Wind shear is the difference in speed or direction of the winds at different atmospheric levels. In the case of the wind shear between 850 millibars and 200 millibars, a strong difference in wind speeds and direction can affect the development of tropical cyclones. Using GFS model forecast data, WorldWinds, Inc. is calculating the average vertical wind shear between these two atmospheric levels. Studies have shown that high levels of shear between 850mb and 200mb hinder the growth of, or weaken, tropical cyclones, and can also impede genesis. Since the interaction of tropical cyclones with their environment can contaminate wind shear calculations, WorldWinds removes the tropical vortex from the model field to portray an accurate synoptic picture of the shear environment around the storm.
Wind shear values are divided into several categories: shear of 0-5 m/s is “No Shear”, shear of 5-10 m/s is “Low Shear”, shear of 10-15 m/s is “Shear”, and any shear greater than 15 m/s is “High Shear”. A storm moving into a “No” or “Low” shear environment will intensify if other factors (a warm ocean and a moist environment) are favorable. However, “Shear” environments, and especially “High Shear” environments can be detrimental.
More details on the method chosen to calculate the wind shear can be found in the following journal article by Dr. Pat Fitzpatrick: http://www.drfitz.net/uploads/tips.pdf