Wind Gusts

Factors involved: Geography, temperature, topography, weather conditions, atmospheric pressure, wind

Regions affected: Cities, coasts, forests, mountains, plains, towns, and valleys

Definition

Wind gusts can be violent, with loss of property and life measured in millions, even billions of dollars. They can occur anywhere on the earth, sometimes without warning. Wind shear, a localized wind gust, can imperil aircraft, causing collisions with terrain on takeoff and landing.

Science

Wind gusts, also called wind shear, occur for a number of reasons, sometimes seemingly at random. No place on the earth’s surface is immune to wind gusts, although some areas are more likely to experience them than others. Gusts may be localized differences in atmospheric pressure caused by frontal weather changes. These occur most often in the spring and fall seasons. Normally, fronts having a temperature difference at the surface of 10 degrees Fahrenheit (5 degrees Celsius) or more and with a frontal speed of at least 30 knots are prone to creating wind gust conditions.

These so-called cold fronts contain a wedge of cold air at their leading edge. This wedge of cold air pushes warm air that is ahead of it upward very rapidly. If the warm air is rich in water vapor, as is seen in the southeastern United States, severe storms erupt ahead of the cold front and may continue until it passes. The weather proverb “If the clouds move against the wind, rain will follow” implies a cold front where clouds in the upper wind are moving in a different direction from clouds driven by lower winds. Most experienced aircraft pilots know how to fly cold frontal boundaries for fuel efficiency, in effect gaining a tailwind both ahead of and into the front.

To determine the strength of wind gusts, a good reference is the Beaufort scale. Beaufort numbers vary from 0, no wind, to 12, which depicts winds in excess of 73 miles per hour. People can start to feel the wind at Beaufort 2. A Beaufort 6 means that an umbrella is hard to control and large tree branches are moving. Serious damage potential arrives with Beaufort 10, when trees are uprooted and considerable structural damage can be incurred by anything in the path of the wind gust.

Thunderstorms, whether a product of a cold front or local air mass heating, are responsible for the majority of wind gusts. Thousands of thunderstorms occur across the earth’s surface every day. Typical of thunderstorms are the “first gust,” the rapid shift and increase in wind velocity just before a thunderstorm hits, and the “downburst,” or rapid downward movement of cooled air in and around the thunderstorm cell. A thunderstorm pulls in relatively warm air near the earth’s surface, then sends it skyward at several thousand feet per minute, rapidly cooling it. The cool air, becoming more dense and heavy, then plummets back down to the earth’s surface. This downward plunge of 7 to 10 miles creates tremendous inertia that can only be dissipated by outflow when the mass strikes the surface. This effect can be compared to dumping a bucket of water on a concrete surface: The “splash” is the same as the outflow from the downburst.

The gusty winds associated with mature thunderstorms are the result of these large downdrafts striking the earth’s surface and spreading out horizontally. Some gusts can change direction by as much as 180 degrees very rapidly and reach velocities of 100 knots as far as 10 miles ahead of the thunderstorm. Low-level gusts, typically between the earth’s surface and an altitude of 1,500 feet, may increase as much as 50 percent, with most of the increase occurring in the first 150 feet. This makes them particularly dangerous for aircraft in takeoff and landing.

The downburst is an extremely intense localized downdraft from a thunderstorm. The downdraft frequently exceeds 720 feet per minute in vertical velocity at 300 feet above the earth’s surface. This velocity can exceed an aircraft’s climb capability, even that of large commercial and military jets. This downdraft is usually much closer to the thunderstorm than the first gust. One clue is the presence of dust clouds, roll clouds, or intense rainfall.

Hurricanes and cyclones also breed large wind gusts. Although winds from these weather phenomena have predictable direction and velocity, tornadoes and whirlwinds imbedded in them can produce wind gusts capable of major damage.

Very local gusting is often referred to as wind shear, and it can be horizontal or vertical. Horizontal shear can move an aircraft off the centerline of a precision approach to an airport. While annoying, it is not usually harmful. Vertical shear, however, is potentially lethal to aircraft. The change in velocity or direction can cause serious changes in lift, indicated airspeed, and thrust requirements, often exceeding the pilot’s and the aircraft’s ability to recover.

A decreasing head wind can cause airspeed and lift of the aircraft to decrease. The pilot reacts with application of power and nose-up attitude of the aircraft. Although overshoots of the intended approach may occur, the pilot is usually able to go around and land safely. Decreasing tailwind causes an increased lift, and the aircraft climbs above the intended approach path.

Modern commercial pilot training devotes significant time to wind shear problems. Using computerized flight simulators, the entire array of wind shear problems can be programmed for flight crews. This increases their awareness and application of wind shear recovery without exposing them to the hazards of ineffective recovery techniques if using actual aircraft.

Geography

Topographic features, both natural and human-made, can promote wind gusts. Most people have experienced this in cities with tall buildings, where the wind intensity is much greater in the gaps between large buildings and swirling winds are expected.

Conditions peculiar to the southwestern United States prompt the formation of temperature inversions. These inversions are caused by overnight cooling, where a relatively cool air mass hugs the ground and is overlain by warmer air in the low-level jetstream. High winds from the low-level jet sometimes mix with this inversion, and significant wind gusts may occur at the interface with 90-degree shifts in direction and 20- to 30-knot increases in wind velocity common.

On a much larger scale are the gusts resulting from high winds in mountain passes, on the leeward side of large mountains, and across valleys between mountain ranges. A weather phenomenon often called a “mountain rotor” results from differential heating across a valley between two mountain ranges. Air flowing down an upwind mountain during the day is heated, traverses a relatively cool air mass in the valley, then moves across the downwind mountain, causing the turbulence at the boundary of the cooled and heated air masses described above. As the air is heated in the morning, a weak rising motion of the cool air is induced and pulls the air currents attempting to climb the downwind mountain back into the valley. This back-rotation creates a rotary motion that contains both horizontal and vertical wind gusts.

At least one commercial aircraft accident has been tentatively blamed on a rotor. Rotors can be seen, unless the atmosphere is devoid of moisture, as nearly round symmetrical clouds in mountain valleys. Pilots undergoing mountain-flying training are cautioned to steer clear of these rotors. A flight into a dry rotor is usually dangerous.

The roughness of the earth’s surface plays a major role in determining wind gust intensity. This roughness can occur from obstacles or terrain contours, called orography. Orography promotes tunnel effects (mountain passes) and hill effects (lee—the side sheltered from the wind—of mountains). Pilots are very familiar with this effect. A very calm outbound flight in the morning after a cold-front passage can mean a bumpy return flight in the afternoon as the frontal wind gains intensity and flows over rough topography.

In general, the rougher the earth’s surface, the more the wind will be slowed. Forests and large cities slow the wind more than lakes and prairies. Surface roughness can be classified as to its ability to slow wind. For example, landscapes with many trees and buildings have a roughness class of 3 or 4, while a large water surface has a roughness class of 0. Open terrain has a roughness class of 0.5. Roughness length is used with roughness class, and relates to the distance above ground level where the wind speed theoretically should be 0.

Prevention and Preparations

The aviation industry has been particularly interested in wind gusts, or wind shear, because of their potential effect on aircraft performance in takeoff and landing. According to National Transportation Safety Board (NTSB) records, wind shears contributed to approximately 50 percent of all commercial airline fatalities between 1974 and 1985. The Federal Aviation Administration (FAA) has required some type of wind shear hazard detection systems on scheduled commercial aircraft since 1995.

Pulsed Doppler radar is the primary means of detection of wind gusts for aircraft crews and ground-based air controllers and weather prognosticators. Doppler radar senses speed and direction in the same manner as police traffic radar. A well-understood Doppler effect is a train whistle that is always higher in pitch when the train is approaching than when moving away.

Doppler weather radar bounces its pulses off raindrops in storm clouds. If the raindrops are moving toward the radar set, the reflected signal is higher in frequency than if the rain is falling vertically. Frequencies are compared, and color displays are created to depict areas of precipitation and wind shear. Some Doppler radar sets create an audible warning to aircrews if wind shear is nearby. Effective though dangerous indicators of wind shear are reports from pilots experiencing it. Air-traffic controllers solicit these reports, and many may be received in a short period of time in areas where pilots are experiencing wind shear conditions.

Aside from aviation and its vulnerability to wind gusts, other modes of transportation are frequently disturbed by wind gusts. Mountain valleys and other gust-prone locations often experience upended tractor-trailer rigs, which are typically top-heavy and show a considerable broadside resistance to the wind. Sailing ships and relatively light watercraft are also prone to upset by wind gusts. Even with no sails in the wind, boats are difficult to steer with changing wind speeds bearing against their hulls.

Rescue and Relief Efforts

Wind gusts produce the same results as tornadoes but are even more localized. Building damage and injury to humans and animals can occur. Trauma-related injuries are typical, including broken bones, excessive lacerations, and imbedded debris. Police and fire officials usually handle the localized nature of wind gust damage, although for widespread damage, the Red Cross and Salvation Army, as well as other relief organizations, may assist victims.

Local authorities also customarily oversee property damage. Clearing may be necessary to restore public utilities and roadways. Insurance adjusters are frequent visitors to damage sites so that they can assess the severity of the damage to client property and recommend compensation.

Impact

Like that of tornadoes, wind gust damage is not long-lasting. It has no significant effect on local topography, but it can cause extensive damage to human-made structures. The famous “Galloping Gertie,” or Tacoma Narrows Bridge, was set in motion by wind gusts and ultimately destroyed by its own harmonic frequencies. Windows and trim in large buildings can be damaged or even removed by wind gusts. Large signs and other similar displays are also frequently damaged or dislodged by gusty winds. These articles pose a risk to passersby on the streets below.

Perhaps most important, wind gusts damage aircraft quite easily. Those aircraft on the ground not secured by tie-down lines may be blown around by gusty winds and extensively damaged. However, the most important damage to aircraft occurs when wind gusts overcome the pilot’s ability to maintain flying conditions in takeoff or landing configurations. Aircraft collisions with the ground can cause minor damage or extensive loss of life and totally destroy aircraft. Literally thousands of aircraft accidents can be traced to wind gusts as the primary cause of or at least a major contributor to the accident. A portion of the avionics industry is devoted exclusively to assessing the severity of wind shear and its effect on the operation of aircraft. Even local television stations proudly advertise that their weather gurus are equipped with the most modern Doppler radar for the safety and convenience of their viewers.

In March 2018, Winter Storm Riley intensified from an nor'easter to what is known as a bomb cyclone, or a storm with a central surface pressure that drops by 24 millibars or more in 24 hours. The bomb cyclone's rain, sleet, and powerful winds grounded some flights on the East Coast. At Reagan National Airport in Arlington, Virginia, the winds forced several planes to abort their take off or landing attempts.

Bibliography

Freier, George D. Weather Proverbs: How 600 Proverbs, Sayings, and Poems Accurately Explain Our Weather. Tucson, Ariz.: Fisher Books, 1992. A very interesting book on weather phenomena, with modern explanations given to ancient weather lore.

Kimble, George H. T. Our American Weather. New York: McGraw Hill, 1955. This is a very readable book, unique in that it depicts U.S. weather by month. Entertaining as well as informative.

Mark, Michelle. “Harrowing Videos Show Wind from Brutal 'Bomb Cyclone' Blowing Planes Around Like Feathers as They Try to Take Off and Land.” Business Insider, 2 Mar. 2018, www.businessinsider.com/bomb-cyclone-planes-video-wind-noreaster-2018-3. Accessed 31 Jan. 2019.

National Aeronautics and Space Administration. Making the Skies Safe from Windshear. http://www.nasa.gov/centers/langley/news/factsheets/ Windshear.html. A series of NASA documents that detail its research into the causes and detection of wind shear as it affects aircraft.

National Transportation Safety Board. http://www.ntsb.gov/ntsb/query.asp. This is the NTSB’s aviation accident/incident database. Although cold and cryptic details are the essence of this Web site, it nevertheless details the mounting toll of aircraft accidents resulting in part from wind gusts.

Palmén, E., and C. W. Newton. Atmospheric Circulation Systems: Their Structure and Physical Interpretation. New York: Academic Press, 1969. Although some knowledge of calculus is necessary to master this book, it still has many readable pages concerning global weather at the lower altitudes that can be understood by most individuals.

Smith, Mike. “Defeating the Downburst: 20 Years Since Last U.S. Commerical Jet Accident from Wind Shear.” The Washington Post, 2 July 2014, www.washingtonpost.com/news/capital-weather-gang/wp/2014/07/02/defeating-the-downburst-20-years-since-last-u-s-commercial-jet-accident-from-wind-shear/?utm‗term=.01b60e342c14. Accessed 31 Jan. 2019.

Wood, Richard A., ed. The Weather Almanac: A Reference Guide to Weather, Climate, and Related Issues in the United States and Its Key Cities. 11th ed. Detroit: Thompson/Gale, 2004. Provides a detailed description of the Beaufort number for wind speed and contains much weather data.