 Hello everyone. So today I'd like to talk about Santa Ana winds. Santa Ana winds are considered catabatic winds and catabatic winds is Greek for flowing downhill. So you already know that the origin of this wind is going to be at some high place and this wind is going to flow downhill and it's going to pick up some speed as it goes and potentially get a little bit warm and dry. So before we even talk about Santa Ana winds or where they occur and ultimately why they occur I'd like to talk about a couple of concepts first. One concept is going to involve adiabatic processes and the other concept is going to talk about relative humidity. Now I've already discussed this in previous video but I want to bring it up again because it's really pertinent to what we want to talk about today and then after that we're going to talk about a third concept which is really important for Santa Ana winds and that's something called the conservation of mass flow. So the first concept involves the 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 then 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 is 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 now that you 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 then 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 it's 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 right 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 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 right we can find those different places in the world but just according to this right now what we are doing in this thought 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 right 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 a hundred percent 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 percent 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 is dropped and is dropped because the temperature is dropped when we hit relative humidity of 100 percent that's going to occur at a particular temperature the temperature at which relative humidity reaches 100 percent 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 percent okay so the last concept I'd like to talk about before we get into the Santa Ana winds is something called the conservation of mass flow and the best way to describe the conservation of mass flow is with a simple analogy let's say that you're going outside and you're either washing a car you're watering the lawn or you just want to spray somebody in the face with water so what do you do you take your hose water is running through the hose and in order to speed the flow of the water that's coming out of the hose you put your thumb over it and then all of a sudden the water just starts spraying faster and faster with a ton more force why is it doing that well it's doing that because of the conservation of mass flow and this is what the conservation of mass flow says if I have a hose like this I've got a certain amount of pressure and this pressure is pushing a certain amount of volume through the hose so the amount of volume going through the hose is based upon the pressure pushing on it and then the size of the cylinder itself and that'll essentially give me a volume or we can talk about mass as well a mass of water coming through the hose the conservation of mass flow says the following that if I cut off the exit of this nozzle and let's say I cut it in half then the pressure is going to increase at such a level that the same exact amount of water is going to flow out of that hose that's the conservation law so the only way that the same amount of water can flow out of that hose is if that water flows out much faster so again the same amount of mass has to flow out of that hose whether I put my thumb over the hose and cut off the exit or whether I leave it wide open here the water is just falling out slower but it's the same amount of mass as the water that's shooting out of the hose over here same amount of mass is just coming out at a different velocity that is the conservation of mass flow so the next question is what does this have to do with anything in terms of the Santa Ana winds well we'll get to that right now so the Santa Ana winds we know is fast flowing air originating from over here which is the great basin of the united states of america the great basin is upwards of 14,000 feet of elevation so it can get pretty high and right around fall spring winter especially in december we get a high pressure cell that develops over the great basin now when we have a high pressure cell air is going to flow out of the high pressure and flow toward low pressure and due to the coriolis effect the air is going to get veered to the right of initial motion in the northern hemisphere and so we get this nice pinwheel effect of flowing air coming from the great basin now as the air flows toward the sea and it picks up speed this essentially are the Santa Ana winds so the Santa Ana winds again they originate from the great basin they flow out of the great basin in this nice counterclockwise fashion and by the way this is called an anti-cyclone and then the air itself flows toward the sea and as it flows toward the sea is fairly dry and it can get pretty toasty so now of course we need to ask a few questions why is it toasty and why is the air flowing so fast so in order to really understand this let's take another look at this whole area in north america but we want to take a look at sort of a side view of it so here's the pacific ocean and here's some landmass and here's the great cascade mountain range which we'll put here then we'll go inward and then here's the great basin right so this is just a rough schematic not to scale probably not my greatest artistic effort but it'll do the job so here's gb the great basin here's c for the cascade mountain range let's put our high pressure cell right there and so we can already see what's potentially going to happen to this air the air from the great basin from this anti-cyclone is going to start flowing toward the sea and when it does it starts flowing downhill this is our definition of the catabatic wind it is the wind that is literally flowing downhill as it flows downhill it's going to pick up speed and it's simply going to pick up speed because it's almost like an avalanche of air flowing but that's not the only reason why it picks up speed as this air flows it's going to find its way through the cascade mountain range if we take a look sort of at a front face picture the mountain range is looking like this and so within this mountain range you've got valleys cuts and crevices and the air coming from back here it's going to flow through these different valleys and crevices as it's going toward the sea now when it does now you got to think of the conservation of mass flow remember with the whole analogy of taking the hose and sticking your thumb over the hose and the water shoots out fast in order to maintain that conservation of mass flow the same is true here with the santa anna winds so as the winds are flowing through this mountainous zone the air is getting pinched and as it's getting pinched it's going to start to move even faster so now we've got fast flowing air coming from a downhill flow of the air and it's increasing its speed because the air is getting pinched in the mountainous zone here as well so now we've got that nice fast flowing movement of air so now we know why it's fast the next question is why is it dry well it's dry because again the air is flowing down and remember when we flow air down when we push air downward there's going to be adiabatic contraction so the adiabatic contraction is going to squeeze the air together the temperature of the air is going to increase because of that in addition to that the air is going to get more dense the capacity of the air to hold moisture is going to increase and because of that the relative humidity is going to go down which now makes the air dry so santa anna winds dry fast flowing air and in places like southern california this can be pretty bad if you have a santa anna flow that's occurring in winter time well that means you've probably just had a relatively dry season in the summer and in the fall and so now if you've got a lot of dry dead brush and then you have a dry fast flowing air that flows over this dry dead brush that can actually lead to spontaneous combustion and lead to things like forest fires you know there's a lot of octobers as well here in southern california where you got to be on the ready just in case a forest fire occurs you got to be on the ready to evacuate because the santa anna winds can promote those type of fires okay that's it well there is one final question does this occur in other parts of the world and the answer is yes of course it that's why catabatic winds can be found in different parts of the world all right that's it for this video and i will see you on the next one