Wind Shear Explainer
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).
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. 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.
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.
Other sources of wind shear information: