 Let's talk about the Indian monsoon. Contrary to popular belief, monsoon doesn't mean big rainstorm, rather monsoon means a reversal of wind. So for example, in the summertime across the Indian subcontinent, air is flowing from the Indian ocean and is going across India. And when it does, it's going to bring that warm, moist air from the Indian ocean across India. And that's going to produce a bunch of rainfall, possible flooding on the Ganges and other locations. So that is in the summertime. But in the wintertime, the wind is reversed. So now the air is going to be flowing across India and going into the Indian ocean. And when that occurs, there's going to be a lot of dry air across India. So now the question is why? And let's explore that question. What I'd like to do is draw 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 in around the sun and see how and why the ITCZ 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 nice 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 winter time. In the winter time, 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 winter time. Let's put in here a picture of the summertime. So in the summertime, the sunlight, 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 Convergence Zone, is a low pressure system. Okay, now let's put all of this in perspective. So I've got my map up again. And I want to draw on this map the tropic of Cancer. So this is T Cancer. And the tropic of Cancer, as we know, is right around 23.5 degrees north latitude. And then the equator is right around here at zero degrees of latitude. So in the summertime, remember, the Intertropical Convergence Zone is shifting northbound. The subtropical high pressure belt, which is up here, here's the subtropical high at around 30 degrees north latitude, that's shifting northbound. And then the subtropical high pressure belt, which is down here at around 30 degrees south latitude, that's shifting northbound as well. So in the summertime, everything is shifting northbound in our picture. So let's put a summertime drawing here. So again, in the summertime, our Intertropical Convergence Zone or ITCZ low pressure is going to be right there across India. The subtropical high high pressure zone in the south is going to be around here. So here's our high pressure zone. That means that the air flow in the summertime is going from high pressure to low pressure across India just like this. And when that occurs, you're going to bring all that nice warm, moist air from the Indian Ocean. You're going to purge it across India. It's potentially going to run into some rough terrain, including the Himalayan mountains, which are around here, going to create a lot of rainfall as well, potentially flood the Ganges River, the Indus River, and some of the other rivers in the area. So that would be the summertime monsoon across India. So remember summertime wet. This reverses in the wintertime. So now let's take a look at the wintertime picture. Let's draw this again. Here is the Tropic of Cancer, right at around 23 and a half degrees north latitude. That's our high pressure zone. And here's the equator right here at zero degrees latitude, which is our low pressure zone. So in the wintertime, everything is now shifting southbound, which means this high pressure zone is going to be right here. It's shifted southbound. And this low pressure zone to the equator, it's going to be shifting southbound as well. So again, air is going to flow from high pressure to low pressure like this. So now it's flowing from India to the Indian Ocean, which means India itself is going to be rather dry. So Indian monsoon, summertime wet, wintertime dry. Now, if you have a really dry winter and you've got drought, when the summertime monsoon rolls around and you get some really heavy rain possibly that goes across India, imagine the potential flooding that could occur then because now the ground is really dry and you've got a hard baked pan. You're getting a lot of sunlight too because you're right around the tropics. So it can be a pretty devastating occurrence, these monsoonal rainy seasons. But that's it. Now, do we have anything similar in anywhere else across the world? Well, yeah, as a matter of fact, we do. If you look at North America, we've got the Gulf, the Gulf of Mexico. And we have a very similar thing that occurs in the Gulf of Mexico, but we have what's called a monsoon tendency. A monsoon itself is a complete 180 degree reversal of wind. A monsoon tendency is a reversal of wind, but not quite 180 degrees. So in the summertime, if you live in Texas, for example, or New Mexico or even Arizona, you're going to get a nice flow of air coming from the Gulf of Mexico. But in the wintertime that air is going to flip. And so now you're going to have dry conditions in Arizona, dry conditions in New Mexico, dry conditions in Texas. So we do see similar phenomena in other places of the world. Okay, that's it. Thanks a lot and I'll see you in the next video.