Wind shear is often the most critical factor controlling hurricane formation and destruction. In general, wind shear refers to any change in wind speed or direction along a straight line. In the case of hurricanes, wind shear is important primarily in the vertical direction--from the surface to the top of the troposphere. The troposphere is the region of the atmosphere that our active weather is confined to, and extends up to about 40,000 feet altitude (a pressure of about 200 mb) in the tropics in summer. Hurricanes fill the entire vertical extent of the troposphere, and are steered by the average wind through this layer. When one hears the phrase, "wind shear is 20 knots over the hurricane", this typically refers to the difference in wind speed between 200 mb (the top of the troposphere, 40,000 feet altitude) and a layer where a pressure of 850 mb is found--about 5,000 feet above the surface. This wind shear is computed over a large area--a circle of 700 miles in diameter centered on the hurricane is one technique used. This 200-850 mb wind shear is a crude measure of the actual wind shear a storm experiences, since only changes in wind speed--not wind direction--are considered. Furthermore, the computed shear does not consider any smaller scale changes that may occur within this large volume of the atmosphere. For example, it is common to find a strong jet of wind at about 600 mb blowing along the edge of the Saharan Air Layer (SAL)--that area of dry, dusty air that frequently lies to the north of developing tropical cyclones in the mid-Atlantic. This jet will create significant wind shear that will not show up on the standard 200-850 mb wind shear plots. Since upper-air measurements are very sparse over the open ocean, wind shear that is invisible on 200-850 mb wind shear analysis charts will often unexpectedly kill or weaken a developing tropical cyclone.
Tropical cyclones are heat engines powered by the release of latent heat when water vapor condenses into liquid water. Wind shear hurts tropical cyclones by removing the heat and moisture they need from the area near their center. Shear will also distort the shape of a hurricane by shearing it (blowing the top away from the lower portion), so that the vortex is tilted. A tilted vortex is usually a less efficient heat engine--the delicate balance of inflowing low-level winds and outflowing upper-level winds that ventilate the storm gets disrupted. Dr. Bill Gray of Colorado State University was one of the first scientists to study the effect of winds shear on hurricane formation. In his classic 1968 paper, "Global View of the Origin of Tropical Disturbances and Storms", Dr. Gray writes:
"In the SW Atlantic and central Pacific, where tropical storms do not occur, the observed climatological tropospheric wind shear is large (i.e., 20-40 kt). This is believed to be the major inhibitor to development in these areas. Large vertical wind shears do not allow for area concentration of the tropospheric distributed cumulonimbus condensation. Large shears produce a large ventilation of heat away from the developing disturbance. The condensation heat released by the cumulus to the upper troposphere is adverted in a different direction relative to the released heat at lower levels. Concentration of heat through the entire troposphere becomes more difficult.
Dr. Gray also discovered that the east-west or "zonal" component of the wind shear was what mattered most to hurricanes. Wind shear in the north-south or "meridional" direction did not significantly affect the storms. This is why one often sees "zonal wind shear" plotted in addition to the total wind shear. (By the way, we now know that tropical storms do occur in the central Pacific, thanks to satellite imagery--and we have also had one tropical cyclone in the SW Atlantic, Hurricane Catarina of 2004)."
Rules of thumb
A general rule of thumb is that the shear must be 20 knots or less for intensification to occur. Most instances of rapid intensification of hurricanes occur when the wind shear is 10 knots or less. However, large and powerful hurricanes can be resistant to shear values as high as 40 knots, as demonstrated by Hurricane Wilma (Figure 1). We often see tropical disturbances under 10 knots of wind shear that do not develop. Why? Oftentimes, this is because cold, dry air aloft associated with an upper level trough of low pressure is interfering with development. Tropical cyclones develop most readily when an upper level anticyclone (high pressure system aloft) is present overhead.
Figure 1. Wind shear plot of Hurricane Wilma at 00GMT October 25, 2005. At time time, Wilma had just intensified to a Category 3 hurricane with 125 mph winds and a 955 mb pressure, despite the presence of 40 knots of wind shear over the storm. Just to the west of Wilma one can see wind shear values of 120 knots, associated with the jet stream.
One excellent web site to diagnose current wind shear values is the University of Wisconsin CIMSS site. They compute upper level winds by looking at cloud motion from satellites. A mean low level wind, averaged over a layer between 925 mb and 700 mb (1500 feet to 10,000 feet), is subtracted from a mean upper level wind, averaged over a layer between 300 mb and 150 mb (30,000 to 45,000 feet). If a tropical cyclone is present, the winds due to the circulation around the storm are removed, so that one can just look at the environmental wind field the storm is embedded in. A sample of one of their wind shear analyses is shown in Figure 1. Yellow contour lines mark wind shear values in knots. Pink lines with arrows show the direction of the shear (which is almost always the same as the direction of the upper level winds at 200 mb, since winds aloft are almost always stronger than winds near the surface). The contour lines are overlaid on a water vapor satellite image. Dark areas show where dry air is. The CIMSS plots are updated every three hours, and available about two hours after the time stamped on the image. All times are Greenwich time (UTC), four hours ahead of EDT.
Shear example: Tropical Storm Gabrielle of 2007
Figure 2 shows a still image of the disturbance that eventually became Tropical Storm Gabrielle of 2007. At the time, Gabrielle had a well defined circulation from the surface to 850 mb, but could not develop any heavy thunderstorm activity near the center because strong upper level winds from the southwest blew away the thunderstorms as quickly as they formed. A 0.5Mb animation of satellite images shows this process quite clearly. The low level vortex of Gabrielle is stationary, and one can see thunderstorms trying to form at the center get sheared away by strong upper level southwesterly winds at 200 mb. The winds carry high, wispy cirrus clouds from lower left to upper right in the animation. These winds blew at about 20 knots (Figure 3), creating 20 knots of wind shear over the center of Gabrielle (where the winds were calm). Just north of the Gabrielle's center, the low-level flow from the surface to 850 mb (5,000 feet) was out of the northeast at about 10 knots, due to the counter-clockwise flow of air around the vortex. As one can see from Figure 3, the shear was thus 20 knots at 200 mb minus negative 10 knots at 850 mb (since the 850 mb winds were blowing in the opposite direction as the 200 mb winds), for a total wind shear of 30 knots. To the south of the vortex, the surface to 850 mb flow was out of the southwest (same direction as the 200 mb flow), so the shear was 20 knots minus positive 10 knots, for a total of 10 knots of shear.
Figure 2. Visible satellite image of the tropical disturbance that eventually became Tropical Storm Gabrielle of 2007. The thick band of clouds extending from southwest to northeast is due to the presence of an upper level trough of low pressure. Image credit: University of Wisconsin CIMSS satellite blog.
Figure 3. Winds shear analysis from the time of the Tropical Storm Gabrielle image of Figure 2. The red box outlines the area of the zoomed-in satellite image from Figure 2. Note that wind shear increases from 10 knots in the lower right portion of the red box to 30 knots in the upper left portion. The pink lines with arrows show that the shear direction was from the southwest, due to upper level winds of about 20 knots that blew from southwest to northeast. Image credit: University of Wisconsin CIMSS satellite blog.
Other sources of wind shear information: