds?
Straight-line winds are generally any thunderstorm wind that is not associated with rotation, and is used mainly to differentiate from tornadic winds.
What causes straight-line winds?
Most straight-line winds are a result of outflow generated by a thunderstorm downdraft.
How common are damaging winds?
Half of all severe reports in the lower 48 states are due to damaging winds. Anyone living in thunderstorm-prone areas of the world is at risk for experiencing this phenomenon.
What is the difference between a microburst and a downburst?
A microburst is less than 4km across, and a downburst is greater than 4km across.
Can a downburst or microburst occur even if there is not a thunderstorm?
Most downbursts and microbursts occur with thunderstorms, but they can be associated with showers too weak to produce thunder.
We had damaging winds in our area – how can we tell if they were caused by a tornado or microburst?
Damage surveys from the air are the most effective way to determine what caused the damage. Tornadoes generally produce a narrow swath of destruction, and if it was on the ground for long, a path. Downbursts or microbursts will have a larger area of damage – often in a starburst pattern. Trees will be uprooted in a direction that points away from the point the downburst struck the surface. Sometimes the damage area has a circular or elliptical shape.
What is a derecho?
A derecho is a fast-moving windstorm that is made up of thunderstorms that repeatedly develop along the leading edge. These lines of storms can move very quickly and produce widespread straight-line winds over long periods of time. Derechos can move anywhere from 35-70 mph and last 8 hours or more. Most derechos that produce severe weather move at speeds greater than 50 mph. Warm season events probably move a little slower than cold season events.
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