Posted - 08/16/2007 : 9:26:13 PM
|WHAT ARE STRAIGHT-LINE WINDS?|
METEOROLOGIST JEFF HABY
There are several terms that mean the same as straight-line winds and they are convective wind
gusts, outflow and downbursts. Straight-line wind is wind that comes out of a thunderstorm. If these winds
meet or exceed 58 miles per hours then the storm is classified as severe by the National Weather
Service. These winds are produced by the downward momentum in the downdraft region of a thunderstorm. An
environment conducive to strong straight-line wind is one in which the updrafts and thus downdrafts
are strong, the air is dry in the middle troposphere and the storm has a fast forward motion.
A storm with a strong updraft will tend to have a strong downdraft. When the CAPE is very high then
strong or severe convective wind gusts could occur. Dry air aloft will entrain into the downdraft. This
promotes evaporative cooling and this further enhances the negative buoyancy of a parcel. A cold parcel of
air surrounded by warm air will sink since the cold air is more dense. The colder the parcel is compared
to the surrounding air then the faster it will sink. Dramatically cooler air is often noticed at the
surface when the downburst air reaches the observer. When a storm has a fast forward motion the rate that
the downdraft is moving is added to the storm motion. This can produce strong to severe winds out
ahead of the storm as the storm approaches.
When the National Weather Service does a storm damage survey they distinguish between straight-line wind
and wind produced from a tornado. Straight-line wind damage will push debris in the same direction the
wind is blowing (hence the creation of the term straight-line). Tornado damage will scatter the debris
in a variety of different directions since the winds of a tornado are rotating violently. This type
of survey can be used to determine if straight-line wind occurred instead of a tornado or vice versa.
Straight-line wind intensity can be as powerful as a tornado. Because of this some people in the general public
will believe a tornado occurred when it reality one did not occur.
To reduce the damage from straight-line wind it is important to secure objects that can be blown by the wind.
It is also important to keep trees well pruned. Tree branches falling on cars or houses produce a significant
amount of damage in high wind events. Also make sure you are in a safe place when the straight-line wind
strikes such as in the interior of a brick home. Storms with severe straight-line winds often also have hail and
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