Flight Training - February 2012 - 43

kinds of storms from the smallest thunderstorm to the biggest winter storm. Intense wind that reduces visibility to near zero in blowing snow is the difference between a blizzard and an ordinary snowstorm. Cold winds from the north make the difference between a snowstorm and a rainstorm. Warm, humid winds blowing from over the Gulf of Mexico or Atlantic Ocean supply the water vapor that a storm turns into rain and snow. The temperature contrast between warm and cold air delivered by the winds is the source of a storm’s energy. The importance of winds to pilots both for practical reasons of flying and navigating airplanes—and an understanding of weather—makes learning a little about wind a good idea.
WIND BEGINS WITH PRESSURE DIFFERENCES. A look at localized winds, such as

farther apart the pressure centers, the more mass of air the force is pushing. Once the air is moving Earth’s rotation and other forces cause wind to flow generally clockwise around high-pressure areas and counterclockwise around low-pressure areas in the Northern Hemisphere. Friction with the ground causes winds at the surface to be slower than winds aloft with similar pressure gradients. For more on the role of pressure differences and wind in weather see “Weather: Nine Keys to Understanding Weather,” January 2011 Flight Training.
WIND TERMINOLOGY. The U.S. National Weather Service and air traffic control use these wind terms in reports of observed and forecast weather: Direction: the true compass direction the wind is coming from. (An exception is that wind directions given by a control tower are the magnetic direction, since runway numbers are based on magnetic direction.) Gust: a sudden, brief increase in wind speed, reported in weather observations when the peak wind speed reaches at least 16 knots and variation in speed between the peaks and lulls is at least nine knots. The duration of a gust is usually less than 20 seconds. Knot: one nautical mile per hour. (Multiply by 1.15 to convert to statute miles per hour.)

the cooling sea breezes that blow inland from the oceans or large lakes on hot days, is a good way to begin understanding what causes winds to blow. Air moving up or down over a location creates pressure differences. The Ls are where air is leaving a location, rising from the ground over land or sinking toward the ocean. The Hs on the figure are locations where air is arriving, such as aloft where air is rising and at the surface where air is sinking, creating air pressure that’s higher than the pressure of the surrounding atmosphere. The differences in air pressure between the “H” and “L” areas create a force pushing air from high toward low pressure. WIND DIRECTION How fast the wind blows depends on both the difference between the pressures 1. As land warms, hot air begins rising. and how far apart they are. Meteorolo2. Cooler air flows in to replace the rising air. gists call this combination the “pressure gradient force.” (“Gradient” refers to the change in pressure measured across a H given distance.) Isaac Newton’s second law of motion, which he published in 1687, explains how 1 this works. It says that how fast something (such as the air between pressure centers) is accelerated is proportional to L the force being applied, divided by the 2 mass of the object being accelerated. The

Speed: the one-minute average speed, known as the sustained wind. Squall: a strong wind of 16 knots or faster than the sustained wind that happens suddenly and lasts for at least two minutes. Variable wind: a wind of seven knots or faster that changes direction by 60 degrees or more. When a pilot is in the air, he or she has to remain aware of whether the airplane is following the planned route. Crosswinds that differ from the forecast could be pushing it off course. A strongerthan-forecast headwind could be slowing the flight. When this happens the pilot must decide whether an unplanned fuel stop would be good insurance against an emergency landing with empty fuel tanks. Pilots have another reason for remaining aware of winds that are different from the forecast used to plan a flight. Since the wind is only a part of an area’s big weather picture, a wind forecast that isn’t turning out to be correct should alert a pilot to watch for other failed forecasts. Differences from the forecast could mean that a storm is moving faster than forecast and that low ceilings and visibility could arrive at you destination forecast faster than expected.
Jack Williams is an instrument-rated private pilot. The

latest of his six books is The AMS Weather Book: The Ultimate Guide to America’s Weather.

3. Air sinks to replace air blowing inland. 4. Air aloft flows over ocean, replacing sinking air.

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Flight Training - February 2012

Table of Contents for the Digital Edition of Flight Training - February 2012