One of the contributors to wind is the Coriolis force. The Coriolis force arises from the fact that the Earth rotates. | Photo: File
Last issue, we talked about how the pressure gradient force is the main driving force of wind in our atmosphere and that it exists due to the unequal heating of the Earth’s surface.
Now, if we were to take a simple look at the Earth, this would mean that the polar regions would have high pressure because they are cold and the equatorial regions would be areas of low pressure because they are warm.
The air would then move directly from the north toward the equator. This doesn’t totally happen due to the Coriolis and friction forces.
The Coriolis force arises from the fact that the Earth rotates.
Anything that moves across the Earth’s surface 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 dependent on the speed of the object — 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, ending up flowing parallel to the isobars.
Let’s tie in friction.
Friction between the wind and Earth’s surface varies depending on the type of surface over which the wind is blowing. It 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, and they spiral inward as they move toward low pressure.
Continuing our simplified view of the Earth, the flow of air moving from the North Pole toward the equator would be deflected to the west, eventually resulting in all the winds blowing from east to west (easterly winds).
If you live in the high Arctic or in the tropics, you would be having no problems with this picture because in these regions, the prevailing winds are easterly.
So, why do we have westerly winds — dynamically induced areas of high and low pressure?
The first of these dynamically created regions are the subtropical highs.
As warm air rises at the equator, it flows northward, gets mechanically or dynamically pushed downward, compressing and heating up.
Once this air reaches the ground, it spreads out once again with some air returning toward the equator and some heading north toward the poles.
It is this northerly flow of air out of the sub-tropical high that gives us our westerly winds.
The second dynamically produced region is an area of low pressure known as the sub-polar low.
This region of low pressure is located around 60 degrees latitude. It is formed when the air flowing northward from the sub-tropical high (the westerly winds) bumps up against the southward flowing polar air, forcing the air to move upward, creating low pressure.
Next issue, I think I will be able to sneak in our monthly weather review and our look at the latest forecasts for this summer, even if it is a day or two early.