Weather

Most people do quite well without paying too much attention to the day-to-day weather. As long as severe weather isn’t a threat, all they need to do is check the temperature and the likelihood of precipitation so they’ll know what to wear when they leave home. Pilots obviously have larger weather concerns before going flying: ceiling, visibility, the danger of icing, and the wind.

Many of the initial challenges to learning how to fly involve wind. This begins with learning to fly the correct paths over the ground in the airport pattern despite the wind’s efforts to push you off course. Ground-reference maneuvers are a pilot’s “graduate school” of coping with wind.

Wind is a big part of aircraft navigation. Student pilots must learn how to use predicted winds aloft to plan flights. They calculate what heading is needed to offset crosswinds that would push their airplane off course. They also plan how long a trip will be and thus how much fuel they’ll need by calculating whether tailwinds will speed the journey or head-winds will slow it.

Most weather involves wind. Winds are also involved in all kinds of weather. Different kinds of wind threats (along with others) characterize all 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 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. How fast the wind blows depends on both the difference between the pressures and how far apart they are. Meteorologists call this combination the “pressure gradient force.” (“Gradient” refers to the change in pressure measured across a given distance.)

Isaac Newton’s second law of motion, which he published in 1687, explains how this works. It says that how fast something (such as the air between pressure centers) is accelerated is proportional to the force being applied, divided by the mass of the object being accelerated. The 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.