 Stretching across the widest part of Africa on the southern edge of the Sahara Desert is the African Transition Zone. This zone is also known as the Sahel, which means border or margin, and includes countries such as Mauritania, Mali, Niger, Chad, Sudan, Ethiopia, and Somalia. The Sahel is the zone where the dry, arid conditions of the desert found up here in the north meet up with the moisture region of the tropics found here along Central Africa. So the question in a physical geography context is what is creating the Sahel? What are the outlooks of the Sahel long term? We know the desertification is occurring, but that's a longer term type effect, but what about seasonal changes to the Sahel? Because that too is going to affect the economic stability of the region. So what we want to do here is discuss the Sahel and try to answer these questions. So the first thing I'd like to do is draw a planet earth. So here's earth and then let's draw the equator on earth. And in this picture, let's also draw the sun. So if the equatorial zone is receiving a lot of direct solar radiation, then we know that this area is going to be pretty toasty and it is. It's going to be warm. We have a lot of surface heating and because we have a lot of surface heating, we're going to create a low pressure zone around the equatorial region. And so we're going to call this the equatorial low. If you have a low pressure zone, then that means air is going to lift up into the sky. Of course, earth has gravity. It's got a gravitational pull. So this air that is lifting into the sky coming from the surface is not going to escape into space, nor is it just going to sit here. Some of this air is going to get pushed toward the north. Some of this air is going to get pushed toward the south. And eventually this air will get forced back down to the surface of the earth. And if you're asking why is that air getting forced back down to the surface of the earth? That's a question for another video, which we'll address a little bit later on. The point where this air is forced back down to the surface of the earth occurs right around 30 degrees north and 30 degrees south latitude. Now, if air is being forced back down to the surface of the earth, then we are creating a high pressure zone. So now we've got our subtropical high pressure zones to go along with our equatorial low pressure zone. Now, air at the surface is going to flow from high pressure to low pressure. And that's why we have winds. And in the northern hemisphere, the air is going to get veered toward the right of initial motion because of the Coriolis effect. And in the southern hemisphere, the air is going to get veered toward the left of initial motion because of the Coriolis effect. So here we see air flowing from H to L. And it's getting veered slightly to the right in the northern hemisphere because of the Coriolis effect. And then here in the southern hemisphere, the air is going from H to L and slightly getting veered to the left because of the Coriolis effect. These winds, by the way, are called the easterly trade winds because they're originating in the east and going toward the west. You'll notice that the air converges and the air converges at the equatorial low. But I don't want to call this the equatorial low anymore. I would rather call this the intertropical convergence zone or ITCZ. And there's a reason why we want to call it the intertropical convergence zone because as we're going to see in a second, this low pressure system actually moves with the seasons. And so a better nomenclature for it is going to be intertropical convergence zone where the easterly trade winds actually converge. Now that we know why we have the subtropical high pressure belts and why we have the intertropical convergence zone and why we have the easterly trade winds, let's take a look at how this whole dynamic shifts during the course of the year. And in order to do that, we need to take a look at the seasons. So what I want to do now is draw a very quick picture of Earth's revolution and around the sun and see how and why the ITCC is going to shift. So here we go. Let's put the sun here right in the middle of the picture. And we've got our solar rays coming out from the sun. And let's also put in this picture Earth. Now as we know, Earth is tilted 23 and a half degrees relative to the plane of the ecliptic. What that means is if this line here represents the plane of the ecliptic, which is the plane that the sun is residing on, then Earth is tilted 23 and a half degrees relative to that plane. So this is 23 and one half degrees. So the north pole is right here. The south pole is right here. And here is the equator. The picture I've just drawn is the summer solstice. And the summer solstice is the first day of summer in the northern hemisphere and the first day of winter in the southern hemisphere. In this same picture, let me draw a person. I'm going to put this person right here on the Tropic of Cancer. And the Tropic of Cancer is located at 23 and a half degrees north latitude. I've just randomly put him here on the Earth in the northern hemisphere. There's no other reason than it's just random and it's sort of cool. You'll see why in just a second. Now the Earth is going to revolve around the sun and it's going to revolve around the sun in a counterclockwise direction. So let's say that a half a year goes by, 180 days. Now we find the Earth over here in this picture. The north pole is still over there. The south pole is still over here. The equator is right here. And notice now that there's something that you really have to notice. The solar rays, the most direct solar rays, that is the portion of Earth that is on the plane of the ecliptic is right here. You'll notice that a half a year earlier during the summer solstice, the most direct rays of the sun, that is the part of the Earth that was intersecting with the plane of the ecliptic, was right here, right where I put this person, right at the Tropic of Cancer. A half a year later during the winter solstice, which is what this picture is, the most direct rays of the sun are residing right here on the Tropic of Capricorn, which is at 23 and a half degrees south latitude. Now our person is right here on the Tropic of Cancer. Now you'll notice this person's relative distance from the most direct solar rays during the course of the year. Here in the summer solstice, if this person goes outside on the first day of summer, that sun is going to be directly over this person's head at solar noon and he or she is not going to see any shadows because the sun is directly overhead. However, half a year later, that's not going to be the case. If this person goes outside at solar noon, the sun is not directly over his head. Rather, the sun is going to be at an angle. So what we notice in terms of seasonal variation is relative to an observer on earth, the position of the sun has seemed to have changed. Now, of course, the sun itself is not moving. Rather, it's earth revolving around the sun. And because the earth is tilted, the height of the sun in the sky relative to your horizon is going to shift essentially on a daily basis. So now the question is, why is this important? Well, you'll recall in our conversation just a moment ago, we said that the equatorial low pressure zone was due to the fact that the solar rays were hitting on top of the equatorial region. And in fact, let's go back to that picture. We take a look at this picture here and you'll notice this is an equinox. This is the vernal equinox or the optimal equinox where the vernal equinox represents the first day of spring and the optimal equinox represents the first day of fall. If we put our person here in this picture standing, well, I put him on 30 degrees north latitude. I meant to put him here right in the tropic of cancer at 23 and a half degrees north latitude. This person goes outside at solar noon. The sun is not directly over that person's head. Rather, if you're standing on the equator, that would be the case. So again, what we're noticing is we have this seasonal shift of where the most direct sun is based upon the season. And what we're also going to notice is that this low pressure zone, which we don't want to call the equatorial low, but rather we want to call the ITCZ or the Intertropical Convergence Zone, this low pressure also has to migrate with the moving sun because that low pressure zone is directly related to the moving sun. So let's go back to this picture. Here we notice that the low pressure zone of the ITCZ has shifted northbound. And here the low pressure zone of the ITCZ has shifted southbound. So let's go back to this picture and let's erase this board and let's draw the earth again. So here is our equator. And we know that during the vernal or the optimal equinox, the low pressure zone of the ITCZ is going to be right there pretty close to the equator. Our subtropical high is going to be right here, right at around 30 degrees north latitude. Our subtropical high in the southern hemisphere is going to be right here around 30 degrees south latitude. So this is a picture of an equinox, first day of fall, first day of spring. Let's use a different color to represent the wintertime. In the wintertime the most direct rays of the sun are going to be here above the tropic of Capricorn in the southern hemisphere. That means that the ITCZ is going to shift southbound. But if the ITCZ shifts southbound, then this subtropical high is going to shift southbound a little bit, and this subtropical high is going to shift southbound a little bit as well. So the entire pressure system around the equator is shifting with the changing season. That's going to be a picture of the wintertime. Let's put in here a picture of the summertime. So in the summertime the most direct sunlight is going to be here residing right above the tropic of cancer at 23.5 degrees north latitude. That means that the ITCZ is going to shift slightly northbound. That low pressure is going to be moving with the sun. The subtropical high down here in the southern hemisphere is going to shift slightly northbound. And then the subtropical high here in the northern hemisphere is going to shift slightly northbound as well. So the ITCZ shifts. And remember, the ITCZ, the Intertropical Convergent Zone, is a low pressure system. Low pressure systems, by the way, produce rain. If you have a low pressure zone, you have a lot of rain. If you have a high pressure zone, you're going to have a lot of dryness. So let's take a look at this picture here of Africa. And this picture of Africa is a nice climate map of Africa. And you'll notice that the Sahara Desert right here is located in and around 30 degrees north latitude. So this is a high pressure zone. And this high pressure zone is creating the Sahara Desert. Down here, we have our ITCZ or Intertropical Convergent Zone or our low pressure system. And this low pressure system is creating a lot of rain in the tropics of Africa. And then down here, we have our other subtropical high. And this is creating the desert region down here. In part, there's other reasons for it, but the subtropical high is creating those deserts by and large. So this is a picture which apparently looks like an equinox period of time, at the either fall or spring. But you'll notice that there's a region associated with this. And so in the summertime, everything shifts northbound. Because remember, in the summertime, the sun is above the tropic of cancer. So this low pressure, this ITCZ in the summertime is now here. And so now we're going to get a fair amount of rain in this transition zone. That's the Sahel. In the Sahara Desert, the subtropical high moves up shift the so moves up north. So it shifts the desert a little bit more north. And that's why you see such a large Sahara Desert region. And here, this subtropical high shifts slightly north as well. And this creates a slightly bigger desert forest down there in the southern hemisphere. Now what about the wintertime? Well, in the wintertime, the exact opposite is going to happen. Now we know here that the ITC is going to shift southbound. And it shifts southbound because now we know that the sun is going to be above the tropic of Capricorn or 23 and a half degrees south latitude. So now we get more rain down here. This subtropical high shifts a little bit southbound as well. And it creates a semi desert region around there. But also notice now, the subtropical high, which created the Sahara Desert also moves southbound and it encroaches upon the Sahel. So here in the wintertime in the Sahel, it's going to be pretty dry. In the summertime of the Sahel, it can actually be a little bit wet. So that's why we have the African transition zone. That's why we have the Sahel. We've got periods of time. We have a season when it'll be dry. We have a season when it'll be wet. Whereas in the Sahara Desert, it's pretty much dry all the time. And the tropical zone here is pretty much wet all the time. So now here's the question. If we know that this creates the Sahel, why is it that we see droughts in the Sahel? It's not far fetched every now and again to turn the TV on and see people, for example, in Ethiopia, starving or in Somalia, starving. And that might have less to do with political reasons and more to do with environmental reasons. What tends to happen periodically in the African area, especially in the Sahelian region, is that when the ITCZ shifts seasonally, sometimes the shifting of the pressure belts stall. And they stall due to other environmental reasons that we won't get into in this video. So you can have a period of time where the ITCZ shifts southbound in the summer, excuse me, in the wintertime. And the high pressure belt of the subtropical high is sitting on top of this region. And then during the summertime, when it's supposed to shift away, and when this region is supposed to get low pressure and some moisture, it doesn't. The earth is still revolving around the sun. The pressure belts are still moving, but not on this part of Africa. And because of that, if that high pressure cell stalls over the Sahelian region, then you can see drought. And it's not unheard of to have years and years of drought in the Sahelian region if this pressure belt actually stalls. Again, why it stalls is a question that we'll say for another day. Okay, that's it. That's the Sahelian region. It's an incredibly interesting place. It's fascinating and a lot of rich history there as well.