Weather

You can do perfectly well as a pilot without really understanding the Coriolis force, which causes winds to follow a curved path across the Earth. In fact, one of the better reasons to understand it is to correctly answer questions on the FAA’s knowledge exam.

The first hurdle in understanding the Coriolis force is figuring out why the wind follows a curved path over the rotating Earth. The pressure gradient force—the differences in atmospheric pressure that cause the wind to begin blowing—pushes air directly from an area of high atmospheric pressure toward an area of lower pressure.

Why does the wind follow a curved path over the Earth? The FAA’s Aviation Weather for Pilots and Flight Operations Personnel advisory circular tries to explain this by using a ruler and pencil to draw a “straight” line on a round piece of cardboard on a rotating turntable. It illustrates how, although the ruler creates a straight line, the line on the rotating cardboard is curved.

Figures 1A and 1B demonstrate how the Coriolis force works. All you need is a globe and a straight edge, such as a ruler or a pencil. The basic idea is to imagine that you could see a “parcel” of air as the Earth spins below.

Figure 1A shows that if the Earth wasn’t rotating, parcels of air initially blowing toward the north, south, east, and west from a starting point at the intersection of a latitude and longitude line would follow lines of latitude and longitude in any direction.

Figure 1B shows that as the actual Earth rotates, northern hemisphere lines of latitude and longitude turn to the left in relation to the fixed stars. A wind continues blowing in a straight line as seen by you from your perch in space as Earth turns below it. Someone on Earth sees the wind curving as the ground stays still.

Why winds curve

Interactions of the pressure gradient and Coriolis forces cause the winds to turn to the left, or counterclockwise, around areas of low atmospheric pressure in the northern hemisphere, and clockwise south of the equator. These interactions also cause the wind to blow clockwise around areas of low atmospheric pressure in the Southern Hemisphere and counterclockwise around areas of high pressure.

Figures 2, 3, and 4 (right) illustrate upper-atmosphere winds in the Northern Hemisphere. Blue arrows show the pressure gradient force that pushes air directly from high atmospheric pressure toward low pressure, black arrows show the Coriolis force, and green arrows show the wind resulting from the interactions of these forces. Thin blue and red lines marked 496 mb, 500 mb, and 504 mb show atmospheric pressures in millibars. Such pressures are typically found roughly 18,000 feet above mean sea level.

Courtesy the AMS Weather Book by Jack Williams: Used with the permission of the American Meteorological Society

Forces affect winds near the surface

In addition to the pressure gradient and Coriolis forces that affect all winds, friction between the wind and the Earth’s surface slows wind speeds near the surface, which in turn affect the wind direction, as shown here. Now, instead of flowing around an area of low pressure, the wind spirals into the center of low pressure and rises. Keep in mind, this doesn’t explain all differences in wind directions as you climb through the atmosphere. At times the patterns of areas of high and low pressure are different at various altitudes aloft and the surface.