Forces For and Against the Wind

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The first step in analyzing the wind direction and speed at a given location is to first identify all of the forces that play a part in moving the air. Some of these forces are more easily described than others. We will start our discussion with two "real" forces--the pressure gradient force and friction.

The Pressure-Gradient Force

The Earth's atmosphere is an ocean of air, and, like an ocean of water, the pressure exerted by air molecules at a weather station can be approximated by the weight of the air (in a column that extends from a predetermined area on the ground to the top of the atmosphere). At sea level, the weight of a column of air on one square inch of area is roughly 14.7 pounds, resulting in an air pressure of 14.7 pounds per square inch (or, equivalently, about 1013 millibars).

The bottom line here is that a low pressure system is a lightweight (the air column above the center of a low weighs less than any of the surrounding air columns) and a high pressure system is a heavyweight (the air column above the center of a high weighs more than any of the surrounding air columns). Granted, the column weights over the centers of most mid-latitude highs and lows are not dramatically different. As we learned in an earlier lesson, the difference between the pressure extremes is only about five percent.

Still, it is this contrast in sea-level pressure (difference in column weights) between highs and lows that drives the wind. To see what I mean, let's perform a simple experiment. A Plexiglas container (pictured below) has two compartments separated by a removable partition. There's more water in the left compartment than there is in the one on the right, translating to a greater weight of water on the left than on the right. Thus, there's higher water pressure on the bottom of the left compartment than on the bottom of the right compartment. If I now remove the partition, there's a flow of water from higher pressure to lower pressure. In other words, the water, initially at rest while the partition was in place, accelerated from rest once I removed the partition.

When the barrier is removed between the two water columns (frame 1) the water flows from the higher column to the lower column because of the pressure gradient force (arrow, frame 2). The result (frame 4) is two columns with equal heights and thus no pressure gradient.

Credit: David Babb

Isaac Newton's second law of motion states that there must have been a net force acting on the water in order to set it into motion. In this case, the catalyst force was the pressure-gradient force, which acted from higher pressure toward lower pressure.

If the amounts of water in each compartment differ by a smaller amount, then the pressure-gradient force (PGF) is much smaller because the weights of the water in both compartments are nearly the same. Now the flow of water will be much slower. Thus, we arrive at the following result: The magnitude of the pressure-gradient force, which, in this experiment, represents the difference in water pressure across the partition, dictates the speed of the flow of water.

Okay, let's switch from my "laboratory" to the real atmosphere. Recall that the gradient of an atmospheric variable such as pressure measures the difference in pressure over a given distance. As a representative value, sea-level pressure might change 20 millibars over a horizontal distance of 500 miles (800 kilometers), which corresponds to a representative pressure gradient of 0.04 mb/mi. So calculated values of horizontal gradients in sea-level pressure "look" pretty small, don't they? Yet, these seemingly paltry pressure gradients can still produce breezes that cause flags to flap and street signs to rattle. And, for extremely strong low-pressure systems like the 944-mb super low over the Bering Sea on November 9, 2011 (see 06Z surface analysis below), calculated values of the pressure gradient appear to belie the strong winds they produce (YouTube video and the relevant meteogram at Nome, Alaska).

The 06Z surface analysis over the Bering Sea and surrounding region on November 9, 2011. At the time, a deep low-pressure system, with a central pressure of 944 millibars, was generating hurricane-force winds east of the low's center.

Credit: Ocean Prediction Center

On surface weather maps, the pressure-gradient force is always measured at right angles to the isobars. Thus, the pressure gradient force, which is drawn as a vector because it has both magnitude and direction, acts from high to low pressure. The magnitude of the pressure gradient force increases as pressure changes more rapidly over a given distance, as shown in the idealized surface weather map below.

The pressure gradient force (PGF) is a vector which points from higher pressure to lower pressure while crossing isobars at a right angle. Its magnitude depends on the pressure gradient, which is a measure of the spacing between isobars.

Credit: David Babb

Thus, the wind should blow faster when isobars are packed relatively close together, as the data from the intense low over the Bering Sea on November 9, 2011, indicate. Want to see an example over the middle latitudes? Check out the 12Z surface analysis on April 30, 2011. Compare the 40-knot sustained winds over eastern Montana and western North Dakota, where isobars are packed pretty tightly, with winds in eastern North Dakota. You should also notice that the wind does not move directly from higher pressure to lower pressure. That's because there are other forces at work other than the pressure gradient force, and they have an impact on wind direction and wind speed as well. Read on.

Friction

Around 0430Z on September 5, 2004, Hurricane Frances (track) made landfall (the eye of Frances reached the coast) along the southeast seaboard of Florida. Below is an analysis of the sustained winds around Hurricane Frances at 06Z on September 5, 2004. Measurements (in knots) were taken by buoys, ships, ASOS, and other instruments.

The 06Z analysis of sustained winds around Hurricane Frances on September 5, 2004. Wind speeds are color-coded in knots. The contour interval is five knots. White arrows indicate wind direction. Note that north of the storm center, wind speeds just off the coast were higher than 80 knots, while wind speeds a bit inland from the coast were between 75 and 80 knots. This rather abrupt reduction in wind speeds over land was a consequence of friction.

Credit: NOAA’s Hurricane Research Division

In the image above, examine the northwest quadrant of the storm, where winds were blowing onshore at the time (the white arrow indicates wind direction). Okay, note that the wind speed just offshore was higher than 80 knots. Just inland from the coast, however, the onshore winds blew at speeds between 75 knots and 80 knots. Without reservation, this rather abrupt reduction in wind speed was a consequence of friction over the rougher land.

It may not be intuitive to you that air in motion near the Earth's surface is slowed by friction. After all, at mundane wind speeds, friction between the air and the ground (or other objects like trees and buildings) is indeed rather small. But once the pressure gradient force puts air in motion, inelastic collisions between air molecules and the unyielding, rough ground cause air parcels to decelerate a bit, a slowing down of the wind that we attribute to a force called "friction."

If my explanation of "inelastic collisions between air molecules and the unyielding, rough ground" seems a bit too abstract, allow me to offer an alternative explanation.

Remember from a previous lesson that when the wind blows, the roughness of the earth's surface causes mechanical eddies to form. These eddies mix faster-moving air down toward the ground, where the momentum of the air diminishes. Meanwhile, mechanical eddies also mix slower-moving air near the ground upward. As a result, wind speeds a bit higher up in a well-mixed layer of air next to the ground don't blow quite as fast. So, when the wind blows, friction at the earth's surface acts to produce the wind profile that increases with height. The bottom line here is that friction reduces the speed of the wind by creating eddies that transport momentum toward the ground (where it diminishes). Moreover, the effects of friction decrease with increasing height above the ground.

Given this explanation, we can rightfully assume that the magnitude of the force of friction increases with increasing speed -- the faster surface winds blow, the greater the force of friction. Force of friction also depends on the "roughness" of the surface. For example, air blowing across the flat plains of Kansas will encounter much less friction than say, air crossing the Rocky Mountains.  Wind blowing over water encounters the least amount of friction. Although, the effects of friction over smooth bodies of water (in a regime of light maritime winds, for example) is generally less than over water that's agitated and "roughened" by persistently strong winds. I point these guidelines out because soon you will have to factor in friction into your analysis of wind direction.

So, we have the pressure-gradient force causing a parcel of air to accelerate and the force of friction slowing it down a bit. The only problem that remains is to explain why, in large-scale weather systems, air does not move directly from high to low pressure. This one is a bit more complicated because it involves a subtlety called "an apparent force" that you will read about in the next section.