Directional wind shear

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Directional wind shear, or the change in wind direction with height, plays an important role in weather and affects everything from the sport of ballooning to storm behavior. A change in wind direction with height that is clockwise as viewed from the surface generally means warm air is moving in. Conversely, winds that change counter-clockwise with height means cooler air is headed your way.

Directional wind shear, or the change in wind direction with height, plays an important role in weather and affects everything from the sport of ballooning to storm behavior. A change in wind direction with height that is clockwise as viewed from the surface generally means warm air is moving in. Conversely, winds that change counter-clockwise with height means cooler air is headed your way.

In ballooning, directional shear (commonly referred to as “steerage”) is generally most evident in the morning when lower layers of the atmosphere are not well mixed. However, with the help of solar heating from the sun, turbulent mixing causes the stratification of the various wind layers to mix into one layer. This layer, which contains similar characteristics, grows throughout the day until the late afternoon hours when the atmosphere begins to decouple from the surface. Research has shown that the depth of this layer is dependant on the square root of heat transport from the surface to the atmosphere. Some other important parameters include the square root of both the length of daytime heating and the initial amount of stratification.

Directional wind shear also plays an important role in storms. If moderate to strong directional wind shear exists in the atmosphere, storms can begin to rotate much easier, increasing the threat for severe weather, especially tornadoes. One of the first ingredients in tornado formation is a rotating updraft, and a change in wind direction helps the thunderstorm updraft rotate. In fact, fairly recent research has shown that directional shear in the lowest kilometer of the atmosphere is a very important factor in tornado formation. If directional wind shear is too weak, then the storm may not be able to rotate efficiently enough to produce a tornado.

Directional wind shear can be observed using various instruments. One of the simplest is a pibal, which is nothing more than a helium balloon that is tracked by sight through the lower levels of the atmosphere. This type of observation is commonly made in Africa. Here in the US, we use something called a rawindsonde to try and get an understanding of the atmosphere at a given point. Rawindsondes, which are instrument packs carried aloft by a hydrogen filled balloon, are generally launched twice a day and take a large sample of data through the atmosphere. These instruments record temperature, dew point, wind direction and speed as they ascend through the atmosphere. As the instrument rises through the atmosphere, the atmospheric pressure becomes less and less and the balloon expands till it bursts. The instruments then fall back to the earth under the guidance of a parachute. These measurements are then used to determine aspects about the atmosphere related to temperature, moisture and stability. Because of the expense involved in launching this type of instruments, they are only launched on a regular basis in 92 locations across the United States. A few locations where these are launched on a regular basis in the Midwest include Green Bay, Omaha, Davenport, and Minneapolis. Once the data is gathered for a particular site, it is input into the forecast models and is plotted on a chart called a skew T log P. These charts can be found on the internet daily at

http://www.rap.ucar.edu/weather/upper .

On the same web page, there is also data that is taken in hourly intervals by wind profilers. Wind profilers are a form of radar that take measurements solely in the vertical. Instead of focusing on precipitation as radar does, wind profilers focus strictly on wind. Wind profilers are prone to errors due to migrating birds. Another drawback to profilers is that they don’t sample the winds below one-half of a kilometer above the ground level.

As you can see, the change of wind direction with height can be observed in many different ways. In the next article, we will discuss land sea breezes and how this affects the sport of ballooning.