Last week we introduced the four main forces that drive our winds; gravity, air pressure, Coriolis and friction. This week we will go into a little more detail on these forces and hopefully come to a better understanding of just what creates wind.
If you remember in Part 1 of this article, we discussed that without gravity there would be no weight to the atmosphere, and without that weight there would be no air pressure. So, in our discussion we will leave out gravity and focus on the forces of pressure gradient, Coriolis and friction, and how they interact together to create our winds.
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Pressure differences exist in our atmosphere due to the unequal heating of the Earth’s surface. Cold air is dense and will exert greater pressure than warm air. We show or measure the strength of a pressure gradient by isobars or isolines. These are lines on a map that connect areas of equal pressure. The spacing between the isobars tells us the intensity of the pressure gradient force — close spacing indicates a steep gradient, while wide spacing is equal to a lesser or gradual gradient.
Now if we were to take a simple look at the Earth, this would mean that the polar regions would be regions of high pressure, since they are cold, and the equatorial regions would be areas of low pressure, since they are warm. The air would then move directly from the north toward the south at right angles to the isobars. We all know that this is not always the case; the question is, why not? The answer lies with our next two forces, Coriolis and friction.
First let us look at the Coriolis force. First of all, I am not going to get into an argument of whether this is a real force or not. We could spend a whole article discussing that point. Instead, for simplicity’s sake, let’s just agree that it is a force. Basically, the Coriolis force arises from the fact that the Earth rotates. Anything that flies or flows across the Earth’s surface (wind, airplanes, ocean currents) will be deflected from a straight path because of that rotation.
In the Northern Hemisphere, things are deflected to the right of their intended path, and to the left in the Southern Hemisphere. The Coriolis force is weakest at the equator and strongest around the poles. The amount of deflection from the Coriolis force is also controlled by the speed of the object that is moving — the greater the speed, the greater the deflection.
So, on our simple Earth, the air flowing from high pressure at the North Pole toward low pressure at the equator would be deflected to the right, eventually ending up flowing parallel to the isobars. While this type of airflow is what we would typically find in the upper atmosphere, we really don’t see that near the Earth’s surface. Why? Well, we have our last force to deal with that is not really felt in the upper atmosphere but can have a significant impact near the Earth’s surface: friction.
Slowing the flow
Friction between the wind and Earth’s surface varies depending on the type of surface the wind is blowing over. We really notice this in the winter, as areas around lakes Winnipeg and Manitoba see higher wind speeds and more blowing snow, because there aren’t any features on the ground to create friction and slow the wind down — whereas, in forested areas and even in large cities like Winnipeg, the trees and buildings create friction, causing winds to slow down.
Typically, frictional forces will affect winds up to an altitude of about 500 m. Friction decreases the speed of the wind and therefore reduces the effect of the Coriolis force. Now instead of the wind blowing parallel to the isobars, it blows at an angle across them. The net effect of this is that winds blowing from areas of high pressure tend to spiral outward, then spiral inward as they move toward low pressure.
This is where it can get confusing unless you remember that the atmosphere is three dimensional. If air spirals into an area of low pressure, where does all the air go? It can’t go into the ground, so the only direction left is upwards. If all that air flows upward, it must go somewhere and just like with thunderstorms, all that rising air will eventually build up, then come crashing back down, wiping out the area of low pressure.
Next article it is time already to look back and see just how cold and snowy February was, and look at just how much snow has fallen across the Prairies so far this winter. We will also look ahead to see what changes the computer models have made for the spring weather outlook. Our continued look into wind will have to wait until a little later in March. Hopefully, I can get right back to it and won’t have to write about a big winter storm; after all, we are now heading into what is often the snowiest and stormiest part of winter.
