With school starting up and gardening and home renos still taking a big chunk of my time, I must admit I am still not finished looking at all the summer weather numbers (I am beginning to sound like some of my students). Next article, I will get to those summer numbers, I promise. So, for this issue we are going to step back into some of the weather school articles I have written and re-examine the topic of general atmospheric circulation, and why hurricanes usually turn eastward once they make landfall in the United States.
We now know that on Earth the strong sunshine along the equatorial regions results in warm rising air and a region of low pressure. This is known as a thermally created area of low pressure. At the poles, the low amount of sunshine results in cold, heavy air that sinks, and results in an area of high pressure — this is known as a thermally created high. Now, if the Earth didn’t rotate our lesson would be coming to an end. These differences in pressure would result in a global pressure gradient force between the poles and the equator and the winds would simply blow from the poles toward the equator.
Fortunately, our planet does rotate, and this ends up creating a much more complex pattern of global winds. The rotation of Earth causes winds to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This means that on our simplified 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 Arctic or in the tropics you would be having no problems with this picture, because in these regions the prevailing winds are easterly. Here in our neck of the woods, we have to start scratching our heads, because our winds don’t seem to follow this simple Earth scenario; our prevailing winds blow from west to east (westerly winds).
So, what’s up? Obviously, something more is going on with the atmosphere. As most of us know, our Earth isn’t a simple place, and this pretty much holds true no matter where you look — and the atmosphere is no exception. We need to add to our simple picture of thermally induced low pressure at the equator and high pressure at the poles, with a couple more regions of low and high pressure. These two new regions are what are known as dynamically induced areas of high and low pressure.
Dynamic high- and low-pressure regions are created not by intense heating or cooling, but by mechanically forcing air to either rise or sink. The first of these dynamically created regions are the subtropical highs. These are the desert regions of the Northern Hemisphere. As warm air rises at the equator, this rising air eventually has to start spreading out. As it flows northward it gets mechanically or dynamically pushed downward. As it is compressed, this air heats 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 subtropical high that gives us our westerly winds.
The second dynamically produced region is an area of low pressure known as the subpolar low. This region of general low pressure is located around 60 degrees latitude. Much like the subtropical high, this area of low pressure is formed when the air flowing northward from the subtropical high (the westerly winds) pumps out against the southward-flowing polar air. This forces the air to move upward, creating low pressure.
Westerly winds win
OK, now let’s look at hurricanes. These systems form over warm tropical oceans, typically between latitudes of five degrees and 30 degrees. This area is at the northern end of the zone of the equatorial low, which means the winds are predominantly easterly — blowing from east to west. This pushes the hurricanes from their locations in the Atlantic or the Caribbean eastward to northeastward and toward the U.S. As these systems slowly gain latitude, they will eventually transition out of the easterlies and get caught up in the dynamically produced westerly winds.
Just how far north this happens depends on the undulations in the boundary between these two regions of oppositely flowing air. But no matter how big these undulations are, the westerly winds will eventually win out. The farther north any hurricane or hurricane remnant has travelled depends on where you are: the farther east, then the farther north they can travel. Along the East Coast, due to the water, they can make it as far north as Newfoundland. For the central U.S., the farthest north one has reached is about Kansas before weakening due to the lack of warm, moist surface, along with the fact that the strong westerly winds tend to rip the storm apart vertically.
Hopefully this helps to make some sense as to why hurricanes and tropical storms can only travel so far north before being whisked off to the east.