hey everybody now that you made it through the forces that govern wind and Global circulation model and talking about all the things related to temperature and pressure on the planet we're going to talk about the actual wind belts that result from the different pressures on the planet so don't forget what we've already talked about when we talked about rising and descending air and the temperature driven cells like the Hadley cells and the polar cells up here okay so rising and sinking air and then the Corola effect is going to help derive our global wind patterns that are consistently related to the dominant high and low pressure systems on the planet so let's talk about some of those wind belts and what we call them and again they're listed here as well for you to review I think this Global di is helpful you'll see the trade winds the Westerly winds and the polar easterly here on this diagram and so the ones we're going to talk about that affect the US most dominantly are going to be the Westerly and the winds so around the equator remember that air is coming together and rising and that warm humid air is going up in this Zone the air is converging and that's where the North and Southeast Northeast and Southeast Trade Winds meet so right around the equator we have something called the itcz or the intertropical Convergent Zone conversion just means coming together so where the trade winds meet we have that equatorial low system air rising up and creating lots of rain and clouds around the equatorial area affected by the itcz so remember that the air is diverging at the upper levels right it's diverging or moving away at these upper levels okay and it's rising in here where the itcz is and that creates a strong low pressure belt right there where the air is rising so the it CZ or that intertropical Convergent Zone will change somewhat seasonally it will float a little bit further North during the northern hemisphere summer and a little bit south of the Equator during the winter but in general that area right around 0 degrees latitude will be very wet however you'll notice that the subtropical High systems we'll talk about is where air is sinking and that sinking air suppresses Cloud development and therefore any type of precipitation so during times where the itcz is overhead that means there's lots of air moving up lots of moisture moving up lots of clouds forming and then that moisture comes back down so when the itcz is present over an area it's very wet and it gets a little bit drier when it moves away from that area our subtropical Highs are like the area over the Sahara Desert I was just describing remember looking down here on this diagram as the air moves out and adx it's going to cool off and it's going to sink so these subtropical High systems are areas where there's lots of suppression of moisture development in clouds and any type of rainfall that air sinking toward the surface is going to heat up so you're going to get warmer drier areas around some of these subtropical High systems okay so many of our deserts occur in these belts around these areas remember where we are around 30° latitude is an exception partially because we're located near a wet warm body of water like the Gulf of Mexico and that changes some of the Dynamics so lots of areas though outside of areas along the gulf have hot dry conditions as a result of that sinking air and that air warming up as it gets closer and closer to the surface so looking at our dominant subtropical High systems around the globe you won't be surprised probably based on what I just said that some of our major world deserts are located near these places so let's look at some of them the Chihuahuan Desert the Sonoran Desert the Mojave Desert in the southwestern us the Sahara Desert in Africa the Outback or the Australian Outback the atakama desert in Chile and then a desert in southwestern Africa or you may have heard of the Kalahari in this area Okay so these areas are dry in part because there's high pressure and then also what we talked about during the first part of class these areas are also located near cold water currents so you have two things happening at once you have air sinking and you have cold water so both things are bad for evaporation and allowing that air to move up into the atmosphere so let's talk about the surface winds that relate to these dominant pressure belts on the planet the first one we talk about are where the trade winds come together around the Equator so near the itcz we talk about the do DRS and the do drums are relatively weak winds that vary a little bit depending on which part of the trade winds is more dominant coming out of the Northern Hemisphere or the southern hemisphere at the time so the trade winds themselves are areas that are going to blow in a general east to west Direction based on the Corola Force the winds are getting deflected remember the opposite way on each side of the Equator to the right in the Northern Hemisphere and to the left in the Southern Hemisphere and you can see that here on this diagram they're getting deflected to the left here and they're getting deflected to the right if you consider their forward direction of motion so you'd have to flip this upside down to see their forward direction of motion and you'd see that they're getting deflected to the right based on that motion okay so the trade wins are important historically because they allowed for exploration they allowed for different things in terms of trade and that's how they got the name the trade wins they are also important in weather and climate because they direct our low press tropical storm waves off the coast of Africa across the Atlantic and potentially into the United States so it is the trade wind belt that helps to steer some of our hurricane and tropical storm system so I want you to remember that so there it is for exploration but again in weather and climate we're worried about low pressure waves that form here getting directed across the Atlantic by this General East to West for of our trade winds in these different areas moving into 30 to 35° we run into the difference in our high and low pressure dominant systems and where our Westerly and easterly meet so where our Westerly and the trade winds meet we wind up in the horse latitudes so that's roughly where we are right we're between 30 and 35ish degrees if you're near campus in Huntsville and so in this area it depends on which pressure systems are more dominant and how things are moving but we can get winds out of basically any direction based on where we're at in between these global wind belts so sometimes we'll have winds and systems come out of the gulf sometimes they'll come straight out of the North sometimes from the West sometimes from the East okay forecasting the weather in the Gulf Coast is somewhat complicated because we're in this belt of variability where there's not one global wind pattern that dominates all the time however if you get further north in the US you do wind up in the Westerly wind belt which you can see documented in this zone right in here and so again things are deflected slightly to the right in terms of their forward motion and you can see that here in the southern hemisphere you'll note that the westes are again deflected slightly to their left relative to the direction of Forward Motion okay and so the West Le Drive much of the weather and climate system over the US except for the very southern part along the Gulf Coast however the Wester Le can also influence us this far south so if I ask you the dominant wind belt in much of the United States this is it it's the Westerly and you'll notice if you watch your weather apps over certain Seasons most of our thunderstorm systems will generally have some flow from the West toward the East so they'll start somewhere over the Great Plains and they'll wind up eventually making out out over the Atlantic in these areas this is a picture on the Oregon coast that's actually looking South so we're looking South this way okay and we know the ocean is over here so we know that this direction is East okay and so looking at this direction that tells us this is West and so there's something called tree flagging which actually has the trees leaned over in a certain direction based on the persisted direction of the wind and looking at this tree we can look at it on the landscape and say even without the wind blowing on a certain day wow this tree is bent over in the same way that must mean the winds keep coming from the same direction in this case from the West off of the Pacific Ocean and so we can also map this out with our pressure systems and combine things we've talked about this close isobar spacing in here indicates that there's a strong pressure gradient force or a big change in pressure over a small distance and that indicates that there is going to be higher wind velocities remember I also told you that our upper level winds flow more parallel to the isobars because of the strong cholis force and the lack of friction so in this case we have the winds moving in off the Pacific and then driving this weather system these weather systems and things to flow across the United States in this general direction note that weak winds are documented in here because there's huge spacing in between the isobars which indicates a weak pressure gradient Force but what I also want to emphasize here is the overall flow dominated by the Westerly winds where much of that circulation is generally heading from a west to east Direction even if part of it is a little more Northwest or a little more from the southwest okay there's still generally that Western flow for much of this United States as you get further north you get into the subpolar low Zone and up in that part of the world we have things moving generally to the east based on air sinking and getting again tugged to the right based on that sinking air okay so now again we're in these general areas here where the air is sinking okay but where the wind front actually meets remember the air is rising because it warms up as it gets away from the poles okay so there's another visualization of this so Westerly there in the middle latitudes over much of the US and then the easterly up here along the polar front and then our Trade Winds further south okay and remember the horse La here where this boundary meets other so there's the polar easterly okay that winds are going to be relatively variable okay and they're Associated again with that sinking or subsiding cold dry air in the polar regions now one thing I want to emphasize here in this part of class is that the jet stream exists along some of these boundaries and you may have heard of the jet stream before from forecasters because it's hugely important in determining how far cold air can sink into the continental US and when it moves further north it helps dictate how warm air can move further north and along these boundaries where the jet stream exists and you have really high velocity winds you you can get big storms that develop and we'll see later in the severe weather part of class that the jet stream being present can enhance thunderstorms and help lead to the development of tornadoes so what is the jet stream well the jet stream is this area along these boundaries of air where we have dominant wind belts and pressure systems where those differing types of air meet we have air that flows in Tighter and Tighter circles and so by flowing in Tighter and Tighter circles we get really high velocities so the definition I have here on the bottom is rivers or tubes of more rapidly moving air moving in the same general direction as the surrounding air so you'll see on this diagram the jet streams generally have that same west of East flow we were talking about with the dominant Westerly wind belts over the United States and we have the subtropical jet and the polar Jet and that helps keep cold air and warm air separated but if this subtropical jet moves further north we can move warmer air into the middle of the continent and likewise if this polar jet stream sinks South you can move colder air further into the southeastern United States and lead to freezes and things that the Southeastern part of the us doesn't always deal with now I told you the wind velocities were high in these jet streams and so during the middle of a jet stream or right in the center of a jet stream you can reach 300 miles hour but the average is lower and winds up being much closer to 60 to 115 mph this has important influences on flight times though for example if you have rapidly moving air at the upper levels of the troposphere 30,000 to 45,000 ft where a lot of jets fly right if you're flying with the wind your flight time will be faster going west to east if you're flying into the wind I'm drawing these arrows in Canada but this is for flights in the US okay if you're flying into the wind in the US then that means your flight time will be longer so how does this work why is the jet streams velocity so high well one you need a steep pressure gradient you need a big difference on each side of the boundary here is that polar front that exist okay so we have cold air on one side we have warmer wetter air generally on the other side and we have the jet stream that's produced by the air flowing around in this direction along that boundary this has to do with something called the conserv of angular momentum which causes the speed of something to go up if the radius of curvature changes so what this formula means down here is angular momentum is equal to mass time velocity times the radius of curvature we're going to assume the mass is the same so if you think about that right here's the mass it's the same in both of these examples what's changing is the radius of curvature around which the air is Flowing we can apply that to a ballerina or an ice skater example if you spin in Tighter and Tighter circles you can go faster and faster whether you're doing ballet or whether you're doing ice skating or something like that right it's the same thing applied to the atmosphere so what's happening is if our radius of curvature smaller like it is here on the bottom example that means to conserve momentum which needs to be consistent right momentum needs to stay the same so you'll see in both these examples it's 30 okay Mass isn't changing right so we can kind of ignore this term okay what's changing again is the radius of curvature so you'll notice to get this number to come out to 30 even right in both examples when the radius of curvature is higher like it is on the top that means velocity has to go down and when the radius of curvature goes down like in this bottom examp example that means velocity has to go up so what's happening around the jet stream is we're flowing Tighter and Tighter and Tighter and Tighter circles which causes the velocity to go up and increase okay so the jet stream velocity is high because of this practical application you can think about with an ice skater or a ballerina and the conservation of angular momentum all right that's it for our global wind belt patterns and how the jet stream works we'll talk more about local and Regional winds going forward in the next lectures