The “Windy” City

Lessons about air pressure.

While the origins of Chicago’s nickname “The Windy City” are not exactly clear, it does provide a nice opportunity to discuss what wind is and how it forms. Before we can do this, first it is important to understand the concept of air pressure.

It may be hard to imagine that air has “weight” to it, but each and every molecule that makes up the atmosphere has mass. Individually, each molecule is quite light, but when all the molecules over one area of the surface are put together, the mass is not insignificant. Gravity (the acceleration downward caused by Earth) pulls the mass of the atmosphere toward Earth’s surface. You might recall from a long ago science class that a force is equal to mass times acceleration (the famous F = ma expression), while pressure is defined as a force applied to an area. Thus, air pressure is the force of the atmosphere applied to Earth’s surface at any point. Atmospheric pressure is greatest at the surface and decreases with increasing distance from the ground.

The average surface pressure across the globe is 1013.25 mb (or 29.92 in. Hg); however on any given day there are areas with higher pressure and lower pressure compared to their surroundings. A higher pressure area contains more mass (and thus more force pressing downward) than a lower pressure area. The flow of air always occurs from an area of higher pressure to an area of lower pressure. Any movement of air on the planet (or “wind”) is ultimately caused by a pressure difference between two locations. These locations can be many thousands of miles apart (creating large-scale flows of air such as the trade winds or the westerlies) or they can be within a few miles of each other (leading to local-scale flows like sea or lake breezes). The important thing is that there is a pressure difference; this is what causes air to move.

Pressure change with altitude. Pressure is greatest near the surface (bottom right of graph) and decreases exponentially with increasing altitude.

Forces that control wind: PGF

So if air moves because of pressure differences, what determines where it goes and how fast it gets there? There are several forces that play a role in this. First, the technical term for what causes the movement of air from higher pressure to lower pressure is the pressure gradient force (PGF). The PGF is responsible for the generation of air motion, its initial direction, and initial speed. The initial direction of the air motion is always towards the lower pressure area (more on the ultimate direction below). Air moves faster from higher to lower pressure when the difference in pressure between the two areas is large. If the PGF = 0, that would indicate no change in pressure and consequently, no air motion. As the PGF increases, the speed of motion will increase. In other words, large changes in pressure over an area (strong PGF illustration) will lead to stronger winds, while small changes in pressure result in lower wind speeds (weak PGF illustration).

Weak PGF

Example of a (left) strong PGF and (right) weak PGF.

These differences are easy to visualize on surface weather maps by looking at the patterns of isobars (lines of equal pressure). Closely spaced isobars indicate a strong PGF, while widely spaced isobars show a weak PGF.

Surface weather map example

The brown lines are isobars. Every location along an isobar has the same pressure. The blue shaded area is an example of a strong PGF, where the isobars are close together (and the resulting wind speeds are around 20 mph). The red area shows a weaker PGF; the isobars have more space between them, and wind speeds are light (5 mph).

Forces that control wind: Coriolis force

If Earth did not rotate, we would be nearly to the end of the story. The rotation of Earth on its axis brings us to the second force, known as the Coriolis force. The Coriolis force is an apparent force ( that causes air (and other objects in motion) to change direction while in motion. In other words, because of the Coriolis force, air will not move directly from high to low pressure. Instead, it is deflected to the right of its initial path in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis force does not impact the speed of the air motion, only its direction. The strength of the Coriolis force is determined by latitude (near zero at the equator, highest at the North/South Pole) and the speed of the wind (higher speeds = more deflection).

Coriolis force example

Examples of air motion on a (left) nonrotating Earth and (center) in the Northern Hemisphere and (right) Southern Hemisphere.

Forces that control wind: friction

Our final force is one that occurs near the surface: friction. Friction causes air to slow down because of a drag on the forward motion due to the roughness of the ground. Imagine rolling a toy car over a smooth table versus a gravel driveway. Given the same initial force (your push of the car), the car moving over the gravel driveway will slow much more quickly than the one moving over the smooth table. It’s much the same in the atmosphere. For two areas with equal PGF, air will slow more over a rough surface (mountains, cities, etc.) than over something relatively smooth like the ocean. For air moving well above the surface (a few kilometers), friction is so small that it can be discounted.

Force combinations to create wind

So putting it all together, the PGF puts air into motion, the Coriolis force alters the direction, and friction slows air down. Well above the surface, only two forces are involved (PGF and Coriolis), which creates what is known as geostrophic wind (we will talk more about this type of wind in a later post). At the surface, where friction is large enough to play a role, the combination of these forces (and their interactions) is what determines the wind direction and speed at the surface. Surface high and low pressure systems are part of this framework as well; the combination of forces creates a clockwise and outward flow around high pressure and a counterclockwise and inward flow around low pressure (in the Northern hemisphere). The overall large-scale air flow is determined by the relative locations of high and low pressure systems, while local variations are caused by changes in surface type (land versus water) or topography.  Now the next time you are out for a long run, imagine these three forces at work creating a refreshing tail wind (or grueling head wind)!

Geostrophic wind

PGF and Coriolis combine to create geostrophic wind.

Surface wind

All three forces (PGF, Coriolis, friction) create surface winds.

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