Why winds curve
Understanding the Coriolis force
When winds blow across the Earth, they follow a curved path in relation to lines of latitude and longitude—counterclockwise in the Northern Hemisphere and clockwise south of the equator. The Coriolis effect is named after French mathematician Gaspard Gustave de Coriolis (1792-1843), who worked out the theory in the early nineteenth century. Others applied it to meteorology later in the century. Understanding the Coriolis effect can be complicated, but a good way to visualize what happens is to realize that as wind blows, the Earth is rotating under it. If you could see moving air from a fixed point in space, wind would follow a straight line as the Earth rotates under the wind. The images below will help you understand why winds follow a curved path across the Earth.
If the Earth didn’t rotate, winds going north, south, east, and west from a point would follow the red arrows (left).
As the Earth rotates, Northern Hemisphere latitude and longitude lines turn to the left in relation to the fixed stars and winds go to the right over the Earth (right).
An instructor might point out the windsock to a student pilot, and explain that takeoffs should be made into the wind, or as close to it as the runway layout permits. That’s just one of the many ways one learns that wind, including a lack of wind, affects all flights. A basic understanding of wind is needed both for safe flying and also for answering some of the questions on FAA knowledge tests.
Wind is 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. On a weather map some areas are marked with an “L” and others are marked with an “H.” An L (low pressure) shows where air is rising, creating lower atmospheric pressure at the surface. This rising air becomes part of the winds aloft, eventually descending somewhere else to create an area of high atmospheric pressure at the surface, which is marked by H on weather maps.
The difference in surface air pressures over Fort Myers, Florida, and the eye at the center of Hurricane Charley created 140 mph winds around Charley’s eye on August 13, 2004, a couple of hours before the storm came ashore north of Fort Myers. While the diagram above shows the differences in pressure between the eye and a place outside the hurricane, similar pressure differences around the storm were pushing wind into Charley.
This diagram shows only the pressure difference part of what causes wind to blow. The other part is how far away the areas of high and low pressure are from each other. This combination creates a pressure gradient force.
An example makes the point: On December 10, 2005, a pressure difference—similar to that with Fort Myers and Charley—existed between Boise, Idaho, and Sault Ste. Marie, Michigan. But, instead of the 60 miles between high and low pressure with Charley, the December 10 difference was 1,560 miles. Winds over the northern Midwest that December day were in the 20- to 25-mph range.
Sir Isaac Newton explained how this works in 1687. Newton’s second law of motion says that how fast something is accelerated (such as the air between pressure centers) is proportional to the force being applied, divided by the mass of the object being accelerated. The farther apart the pressure centers, the bigger the mass of air the force is pushing.
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