Wind Flows
Keith Thomassen, PhD, CFII

As pilots we are affected by winds in many varied ways and flight regimes, so it behooves us to understand them. Whole textbooks are written on the role of wind in meteorology so this article is necessarily brief and very selective. Its simple intension is to give some insight in what drives the wind. Perhaps it will stimulate further study on your part.

Wind is air in motion, so to study wind is to first understand the forces that drive the air masses; these forces are pressure gradients and gravity. Vertically, air rises to a level at which these two forces are in balance, but horizontal flows are driven by pressure gradients and flows result when different forces come in balance

In an inertial frame (non-accelerating) Newton’s laws describe air motions owing to the forces of gravity and pressure gradients. But since the earth rotates, so does our frame of reference. Then, additional (pseudo) forces are at play, and motions in this accelerating reference frame must be analyzed by including them. They are needed to explain how motions appear in this frame compared to looking from a fixed frame in outer space (which simply uses the pressure gradient and gravity forces).

These new features in our rotating frame are centrifugal and coriolis forces. The coriolis force is proportional to the earth rotation speed and is perpendicular to both the axis of rotation and the wind speed vector, so it forces the wind to curve in the horizontal plane. In the Northern hemisphere it curves it to the right (down under it curves left).

Figure 1

In a low-pressure system there are lines of constant pressure (isobars) encircling the low. The pressure gradient is inward, perpendicular to these lines, but the coriolis force curves the air right, towards the isobar. An equilibrium is reached when the pressure gradient and coriolis forces are nearly in balance, there being a small net inward force (to balance centrifugal force) that keeps the air moving counterclockwise along the isobars. Both forces are in opposite directions around a high-pressure area, so the result is that air flows clockwise around the high.

Figure 2

Within the 1-2 km layer of air above the earth, friction between air layers changes the flow pattern. This region is known as the planetary boundary layer (PBL). The layer next to the earth has the greatest friction, so it slows the wind speed the most, but each layer drags on the one above it up to the top of the PBL. It also changes its direction as in the graphic below. Coriolis force decreases as the wind slows, changing the balance with the pressure gradient force, pushing the wind towards low pressure (and away from a high).

Figure 3

So in the Northern hemisphere, looking down from above, friction forces the air flow into a low and away from a high. In each case, if you look at a wind vector at the surface, then imagine the change with altitude, it grows larger (less friction) and rotates right until it aligns with the isobar at the top of the PBL. So if you’re in the PBL (up to 6500 ft), keep this picture in mind to visualize wind speed and direction above or below you.

Figure 4

So the understanding of wind flows as presented here is offered simply as a supplement to help in understanding what’s driving the wind. In planning a flight, you need to know what is actually happening. Your iPad includes forecast winds at altitude to use in choosing altitudes for flight segments. In flight, your GPS may display wind information, and your EFIS may display winds at altitude from a satellite weather service (as does my Chelton). For the airport environment AWOS is often available in giving you wind information for landing. But viewing this data with the broad principles discussed here can be helpful.