What is shear?

May 9, 2006

Growing up as an avid watcher of hurricanes, I heard references to shear hundreds of times, but it wasn't until my meteorology courses in college that I actually knew what shear was.  To spare kids such a fate and to enlighten the adults who were on a different path from me, I offer a discussion.

Like last Tuesday, we start with the definition offered by the AMS' Glossary of Meteorology 

 shear—The variation (usually the directional derivative) of a vector field along a given direction in space. The most frequent context for this concept is wind shear.

Indeed, when we are talking about hurricanes and shear we are talking about

wind shear—The local variation of the wind vector or any of its components in a given direction. The vertical shear can be expressed in terms of height ∂V/∂z or of pressureV/∂p as the vertical coordinate. If the wind is geostrophic, the vertical shear is given by the thermal wind equation. The wind shear at a point is said to be cyclonic or anticyclonic according to whether the sense of rotation from the wind vector to the shear vector at that point is cyclonic or anticyclonic.

Crikies, Greek letters! Fear not, it's fairly simple to understand.

The first part of the definition that may be unclear to some people that is essential to the disucssion is the term vector.  A vector is simply a quantity that has a magnitude and direction.  Wind velocity is a vector because is has a two part description: its direction and its speed. A westerly 10 knot wind, for example. 

The second sentence is the meat of the definition. When talking about shear and hurricanes the second expression, ∂V/∂p is used.  It is the change of velocity with respect to the change in pressure. This means the velocity at some point in the atmosphere minus the velocity at a lower point. Usually 200 millibars is used as the higher point and 850 millibars is used as the lower point.  So, if there's a 30 knot westerly wind at the 200 millibar level and a 10 knot westerly wind at the 850 millibar level, there is a westerly shear of 20 knots.

In general, low values of shear (less than 15 knots) are considered to be favorable for the formation and development of tropical cyclones while higher values are hindrance (with 40 knots and above being absolutely destructive).      The exact details of the effects of shear on tropical cyclones is an area of considerable ongoing research.

The Goes 12 Wind Shear analysis from the University of Wisconsin is the product I refer to most when looking at shear. At the moment I am writing this, the analysis shows 30 to 70 knots of shear across the Gulf of Mexico with the only low shear area being just east of Panama. In general, that is what one would expect to see at this time of year.

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