Air pressure differences cause severe winds

This past fall saw more than its fair share of windstorms. The last one brought 90-plus mph winds to the coast and mountaintops, and severe winds across the Inland Northwest as well. What makes the winds blow with such severity?

In the Midwest, most severe wind events are due to strong thunderstorms and/or tornadoes and are accompanied by torrential rains and frequent lightning. The individual storms usually affect a relatively small area. Here in the Inland Northwest, in the absence of thunderstorms, weather conditions can come together in a different way to produce widespread damaging winds across an entire state.

Horizontal movement of air is caused by differences in air pressure. Air moves from higher pressure to lower pressure. I think the best way to visualize this is to think of a balloon. When you blow up a balloon, you are forcing air molecules into a smaller space, restricted by the surrounding balloon. The air molecules are packed closer together inside the balloon than they are outside of it. Thus, the pressure of the air inside the balloon is higher than on the outside. If you release the balloon without tying off the end, the air inside the balloon will rush out. The more you inflate the balloon, the higher pressure you create inside, which results in a larger pressure difference from the inside to the outside of the balloon, and the faster the air will rush out once you release it.

Differential solar heating across the globe causes areas of higher and lower pressure to develop, with the air moving from the highs to the lows. Near the Earth’s surface, however, the air does not take a direct path from high to low pressure. Friction is one of the factors that alters the path of the air. Because of the rotation of the Earth, two other forces, the coriolis force and centrifugal force, also alter the movement of the air. In general, the end result is air that moves counterclockwise and toward a low pressure area, and clockwise and away from a high pressure area. This, with the added factor of terrain influences, determines the direction of the wind.

The strength of the winds is determined by the pressure gradient – the change in air pressure over an area. Meteorologists can forecast wind intensity by looking at a surface air pressure contour map. The tighter the gradient (or the closer together the contour lines are), the stronger the winds will be. During the most recent windstorm, a strong (or deep) low pressure area moved eastward along the Canadian border. The computer models did a good job depicting the tight pressure gradients several days ahead of time so that meteorologists were able to give a heads up about the imminent windstorm.

One of the local terrain features that affects winds in the Coeur d’Alene area is the Rathdrum Prairie, which is flanked by southwest- to northeast-oriented ridges. Whenever the winds are out of the northeast, they are accelerated by the “funneling effect.” Northeast winds in this area can be 10 to 15 mph stronger than the northeast winds felt in the Spokane area, where the topography is flatter.

Though there has been sufficient cold air this month, the little moisture we’ve had has not been in sync to produce any significant snows. With less than 2 inches of snow as of Dec. 21, we are well below the monthly average of 19.3 inches. December 1940 is on the record books as the least snowy December, with a measly 1.1 inches.