Weather

Someone’s first takeoff, even if it’s only on an introductory flight, is a good time for a flight instructor to introduce the importance of winds. Unless it’s one of those rare days with nearly calm winds, the instructor can point out that the direction the wind is coming from determines the runway being used.

Wind affects pilots in other ways, including turbulence and pushing the airplane in directions that pilots don’t want to follow.

Students learn to cope with winds early in their training as they master flying a traffic pattern to land and take off, and fly ground reference maneuvers that require following particular tracks over the ground as wind tries to push the airplane off course.

The importance of winds to pilots—for practical reasons of flying and navigating airplanes—and an understanding of weather make learning a little about wind a good idea.

SURFACE WINDS. Winds are nothing but the movement of air from areas of relatively high atmospheric pressure toward areas of relatively low pressure at the Earth’s surface or aloft.

When you look at a weather map, you see some areas marked with an “L” and others marked with an “H.” An “L” shows where air is rising, creating lower air pressure at the surface. This rising air becomes part of the winds aloft, eventually to descend somewhere else to create an area of high atmospheric pressure at the surface, which is marked by an “H” on weather maps.

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 in the high and low centers 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.

WHY WINDS FOLLOW CURVED PATHS. In the atmosphere, air begins moving straight from higher pressure toward lower pressure. But the Earth is turning under the moving air, which means the moving air—wind—follows a curved path in relation to the planet. This effect of the rotating Earth on wind is called the Coriolis force, for the French scientist who first described it mathematically in 1835.

An instructor and a student easily can envision why winds follow curved paths across the Earth. First, you have to realize that air isn’t attached to the Earth.

One person holds a ball and slowly turns it, while the other uses a ruler and a marker pen to trace “straight” paths in various directions on the ball while it’s turning.

The marker pen traces a curved path across the ball, even though the person with the pen held it against the straight edge while drawing the line. The line on the ball will curve no matter which direction it’s going, unless it’s going directly east or west along the equator. On Earth, the Coriolis force is zero right along the equator, with the effect increasing as you move toward the north or south poles. It is strongest at the poles.

The strength of the Coriolis force also increases with the wind’s speed: The faster the air is moving, the more it curves.

In the atmosphere the combination of the pressure gradient and Coriolis forces cause winds to flow counterclockwise around large storms such as hurricanes and winter storms in the Northern Hemisphere, and clockwise around similar storms in the Southern Hemisphere.

For practical purposes, pilots don’t have to worry about correcting for the Coriolis force to reach their far-away destinations. They stay busy making adjustments for the winds that are usually pushing their airplanes one way or another, sometimes very gently.

WINDS ALOFT AND NEAR THE GROUND. Pressure gradient and Coriolis forces cause the winds, both aloft and at the surface—but there are differences. First, the patterns and strength of air pressures are different at different altitudes. Another reason winds aloft are faster than winds close to the ground is that in addition to the pressure gradient and Coriolis forces that affect winds aloft, winds near the surface are slowed by friction, which is a force that slows winds over land more than those over oceans or large lakes.

Aloft, the pressure gradient and Coriolis forces are in balance, which causes the air to curve around areas of high and low pressure at their altitude in the wavy patterns seen on upper-air charts.

The friction that slows winds close to the ground also weakens the Coriolis force, since it depends on wind speed. Now the pressure gradient force is stronger than the Coriolis force, and wind spirals into the low-pressure area.

By the way, the air rising and cooling as it rises in a surface low-pressure area causes clouds and precipitation when the air cools enough for condensation to begin. This is why surface low-pressure areas are the centers of storms, except in very dry regions. The lower the pressure, the stronger the storm, because the greater pressure gradient force pushes more humid air into the storm.

Air rises from surface low-pressure areas because various wind patterns aloft encourage air to rise, while others cause it to sink. When a pattern that causes air to rise is over a surface low, air continues rising and the storm rages. When the pattern aloft that’s been encouraging air to rise moves away or weakens, air flowing into the low center remains there instead of rising, which increases the air pressure until the low-pressure area vanishes and the winds die.

All pilots learn that even though the wind is invisible—unless it’s carrying snow, rain, or dust­—it can’t be ignored. Wind always has to be considered in planning trips. Will the headwinds slow the airplane so much that you’ll have to make an extra fuel stop? In addition to planning for forecast winds, pilots should always be ready for surprises, such as being alert to sudden wind shifts.

BE MINDFUL OF WINDS. 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 stronger-than-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 to remain 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.