What Is Wind Shear and How Is It Detected and Measured?

Some weather phrases naturally tend to make you perk up and listen, like when you hear “tornado” or “hurricane” mentioned during a TV broadcast. Trained meteorologists might get the same reaction when they discover wind shear in their analysis. Let’s discover what wind shear is, what it does, and why it’s so important in severe weather forecasts.

What Is Wind Shear?

Wind shear is the change in airspeed or direction with distance; it can be a vertical change, a horizontal change, or some combination of the two. Wind shear tends to cause an air mass to rotate, so if other factors like moisture and rising air are also in place, severe storms have an easier time producing dangerous weather, including large hail and tornadoes.

Vertical Wind Shear

First, let's consider how the wind can change vertically, from the ground to where the airplanes fly. If the air is blowing from the south at 40 mph at 5,000 feet and 60 mph from the south at 10,000 feet, then we have 20 mph of vertical wind shear between these two levels. When the wind direction does not change we have speed shear between these levels. When the wind direction changes it’s called directional shear.

Horizontal Wind Shear

Horizontal wind shear occurs when the wind shifts direction over a short distance at the same level in the atmosphere, such as along a cold front when it quickly changes from the south to the north. Combining horizontal and vertical wind shear gives meteorologists a complete picture of how the wind changes with distance.

How Is Wind Shear Detected?

Meteorologists use several sophisticated electronic tools and equipment to identify wind shear. Each of these tools helps fill in a piece of the puzzle:

• Radiosondes: Specialized sensors attached to balloons radio back temperature, humidity, pressure, and wind information as they ascend. This information tells us how the atmosphere changes with height at any given location, and computers then generate a national map of this data for 69 locations across the U.S.
• Wind profilers: Specialized radar antennas called wind profilers give accurate wind readings, such as speed and direction with height, at up to about 40,000 feet.
• Doppler radar: This specialized weather radar uses a computer program to identify areas of rapidly changing wind, making detection of high winds and tornadoes (rotation) possible, usually for multiple layers of the atmosphere.
• Surface reports: Wind readings from hundreds of airports across North America are examined in real time to identify fronts and storm outflows that produce wind shear.

How Is Wind Shear Measured?

Wind shear is usually factored through a layer of air anywhere from 1 to 4 miles thick for weather-forecasting purposes. For instance, a common technique is to measure the changes in wind shear between the ground and an altitude of about 10,000 feet.

• Wind shear measurements are expressed within a layer; a ground-level wind shear report might read “40 mph of shear from 0 to 1 kilometer.”
• Think of it this way: A supercell is a giant corkscrewing thunderstorm — it may end up producing hail, tornadoes, straight line winds, or a combination of these. It all starts with wind shear, which helps to create rising and spinning air; when strong wind shear gets near the ground, it can maximize the potential for a tornado. Low-level shear, within 10,000 feet of the ground, gives the best estimate of tornado potential — this spin factor uses a fancy weather word, “helicity.”
• Helicity runs on a scale from 0 to 600 m²/s² — values over 200 start to get the attention of forecasters, and if they are over 300, things might start to get dangerous!

Tornadoes Need Wind Shear

The Storm Prediction Center issues daily outlooks for severe weather up to eight days out. Expected wind shear is a main component in developing these outlooks.

• Wind shear allows for an efficient feed of moisture into the storm.
• Wind shear can help thunderstorms remain intense even overnight.
• Wind shear is responsible for most rotating thunderstorms, called supercells, which are the ones that produce most large tornadoes.

Hurricanes and Wind Shear

Tornadoes thrive on wind shear, but it’s a calamity for a hurricane. A strong wind shear pattern disrupts the evenly stacked structure of a hurricane, which basically rips it apart. An exception is when the hurricane is moving over land. Hurricanes that make contact with land produce their own wind shear as the storm air flows over hills, trees, and buildings.

• Land obstructions create a lot more friction than the flat sea surface, causing low-level wind shear and the threat of hurricane-spawned tornadoes.
• Helicity in hurricanes over land can be over 300 m²/s², the threshold for serious trouble!
• Some hurricanes produce dozens of tornadoes, others spawn very few.

Wind Shear in Your Backyard

If there are several layers of cloud visible, try watching them for a few minutes. Are they moving in different directions? If so, that’s wind shear! One sign of possible wind shear is increasing surface wind and quickly falling pressures. A reliable home weather station with online reporting capabilities to the My AcuRite® app and website allows you to track wind and pressure patterns, will help you track pressure patterns, and perhaps help you predict when winds will increase or decrease, making for greater or lesser wind shear. Try it on your own and share your observations below!