Howdy everyone and thank you for continuing on with module 8. In this video, we'll continue exploring some of the basic processes that occur in our atmosphere. Starting with atmospheric circulation, which is not only important when we study the Earth's climate in our next module, but is super important when we start be beginning our study of inclement weather because this is ultimately what's going to control the weather patterns we see on planet Earth. Atmospheric circulation is ultimately powered by sunlight and only about one part in 2.2 billion is actually intercepted by our earth. But that tiny part of 2.2 billion actually amounts to on average 7 million calories per square meter per day at the top of our atmosphere. That is an incredible 17 trillion kilowatts for Earth as a whole or 23 trillion horsepower. That is a lot of energy. And again, this is what's going to power our atmospheric circulation. Now, every day we have 7 million calories per square meter of incoming shortwave solar radiation, also known as light, reaching the Earth. On a global basis, about 51% or half of that incoming energy gets absorbed by the Earth's land and water surface, as seen here in Earth's heat budget. The rest is either absorbed by clouds in the atmosphere or gets reflected back into space by the atmosphere, clouds, and the surface of the Earth. Now that 51% that is absorbed by the land and oceans then gets converted to longer wave infrared radiation or heat and then transferred into the atmosphere by conduction, radiation and evaporation. Now the atmosphere just like the land and oceans eventually radiates this heat back into space in the form of longwave infrared radiation. And over long periods of time, the total incoming heat equals the total outgoing heat, which makes the Earth in thermal equilibrium. However, over shorter periods of time, we do have changes in heat balance called global warming, which we'll get into later this semester in the next module actually. Although the heat budget for the earth as a whole over long periods of time is in balance, the heat budget is different for different latitudes. That's not in balance. And this gives rise to atmospheric circulation. As shown here, the solar heating of the Earth does indeed occur unevenly and it varies by latitude. Notice how the sunlight is spread over a greater area near the poles as opposed to the tropics. Here I'm just showing the concentration of light over the Earth using a flashlight as an example. The concentration of light is greatest at the equator because it's at a 90° angle. However, closer to the poles, it's about a 30° angle, resulting in the light rays being dispersed over a larger area, about two times as large, resulting in less concentrated energy. Now, this makes sense knowing that the poles are cold and the equator is warm. Now, we also know that there are variations of solar heating, not just with latitudes, but with seasons as well. Now here I have shown the average annual incoming solar radiation which is the red line in this graph and that's being absorbed by the earth along with the average annual infrared radiation which is the blue line. The blue line is what's being emitted by the earth. So the red line's being absorbed, the blue line's being emitted. And you could see here that the polar latitudes lose more heat to space than they gain. And tropical latitudes are gaining more energy. They're gaining more heat than they're actually losing. Only at about 37° north and south does the amount of radiation received equal the amount that's lost. So then why doesn't the polar ocean completely freeze solid if it's losing more energy than it's gaining? And why aren't the tropic oceans not boiling away if they continually receive more energy than they're emitting? Well, the main reason is the water itself. Now, water has a great thermal properties that make it an ideal fluid to equalize the polar tropical heat imbalance by moving huge amounts of that heat between the poles in the tropics. And water's heat capacity moves heat pward in ocean currents, which we'll talk about in a couple modules. But water's exceptionally high latent heat of vaporization means that the water vapor which is in our atmosphere actually transfers much more heat per unit of mass than liquid water. So that means masses of moving air within our atmospheric circulation actually accounts for 2/3 of this poleward transfer of heat and the ocean currents account for the other third. Again, it's this uneven solar heating that results in our large scale atmospheric circulation. Now, that brings us to convection currents. You might remember how convection works from maybe your high school science class or maybe you've taken another college physics course already on campus. But convection currents is just telling us how warm air rises, warm things like to rise, and cold things like to sink. So, we have this convection current where we have warm air. It's rising and as it cools, it begins to sink again, creating this nice circulation. It's a convection current and this is occurring all over our planet. Now, remember plate tectonics earlier on in the semester? Well, plate tectonics is thought to have been controlled by convection that's occurring in the Earth's mantle. Except for here we're going to focus on the convection occurring in our atmosphere. Now there are three atmospheric circulation cells or convection currents that circulate the earth in each hemisphere. We call them Hadley cells, feral cells and polar cells. Now Hadley cells are the atmospheric circulation cell nearest the equator in each hemisphere. So you'll see one in the north and in the south. And air in these cells rises near the equator. Why is that? Well, we have strong solar heating there. So hot air likes to rise. A lot of solar heating there. So we have a lot of air rising and then it falls eventually because it cools as it rises in the atmosphere. And that falling actually happens around 30° latitude. Now, feral cells are the middle atmospheric circulation cell in each hemisphere and air in these cells rises at about 60° latitude and fall at 30° latitude. And then our polar cells are the atmospheric circulation cells centered over each pole. Now, here is showing the three circulation cells, the Hadley, the pharaoh, and polar cells, but just a cross-sectional view. So here we're looking at our equator and moving north. So this is 30° north, 60° north, and this is our north pole just to center yourselves here. So here's our equator. So what's happening at the equator? We have a lot of incoming solar radiation. So a lot of air rising and the polar ferro and Hadley cells. You can see here in a little bit more detail, but notice that the troposphere is actually higher in the tropics than it is at the poles. And we have these things called jet streams which are rivers of fastm moving air that occur at these cell junctions. So in between the polar and fereral cell we see our polar jet stream. Again this is a cross-sectional view. So this polar jet stream is going to go all around the earth around 60° north. And we know it it fluctuates and moves. Same with our subtropical jetream occurs between our pharaoh and our hatley cells. Now that uneven solar radiation that reaches our planet is what's controlling these circulation cells. But our planet is also rotating and that rotation of the earth causes something called the corololis effect. And the rotation of the earth the corololis effect also is going to influence the movement of air in these atmospheric um circulation cells. So you can see here in the northern hemisphere we have our northeast trade winds that's going to bend the winds in this direction. Same with our southeast trade winds. And then you'll see between 30 and 60° latitude. These are our prevailing westerlys and our prevailing westerlys in the other hemisphere and so forth. So this is actually playing a role in the direction of air movement. Not just the uneven solar radiation, but also the fact that our planet is spinning. Now, here's a probably better figure to show you this atmospheric circulation that's taking place. So there at the equator, lot of warm air rising. This leads to low pressure because the air is going up. It's not pressing against our our planet. So, it's got low pressure and it's going to rise. It's going to cool and eventually sink at around 30° latitude. So, that sinking air is going to cause high pressure systems because the air is pressing on the Earth's surface. And then you see the corololis effect kind of bending or or refracting the movement of our air. So, this is the movement that most of our weather patterns take here. And you'll see something called the ITCZ right around the equator. We're going to talk about that eventually. That's the intertropical convergence zone. That's almost a continuous layer of clouds that exists on our planet. And then you'll see that our polar easterlys, our high pressure center zones, we have a lot of sinking cold air at the poles is causing the air to move. We have another layer of low pressure right around 60°. And I'll let that play one more time so you can see those how those atmospheric circulation cells are playing with these weather systems and these air pressure centers and how the coralis effect is also changing the way that moves and we'll get to it eventually. But low pressure you could see is associated with cloud cover, precipitation. High pressure shown in yellow here is associated with fair weather, dry weather. Okay, moving on. So, here's just showing those typical pressure cells with some of the terms that you might hear um that are used to describe some of them in your weather reports. But here you can see that ITCZ that exists right around the equator, that intertropical convergence zone. This is a zone of low pressure, equatorial low pressure. It exists between 10° north and 10° south. And because this is a low pressure system, because we have a lot of air rising, this is going to promote warm and wet conditions. So you probably know the equator is full of warm wet places, tropical places, right? And this is exactly why a lot of air is rising that's promoting that cloud cover and precipitation plus a lot of solar heating which makes it very very warm. And that's our intertropical convergence zone. Then we have our subtropical high pressure zone which occurs between 20 and 35 degrees north but it also occurs in the south too. I don't know why I just have it in the north but you can see it occurs in the south. Um and high pressure systems we have a lot of air sinking. So all that air that's rising at the equator. It's going to cool and sink at about 30° and that sinking air is going to press the air down into the earth's surface and cause high pressure. And as we'll soon learn, high pressure does not promote cloud formation. So this is usually clear, sunny skies and no clouds, no precipitation. So dry. And we're still pretty warm at 30°. So this is going to promote hot, dry conditions. And as you'll soon see, a lot of the Earth's deserts actually occur in this subtropical high pressure zones. You might also see them referred to as the Azor's high. Then we have our subpolar low pressure centers that occurs at about 60 degrees north. Technically it occurs in the south also, but things are a lot more prominent in the in the northern hemisphere. We have a lot more land mass up here. So this promotes cold and wet conditions. Why is that? Well, we got low pressure. Again, this is where we have a lot of air rising. Anytime you have air rising, that's going to promote cloud formation and precipitation. So that's wet. 60° is fairly cool. We're getting closer to our pool. So instead of instead of warm and wet like our tropics, we get cold and wet conditions in our 60° latitude. And this promotes a lot of forest formation. So you might see a lot of forests around 60° north, which you do. And atmospheric pressure is or atmospheric circulation is to thank for that. And you might also hear them referred to as Icelandic lows. And then finally, finally, we have our polar high pressure centers that occur at each poles. They occur about 60° to 90° north. Now, high pressure, a lot of air sinking. That's what's causing the pressure to be high here. And when we have sinking air, we do not get cloud formation and precipitation. So, dry conditions here, but we're at the pool, so we're also not receiving a lot of heat. So, cold and dry. This is why we have the Arctic and the Antarctic, which are frozen deserts. And you'll also see them referred to as polar highs. Now, here is our intertropical convergence zone. This is a map showing average rainfall for the last 30 days where our warm colors is where we see the most precipitation. And you could see very clearly in the data lots of precipitation right around the equator. That's because this is our intertropical convergence zone. This is where we have a lot of warm air rising which promotes cloud formation and precipitation. So this ITCZ which bounces north and south of the equator depending on which season we're in is literally a constant cover of clouds. So, anytime you see an image from the space station or a satellite of our planet, you will see right around the equator a band of clouds, a band of white that always exists because of this intertropical convergence zone. And you will never be able to unsee it. I promise. So, here's a picture of planet Earth. You see that band of clouds? That's where we have all that warm air rising, forming clouds and precipitation or ITCZ. And if you didn't believe me, here is another image. And you can also see our northeasterly trade winds bringing weather patterns in this direction and our southeasterly trade wings bringing weather in this direction converging at this intertropical convergence zone. We have a convergence of weather here. And just for completeness, a completely different image of planet Earth. But again, you can see the ITCZ. I promise you guys, you'll never be able to unsee this. Now, and to finish up this video of atmospheric circulation, this is just showing the distribution of deserts on our planet. So, we saw the constant cloud cover that occurs on the equator. Well, we have high pressure around 30° where all that air starts to sink again. And you can see a lot of the Earth's deserts occur right around this 30°ree mark. And that's exactly why we do not promote cloud cover or precipitation where we have high pressure weather. So atmospheric circulation is also controlling a lot of the geography that we see. And with that, I will meet you in our next um lecture video where we'll talk more about what exactly this air pressure means. What is it? How do we measure it? And so forth. I'll see you there.