 Rain Shadow effect. What is it? What causes it? That's what we're going to talk about here. So what is a rain shadow? Well, let's say we've got a mountain range and we've got air flowing toward the mountain range. We would call this side of the mountain the windward side, and as the air continues to flow over the mountain and go down the other side, we would call that side the leeward side. The windward side of the mountain is going to have some cloud cover and rain. The leeward side of the mountain will be dry. The question is why? And by the way, this is going to get even more severe the higher the mountain range that you have. So if you look at, let's say Mount Everest in the Himalayan mountains, 29,000 feet high, you're going to have some serious rain shadow effect. Sierra Nevada mountains in North America, you've got some pretty high mountain ranges there as well. Big rain shadow effect, Rocky Mountains, big rain shadow effect, Atacama, South America, big rain shadow effect. So again, anywhere where you've got large mountainous zones of the world, you're going to have big rain shadow effect. What I generally like to do when I talk about the rain shadow effect is just give an example problem with numbers on the mountain. And when you see the numbers on the mountain, it'll make all perfect sense. But before we get into that, we have to talk about a couple of concepts first. So the first one involves a helium balloon. So here you are, you got yourself a nice big balloon, and it's full of helium. And so here we go, we got a bunch of helium atoms inside of this balloon. Now you'll notice that if you're standing on the ground, because the balloon doesn't take you away, and you hold this balloon, the balloon is not expanding or contracting, the balloon is in equilibrium. And the reason why the balloon is in equilibrium is because the helium inside the balloon is pushing outward at the same exact pressure that air pressure is pushing inward. So the balloon stays in equilibrium. Of course, the next question is, well, why is the helium balloon lifting up into the sky at all versus a balloon that you blow up with your own air? And the answer is helium is a lightweight gas. And so the helium weighs less than the surrounding air, even with the rubber balloon. So the helium itself wants to lift up into the sky. Now let's say for sake of argument, you release the balloon. So now the balloon actually does lift up into the sky. And as it lifts up into the sky, the balloon is actually going to expand. So the higher it goes, the balloon will get bigger and bigger and bigger until eventually it might just pop. And the question is why? And the answer is your helium atoms inside of the balloon, no matter where the balloon is, is pushing out at the same pressure it was pushing out down here. But what's changed is the air pressure pushing in is becoming less and less. And that's because there's less and less air that's now surrounding the balloon. And because there's less air surrounding the balloon, there's less air pressure. So there's more pressure pushing outward than there is pressure pushing inward. And so we see this nice expansion of the balloon. Now if I were to stick a thermometer inside of the balloon and try to measure the temperature inside the balloon, what's going to happen is the temperature inside is going to become less and less and less. The temperature drops as the balloon is lifting up into the sky. And it's dropping in temperature not because any heat is being lost by the balloon, but rather the helium atoms or molecules are expanding. They're becoming further apart from one another. And because they're further apart, there's going to be less collision of the helium atoms and molecules inside of the balloon itself. So less collision means that the temperatures are going to drop because you just simply don't have that energy transfer anymore. This process by the way is called adiabatic expansion. And so in the adiabatic expansion process, the temperature of our parcel of air or helium in this case drops in temperature not because we're removing any heat from the system but simply because we are expanding the parcel of air itself and we're getting less molecular collision. So that's the analogy. So let's say that we're down here and we actually have a parcel of air and this parcel of air has a bunch of air molecules in it. We're talking like oxygen, nitrogen, stuff like that. By the way, you might be asking yourself, well, how can you have a parcel of air in air? And I'll just answer that by asking you, have you ever been in a swimming pool before? The water is relatively warm and all of a sudden you get this push of cold water that strikes you. Yes, you have and what you felt is a parcel of water moving within the pool and that parcel of water might have a different temperature and might have a different density. Well, the physics of air and the physics of water are pretty much the same. So you can be outside and you can have a parcel of air that happens upon you and that parcel of air will have a different density and temperature than the surrounding air. So this is not inconceivable at all. So let's say that we have a parcel of air down here, let's say around sea level and for one reason or another, this air lifts up into the sky. Well, as the air lifts up into the sky, it will expand for the same exact reasons that the helium balloon expanded. The pressure outside is getting less whereas the pressure inside stayed the same. So the balloon continues to rise or excuse me, the air parcel continues to rise. The air molecules are further apart now because we continually have this expansion. And if I were to measure the temperature inside the parcel of air, the temperatures are dropping. And again, that is because of adiabatic expansion, which we can also call adiabatic cooling. Now by the way, the opposite is true. If I push this air downward, the air will start to contract. And it will contract because now the pressure outside begins to increase relative to the pressure inside pushing outward. So the exact opposite effect occurs and the air will contract. And if the air contracts, then the temperature inside the parcel of air now begins to increase. So again, in this case, we're not adding any heat into the system. We're just squeezing the air molecules together. And when we squeeze them together, there's going to be more collision of air molecules. So there's more of a exchange of energy. And so now the temperature begins to increase. That is called adiabatic contraction or adiabatic heating. Okay, so that's one concept. The next concept that we want to talk about before we get into the rain shadow effect is something called relative humidity. Relative humidity is basically the amount of water vapor that you have in the air divided by the amount of water vapor that you can possibly have in the air, which we'll just call capacity. So capacity is the amount of water vapor that you can have in the air. Water vapor, the actual number is the actual amount that you do have in the air. And we multiply that by 100 so that we can get a percentage. So let's say you've got a parcel of air. And let's say that the capacity of this parcel of air is like 10 water vapor molecules. But as you can see in this little diagram, I only put five. So my actual water vapor content is five. The capacity is 10, which means that my relative humidity would equal 100%. So once again, relative humidity equals content of water vapor divided by capacity times 100%. Now of course, the physics behind this and the chemistry behind this is much more complicated. But we're just going to use this as a simple example. Okay, now here's one thing that you need to know. Capacity and temperature are directly related. So if the temperature of air increases, then the capacity of that parcel of air increases as well. But if the temperature of the air decreases, then the capacity of that air to hold moisture decreases as well. So temperature and capacity are directly related. But notice capacity and relative humidity are invertly related. So if the temperature goes up and the capacity goes up, then the relative humidity will go down. If the temperature goes down and the capacity goes down, then the relative humidity goes up. What does that mean? That means that typically hot air is dry. Cold air is wet. Is that the rule? No! Please don't write that down in your notebook and say this is the rule. That's not the rule. Because we'll find places in the world where air that is hot can be wet. We'll also find places in the world where air that's cold can be dry. We can find those different places in the world. But just according to this right now, what we are doing in this analysis is that if the temperature of a parcel of air goes up, its capacity to hold moisture will go up. And if that happens, this relative humidity will go down. So I'm only talking about a singular parcel of air. If I change the temperature of that parcel of air, I can change its relative humidity. And the same is true if I lower the temperature. If I lower the temperature of a parcel of air, and if its capacity goes down, then I can increase its relative humidity. So I can take a parcel of air, I can chill it, and then I can make that relative humidity go up. Now why is this important? It's important for the following reason. If relative humidity gets to 100%, then the air is saturated. And if the air is saturated, then we can get cloud cover, we can get condensation, we can get fog, we can get potential rain. So this is how we make clouds. We make clouds by lifting air up into the sky, lowering the temperature so that we lower the capacity. And when we lower the capacity, we increase the relative humidity. And if we increase that relative humidity to 100%, we can make a cloud. That's it. Now keep this in mind. Remember, relative humidity is invertly related to capacity. So when relative humidity is going down, that's because the capacity of the air to hold moisture has gone up. But remember, capacity of air to hold moisture is directly related to temperature. And if we have a capacity going up, that means that the temperature has gone up. Flip that. If we want to increase the relative humidity, then we know that the capacity has dropped and has dropped because the temperature has dropped. When we hit relative humidity of 100%, that's going to occur at a particular temperature. The temperature at which relative humidity reaches 100%, that is called the dew point temperature. So dew point temperature is the temperature where we're going to saturate air or air reaches a relative humidity of 100%. Alright, I think you're ready. I think you're ready for the rain shadow effect problem. So let's do it. So let me draw a mountain. There it is. And on this mountain, I'm going to have city A. And on this mountain, I'm going to have city B. City A is at 0 meters of elevation or sea level. City B is going to be at 0 meters of elevation as well. Here we've got air flowing into city A. So this is our windward side of our mountain. And over here is the leeward side of the mountain. So the air is going to go up the mountain, down the mountain, over there to city B. Let's also put some numbers on our picture. So here at city A, let's say that the temperature is 30 degrees centigrade, which is 86 degrees Fahrenheit. That's pretty toasty. By the way, let's put the peak of the mountain at 5,000 meters. Okay, so now what we want to do is follow the path of air up the windward side of the mountain. And as we know just from our previous discussion, as the air lifts up into the sky, it's forced upward because of the mountain. And by the way, this is also called an orographic process. An orographic is in reference to the mountain. So we've got an orographic process occurring or orographic lifting. So the air is being pushed up the windward side of the mountain. This is an orographic lifting process. And if the air lifts up into the sky, it gets colder and colder and colder. Remember, as the air gets colder, the relative humidity goes up because the capacity of air to hold moisture goes down. Eventually, we will hit the dew point temperature. And when we hit the dew point temperature, relative humidity is going to be 100%. The level where this occurs on the mountain is called the LCL or the lifting condensation level. So the lifting condensation level or the LCL is where we've got a cloud. And let's just say for sake of argument that in this picture, the height of the LCL is at 3,000 meters. Okay, so here's our picture. City A, 0 meters, 30 degrees centigrade. The lifting condensation level is at 3,000 meters. The peak of the mountains at 5,000 meters. City B on the leeward side, 0 meters. We don't know the temperature yet. First thing we want to do is figure out the temperature of the lifting condensation level. Let's clear the board. We'll draw it again to make it all pretty. Here's City A at 0 meters. City B also at 0 meters. Remember the temperature at A, 30 degrees centigrade or 86 degrees Fahrenheit. Here's the LCL at 3,000 meters. We want to know what the temperature at the LCL is. We basically want to know what the dew point temperature is. Now there's a variety of ways to calculate this, but we're just going to go way simple. There is something called the adiabatic lapse rate. Remember in our previous discussion that we just had, I said that when air expanded into the sky, it is expanding and getting lower in temperature and that was called adiabatic expansion or in that case adiabatic cooling. So adiabatic is the process of changing the temperature of our parcel of air without adding or removing heat to the system. So we know as the air is lifting up this mountain and up into the sky, it is cooling off. And the rate that it's cooling off is called the dry adiabatic lapse rate. That's a mouthful. So we're just going to call the dry adiabatic lapse rate the dull R. And the dull R is 10 degrees of centigrade change for every 1,000 meters of elevation. That's it. That's the dull R. So that's sort of the average rate of temperature drop as you expand air, dry air and dry air is air that has a relative humidity of less than 100%. So it's just simply not saturated yet. So here we're going from 0 meters to 3,000 meters. So we have a 3,000 meter elevation change. So the temperature change from sea level to 3,000 meters is going to be the dull R times 3,000 meters which equals 30 degrees centigrade. So if the temperature down here at sea level is 30 degrees when it goes 3,000 meters high it'll drop by 30 degrees centigrade which means that the temperature at the LCL is 30 minus 30 or 0 degrees centigrade which is 32 degrees Fahrenheit. That's the freezing point of water. So here at the lifting condensation level we have a temperature of 0 degrees centigrade so not only do we have cloud cover but we've probably got some ice crystals developing in this cloud as well and we might even potentially get some snowfall, right? As the air continues to lift up into the sky it's going to continue to cool off because it's still lifting up into the sky but guess what? The air is not dry anymore. The relative humidity is 100% and as it continues to lift up into the sky it might even get a little bit higher than relative humidity of 100%. So it's cooling off but the air, the chemistry and the physics and the air is now different because the air is saturated. So the air will cool off but it'll no longer cool off at the dull R which is the dry adiabatic lapse rate. It'll actually continue to cool off at the wet adiabatic lapse rate or wall R. I know, funny. So the wall R is 5 degrees centigrade per every thousand meters. So here the lifting condensation level is at 3,000 meters. The peak of the mountain is at 5,000 so we've got 2,000 more meters to go. So we want to know what the temperature drop is. It's just simply going to be the wall R, 5 degrees centigrade per every thousand meters times 2,000 more meters to go. That equals 10 degrees centigrade drop. So if the temperature here at the lifting condensation is zero then the temperature at the peak of the mountain is zero minus 10 degrees centigrade or negative 10 degrees centigrade. And negative 10 equals 14 degrees Fahrenheit. So at the top of the mountain, at the top of this 5,000 meter mountain the temperature is below freezing by 10 degrees centigrade. So it's cold. It's pretty icy. This is everything that's occurring on the windward side. The air is lifting up into the sky. We reach the lifting condensation level. We reach dew point temperature. Relative humidity is 100%. We get some rainfall. The air continues to lift up into the sky. It continues to cool off but it cools off according to the wet adiabatic lapse right now. And so now the temperature at the peak is negative 10 degrees centigrade. Okay, that is the windward side. What about the leeward side? This is where it gets interesting. So let's draw this picture one more time. I'll save this stuff. We've already done it. We'll just take a look at now as the leeward side. So on the leeward side we are at 5,000 meters elevation to start. We know the temperature is negative 10 degrees centigrade or 14 degrees Fahrenheit. We know over here city B is at zero meters. So the air will drop and it's going to drop 5,000 meters of elevation. And we also know because the air is being forced downward, we're going to have adiabatic contraction. And adiabatic contraction means that the air is heating up. It's not cooling off anymore. It's not expanding. The air is squeezing together. It's getting more dense. And so now it's heating up. The question is at what rate does it heat up? First intuition would say, well, if the air is wet up here, shouldn't the air heat up according to the wet adiabatic lapse rate, even though intuitively that might make some sense, that's not what's happening. As long as the air is dropping, as long as it's going downward, it will heat up always according to the dry adiabatic lapse rate or dolar of 10 degrees centigrade per every thousand meters. So we start here at 5,000. We're going to zero. The temperature change on the way down is 5,000 times 10 over 1,000 is 50 degrees centigrade. So if the temperature up here is negative 10, that means that the temperature down here is negative 10 plus 50 or 40 degrees centigrade, which equals a whopping 104 degrees Fahrenheit. Let's compare that again to city A. Just to remind you, city A was at 30 degrees or 86 degrees Fahrenheit. So that's it. Here's the leeward side. Air is going down. Here's the windward side. Air is going up. We're not adding heat or removing heat to the system. Nevertheless, the temperatures are changing with the air as the air is moving up and down. We went from 30 degrees centigrade to 40 degrees centigrade because of the mountain. Not only that, the air over here is wet and the air over here is dry, because as the air is being pushed down, we're no longer going to be at relative humidity of 100 percent because the air starts to heat up in temperature and remember when the air heats up in temperature, the capacity also goes higher and if the capacity goes higher, that means relative humidity goes down. So as we go down the mountain, the air gets drier and drier and drier and you see that in the relative humidity. Now you go to a place like Death Valley in California and Death Valley is actually below sea level. So on its leeward side, the air continues to get pushed down. It gets even drier and hotter. That's why it's called Death Valley because a whole bunch of stuff just doesn't live there. It's just really dry and hot. And again, you go to other places of the world like the Himalayan Mountains. Pretty insane rain shadow effects because those mountainous zones are so large and vast. All right, that is the rain shadow effect. But in this discussion, we not only talked about rain shadow, we talked about adiabatic expansion, adiabatic contraction, relative humidity. That's it. Thank you and we will talk to you next time.