in this video you should learn about why the wind blows in the previous video we talked about the energy imbalance on earth and how the winds help to even out the differences in heat between the equator and the pole air flows because of differences in pressure from place to place around the earth let's unpack this idea a little bit we already discussed air pressure and said that air pressure was highest at the surface and lowest as we ascended into the atmosphere but what we didn't talk about is the fact that air pressure is not the same at every location on the earth there are some places where air pressure is relatively higher and some places where it's relatively lower this is related to those same differences in temperature that were caused by the different sun angles from place to place around the earth the atmosphere is constantly trying to even itself out and make the amount of air molecules the same in every place this is what causes the wind to blow in order to understand why the wind blows in the places that it does we need to define a couple of terms high pressure center and low pressure center a high pressure center is a location on earth's surface where the air pressure is relatively high compared to other areas and we depict it with a blue h on a weather map the low pressure center is a location on earth's surface where air pressure is low relative to other areas and we depict this with a large l on a weather map in order to understand why the wind blows we need to discuss some fundamental forces in the atmosphere remember the importance of a force from your physics classes if a force is exerted on an object the object will accelerate meaning its speed or its direction will change in this case the object that we are talking about are the air molecules in the atmosphere if a force is exerted on an air molecule it will start to move and the wind will blow the three fundamental atmospheric forces are one the pressure gradient force two the coriolis effect sometimes called the coriolis force and three the frictional force we will focus on the first two forces in this class the first force is the pressure gradient force or the pgf when we're thinking about pressure gradients we're talking about pressure differences from one place to another in the atmosphere a simple example might be a beaker of water that has a divider in the middle the water is higher on one side of the divider than on the other side which side has a higher pressure at the bottom of the beaker think about it for a minute and decide what your answer will be if you said that the higher pressure would be on the side of the beaker with higher water level you're correct the higher pressure that exists at this location is because there is more water exerting a force on the bottom of the beaker on the side that's filled higher what do you think will happen when the divider is removed most people usually guess that when the divider is removed the water will spill from the higher side to the lower side evening out the water level in the beaker this is absolutely correct and this is a great demonstration of the pressure gradient force the only difference is the pressure gradient force is invisible to us because we can't see the air moving all around us but it's the atmosphere's way of balancing out the pressure field causing the wind to blow always from high to low pressure just like in our beaker when we look at a map of global winds like this one shown here we see that winds don't actually blow straight from high to low pressure so let's look at the diagram for a minute and see what we can find first notice the latitude around the edge of the globe from 0 degrees or the equator up to 90 degrees north which is the north pole and we can see that there's low pressure found at the equator high pressure at 30 degrees north low pressure at 60 degrees north high pressure at the north pole if the pressure gradient force was the only force that was operating we would see wind arrows that pointed straight from high to low without curving so for instance the wind would blow straight from 30 degrees north to the equator without curving at all we see that the winds on the globe are actually curved with winds blowing from 30 degrees north to the equator with a curve to the right what's going on here well the answer is that we need our second atmospheric force which is the coriolis in this class we might sometimes call it the coriolis force because based on our perspective it will look like a force in a physics class you would never call this a force because it's simply an apparent motion that's due to the rotation of the earth this is because we are situated on the rotating earth and therefore we're rotating along with earth and so sometimes objects will appear to curve to us as observers who are also rotating an example of this that you would see in any physics class is the merry-go-round if you are sitting on a merry-go-round and you can try this at home if you want sit across from a friend and try to throw a ball to them on a non-moving merry-go-round this is pretty easy because usually a merry-go-round is small we can throw the ball easily and they'll catch it now here's the challenge start rotating the merry-go-round and try to throw the ball to the person sitting across from you you will find that when you try to throw it straight to them you are rotating but the ball is going to move straight ahead through the air as soon as it releases from your hand therefore will look from your perspective as if the ball curves and misses the other person when really you and the other person are rotating missing the ball which was moving straight ahead this is the coriolis effect in the atmosphere this means that air particles that are moving above the rotating earth are actually moving in a straight line but to us as observers from the earth it looks as if they are rotating because we are rotating underneath the atmosphere here are three really great videos that help demonstrate the coriolis effect i would recommend watching them so that you can get a better visualization of what's happening there are many more videos on the internet also spend some time looking around until you feel comfortable with this concept also if you would like more information about the math that supports the coriolis effect i have a really interesting information sheet if you ask me for it here's another example of the coriolis effect this picture of the globe shows three starting locations at 0 degrees north 30 degrees north and 60 degrees north indicated by the little purple line it also shows three destinations the destinations are to the east of the starting points along the same line of latitude if an airplane takes off and travels due east from each of those locations you would think that they would easily meet their destination but don't forget as the airplane is flying in a straight line the earth is rotating underneath them and therefore if they do not correct for the motion of the earth they will end up to the south of where they intended to go you'll notice from looking at this diagram the deviation is larger the farther toward the pole that you are so the 60 degree north location had a large deviation from their destination whereas the location on the equator really didn't deviate at all so this gives us our final information about the coriolis effect that we will use when determining which way the wind will blow first of all the coriolis effect increases with latitude meaning it's stronger toward the poles it increases with wind speed so if wind is moving faster it will be more impacted and it also can change with planet rotation speed we don't have to worry about that on planet earth because earth will always rotate at a constant speed on the time scales that we are discussing one important thing to note is that the coriolis effect is always zero at the equator the diagram shows what happens or which direction the coriolis effect points at the star so at the star if we have wind moving from south to north the coriolis effect will act exactly at a 90 degree angle to the right of the wind and so in this case since the wind is blowing from south to north if you take a 90 degree angle to the right the coriolis force will act to the east imagine you're walking along the wind line and you make a hard right turn at a 90 degree angle that will tell you which way the coriolis effect will be oriented in the southern hemisphere the coralis effect acts 90 degrees to the left of the wind instead and this is because the relative rotation of the earth is opposite in the southern hemisphere when we take the coriolis effect into account we can suddenly understand the global surface wind patterns that we looked at a moment ago let's go back to looking at our example between 30 degrees north and the equator we had said that the winds should blow directly from high to low if the pressure gradient force was the only thing to take into account however we see that the winds actually blow they start to blow from north to south but then they curve to the west a little bit imagine you're walking along that wind line what's happening is that you're actually curving to the right and this is because the coriolis effect acts at a 90 degree angle to the right of the wind in the northern hemisphere we see the same thing happen between 30 degrees north and 60 degrees north the wind if the pressure gradient force directs the wind directly from south to north but when the corollas effect is added that diverges the wind to the east a little bit causing it to curve throughout its motion this is how we can figure out which way the winds blow on earth so let's take a moment for you to test yourself and see if you can use the diagram of our surface winds to figure out which way winds blow at different locations so the first one on average which way do the winds blow at the equator number two on average which way do winds blow at 45 degrees north take a moment and use the diagram to see if you can figure out the answer pause the video hopefully you found that at the equator winds tend to blow from east to west or from northeast to southwest northeasterly or easterly winds on the north side of the equator if you put the that they were blowing from southeast to northwest that would also be true if you were talking about winds that were on the south side of the equator generally we we talk about the winds at the equator blowing from east to west and we call them our easterly trade winds the second question asks which way the winds blow at 45 degrees north this is where we live and you should have found that in general the winds blow from west to east westerly winds and actually the word westerly winds describes the winds in a lot in a large portion of the mid-latitude world that we live in let's take a side view now and look at the vertical circulation of the winds so this diagram is showing latitude on the x-axis so the equator is on the right side of the picture and the north pole is on the left side and you can see the places where pressure is low and high low at the equator high at 30 degrees north low at 60 degrees north high at the north pole just like in the last diagram but this time instead of looking at at longitude also we're going to look at altitude and we can see that at the equator the air is rising at 30 degrees north the air is sinking at 60 degrees north the air is rising and at the north pole the air is sinking this vertical circulation makes up our three cell model of the global circulation with the hadley cell between the equator and 30 degrees north the feral cell between 30 and 60 degrees north and the polar cell between 60 degrees north and the north pole originally scientists thought this circulation cell was all one cell but it turns out the globe is just too large to support one cell one cell and it becomes unstable and actually breaks into three pieces it's important that you can remember which way the winds blow in each of the three circulation cells why are we learning about the direction of the winds anyway we know that the winds help to transfer heat between the equator and the pole but what else about this is going to provide interesting for learning about global climate one of the big things that we're learning about is the idea of air pressure and it turns out that air pressure has a strong influence on the climate of a given location because air pressure is directly related to whether or not places will be rainy or whether they'll be sunny places that have high air pressure tend to have sinking air or air that's sinking down on top of them and the sinking air leads to clear sunny cloud-free and rain-free climates places that are located under low pressure will tend to have rising air which produces clouds and rain and wet climates if we change our map to look at just our low and high pressure zones on earth we see that they line up very nicely with where our major rainforests and deserts are located the band of low pressure between 0 degrees and 30 degrees in both the north and southern hemispheres which sometimes we call the tropics this area is characterized by low pressure meaning rising air and rain and should be a wet area the area around 30 degrees north is going to be characterized with high pressure in that subtropical location which will produce dry deserts etc etc remember high and low pressure are important because they produce the major climate differences between those regions high pressure causes sinking air low pressure causes rising air therefore high pressure causes clear skies where low pressure causes cloudy skies let's look at some examples near the equator we have the inter tropical convergence zone this is because that's the zone where the easterly trade winds are all converging on the equator if you look back to the slide showing the global winds you'll be able to see this very clearly when these winds all converge they have nowhere to go but up and we end up with rising air and low pressure at the equator because of that the equator constantly has a band of clouds and in this satellite image you see it looks like a belt strapping all the way around planet earth right on the equator that belt of clouds shows a location on earth that tends to be rainy all the time this is where a lot of our major rainforests are located or i should say our tropical rainforests are located on planet earth for example the amazon rainforest if we move to 30 degrees north and south we see that high pressure dominates and dry clear climates this fits in perfectly to where the location of many of our subtropical deserts are found the most notable being the sahara desert also the desert south of the southwestern united states australian deserts and many more as you can see on this chart deserts tend to be located near 30 degrees north and south because of that high pressure located there at 60 degrees north and south we have rising air and low pressure and this is actually related to mid-latitude cyclones which are major storms that roll through one after the other in our mid-latitude locations this is actually very similar to the weather that portland has because while we're not located at 60 degrees north we we feel the effects of this weather pattern since we're at 45 degrees north and so we do see that storm after storm will roll through those storms are characterized by rising air cloudy skies and rain finally if we go up to the poles to the north pole or down to the south pole we find extremely high pressure where it tends to be clear and cold all the time and if you look at these facts about antarctica this is just um some example temperatures from july 4th which would be winter in antarctica and we can see that the temperature was minus 78.3 degrees celsius or minus 108.9 degrees fahrenheit with a very high wind chill very windy skies and and that is because of the high pressure that happens at the poles not only can we see this pattern in examples but we can look at it with a map showing all global precipitation so this map is showing the yearly average precipitation or rain across the globe over the 1980s and 1990s the takeaway message here is that rain is going to follow low pressure around the globe and is greatest near the equator where it's warm and there's lots of water available and so if we relate this back to what we've already talked about we know that near the equator we're going to have bands of rain and also near 60 degrees north and we see that this is indeed true obviously looking at this graph of the real world it's not as as pretty as the animations you might see from a textbook image there's definitely more variation and there's more complexity to it than we see just from our idealized examples but the main idea still holds low pressure is what causes rain high pressure causes clear climates to occur or dry clear climates and so if you want to know in general where it's most likely to rain follow the low pressure and you'll find that that is at the equator and at 60 degrees north now you're going to have a chance to test yourself with a few practice problems i would highly encourage you to stop the video at each problem and write down your answer before moving on and seeing if you're correct first of all draw the air motions associated with the hadley cell mark where surface pressure is high and low use this as a template you'll see that along the x-axis of this diagram we have the equator on the left and 30 degrees north on the right altitude is going up on the y-axis if you drew air rising at the equator and sinking at 30 degrees north that is correct then you just need to connect the arrows to form a continuous airflow in this region in terms of where air is high and low we know that air pressure is low at the equator and high at 30 degrees north and if you want you can even label the trade winds at the surface and the subtropical jet stream aloft second question draw the surface air motions associated with the hadley cell mark where the surface pressure is high and low this is just meant to be a map like one like one that we already looked at north south east west orientation with the equator toward the bottom of the page and 30 degrees north toward the top of the page which way do the winds blow between the equator and 30 degrees north if you drew the winds blowing from 30 degrees north toward the equator with a curve to the right you're absolutely correct now i want you to test yourself on a problem that's a little bit more complex start off by drawing a blank globe label the equator 30 degrees north and south 60 degrees north and south and the poles next label which latitudes have high and low pressure decide what the two atmospheric forces are and which one is related to high and low pressure once you've done that draw a few arrows on your globe that show the pressure gradient force then draw arrows that represent the real atmospheric winds why are these two pointed in different directions finally draw the three circulation cells the polar hadley and feral cells on the left side of your globe in the northern hemisphere where is the air rising and sinking once you're done with this exercise go to the example picture of the surface ones on earth and compare your sketch to the answer how did you do here's the answer