hi everybody Welcome this is Earth and space science 102 and I'm going to be the instructor for this class my name is Stephanie Welch I teach here on campus in the Department of Chemistry and physics so in Earth and space science 102 we pick up where we left off in Earth and space science 101 in the 101 class we cover geology and a little tiny bit of oceanography and so in that class we are firmly rooted to talking about the Earth's interior and near surface now you don't have to have taken that class first that is not a prerequisite for this class but you'll see a couple of things that in this class that we have talked about in 101 I might be referencing it you know just a little bit but you're okay if you're just coming in to the specific class in Earth and space science 102 we are going to talk about the Earth's atmosphere and the subject of meteorology that's going to be the first of three units then we're going to broaden our perspective a little bit and talk about in the second unit the solar system so we'll go out to the very edge of our solar system we'll talk about other planets in our solar system other moons besides our own our sun and the basic processes that are really sort of holding our entire solar system together finally my favorite unit the third unit we're going to be broadening the perspective further to Encompass the rest of the universe so we'll look at conditions outside of our own solar system we'll look at stars compare those stars back to our own sun and we'll talk about galaxies the overall fate of the universe so it's really big really cool stuff and I'm you know going to try my best to make it as interesting as possible but I'm very very excited to be teaching it so any kind of logistical details for this class anything like exam dates the syllabus the basic structure of the class you'll just go to your Moodle page for all of that if you're just joining us on the the TV then you don't need to worry about any of that at all you can just enjoy the class so without any further Ado I'm going to get right into it and we're going to start off with the basics of meteorology so this first lecture today will be one of seven lectures that will Encompass not just weather and our day-to-day conditions in the Earth's atmosphere but we'll go on to talk about climate and global climate change as something that happens naturally and as we'll see something that happens due to man-made influences due to the burning of fossil fuels so we're going to start off with the very very Basics today the basic elements of weather and climate as you can see are going to include a lot of things that you think about in your everyday life things that are going to be very familiar to you and then one thing on this list is not going to be terribly familiar to you that's going to be air pressure so all of the other parts and pieces of weather and climate all of the other elements of weather and climate are things that you think about when you decide how to dress on a particular day and whether to bring an umbrella with you and all that kind of stuff things like temperature humidity clouds precipitation wind you're factoring on all of these things but the one sort of overriding control on anything that's going on with weather and climate on Earth is really going to be atmospheric pressure air pressure air pressure is going to be determined by a couple of these factors temperature and humidity they're going to control air pressure and then air pressure in turn controls where you get cloud formation and precipitation on the surface of the planet and where you're going to have winds coming from because air is always going to move from high to low pressure so as we look at this one really important characteristic a little bit further we're going to see that air pressure is really just a pushing force of the weight of the atmosphere onto the surface of the Earth so we don't really think about gas as having a particular weight we don't think of the nitrogen and the oxygen and the Earth's atmosphere as having any major influence on the surface of the planet but they absolutely do gravity holds the Earth's atmosphere to the surface of our planet and because of this the atmosphere exerts a pressure on the Earth's surface we walk around on the Earth's surface like we're not even having to deal with this weight but our bodies are a creature on earth we are evolutionarily successful at being able to kind of cope with this this air pressure now Standard air pressure at sea level is 1013.25 millibars so that's just going to be one way that we can actually measure air pressure and I'll talk about a couple other ones millimeters and inches of mercury and and other units for air pressure but I'm mainly going to con stay constrained and talking about air pressure in terms of millibars so even though there is this average sea level air pressure air pressure ranges significantly from higher air pressure than that which is normal at sea level to lower air pressure it's also going to range with elevation with altitude into virus atmosphere all of these factors are going to affect air pressure so the reason why we have variation in air pressure from one point on the surface of the Earth to another point on the surface of the Earth is because temperature and humidity are the main controlling factors they're the main determining characteristics in air pressure so we're going to look at those individually and most of this lecture today is going to be drying out let's say first temperature and we're going to see how temperature is going to control air pressure and then we're going to talk about how humidity is going to control air pressure so first I wanted to just give you a full feel for the total range in air pressure that's possible naturally on the surface of the Earth we can measure air pressure by doing something as simple as seeing how air pressure is going to change the height of mercury in a tube as it's suspended in the tube and the height that is suspended in the tube gives you an estimate of how much air pressure there is and so that in turn is going to be controlled by temperature and humidity so think about really really high air pressure as as essentially sort of a good thing especially from our point of view in south Louisiana really high air pressure is going to be associated with very very little cloud formation very little precipitation and colder and drier conditions that's going to equate to higher overall air pressure think of low air pressure lower than that standard sea level air pressure is essentially sort of a Bad Thing low air pressure is always going to be higher temperatures and higher humidity higher amounts of water vapor in the Earth's atmosphere and so that air lower air pressure is going to mean the air is moving towards that point source on the surface of the planet and the lower the air pressure the faster the wind speeds are also when all that air gets into the center of a low pressure it has to then move up into the Earth's atmosphere and that's when you start to form clouds and precipitation so think of low pressure as bad weather in a sense storms whether it be upper level lows or hurricanes these are all just essentially centers of low pressure and think about high pressure is a good thing so I've got a couple of accompanying pictures here just to give you something to kind of take with you to recall what high pressure systems versus low pressure systems might look like in this first picture you have your classic crisp winter day and somewhere farther north than we are here because there's a nice low layer of snow on the ground this is what conditions would look like if you had high air pressure that means that you're going to have drier conditions most likely you're going to be further away from the oceans and deeper into the interior of the continent and you're going to have colder overall temperatures so certainly air pressure is something that ranges seasonally and throughout the next few lectures we'll look at that as well so this is the picture I kind of want you to take with you of what high pressure systems look like cold dry air a crisp clear winter day the opposite of that would be something like a hurricane this is a picture of Taken during Hurricane Katrina of the French quarter and the recorded air pressure at the center of that hurricane as it made landfall was a very very low 920 millibars of air pressure so that's almost 100 millibars less than what Standard air pressure looks like and the result is that air has to move very very fast to the very center of that low pressure system and as a result you form what we call in the North Atlantic a hurricane and what you might call in the Western Pacific a typhoon they're essentially the same just different names for the same thing the same phenomena so low pressure you want to associate with bad weather the lower the pressure the worse the weather in a sense and high pressure you want to associate with with nice weather good conditions now the next thing that we're going to do is look at the factors at play that actually control air pressure and the most important control is going to be the air temperature so the temperature of the air is something that I want you to start kind of thinking about a little bit differently than you probably do at this moment you typically want to think about temperature is whether or not you're actually comfortable whether or not you want to switch on or off the air conditioner or whether you want to grab a coat on your way out the door temperature especially when you're considering it in terms of the atmosphere and a gas is actually nothing more than the average kinetic energy of all of the particles that make up a substance so essentially when you're thinking about a room being really really cold for instance that's because those individual particles are moving slower if it's really warm to you in that room that means relatively speaking those particles all of those little molecules of nitrogen and oxygen that make up the atmosphere they're moving a little bit faster and so the rate in which all those particles move on average when you're talking about billions and billions of particles that makes up what we consider to be the temperature and so that also introduces another complexity what happens when those particles stop moving Kelvin invented Lord Kelvin of the late 19th century invented an entire scale around the idea an entire temperature scale based on the idea that at some point you could actually make all of these particles stop moving at all and he called that absolute zero so when you think about it being really warm outside if it's maybe August outside think about it in terms of the speed of all of those individual particles and that's going to mean that those particles are moving fast so it's not warm outside it's fast outside you got to think about it like that if it's cold outside that means that those particles are moving much much slower think about it as being slow kind of the speed in which you might want to get out of bed on a really cold morning you'd be moving around a lot slower and the air is essentially doing the same thing all of those particles are moving much slower so in addition to controlling the rate of the motion of these particles the temperature is also going to control how closely they are spaced and that's going to be how temperature affects air pressure as we'll see in just a minute but before I do that I want to talk a little bit more about these temperature scales um I'd find that most people are fairly relieved that at least in terms of the subject of meteorology I tend to stay in Fahrenheit and the reason I do that even though the Celsius scale is a much more scientific scale it's based around the boiling and freezing point of water the Fahrenheit scale is the one that we actually use and I find that I get a little bit more out of talking to people in terms of keeping it all in Fahrenheit and keeping it in a number that everybody's fairly used to it won't be until we get into the very very end of the semester we can talk about the kind of temperatures that exist in the core of stars that I'm going to deal with the Kelvin scale because that scale is going to become more useful to us for that that subject okay so back to how temperature is going to affect air pressure I'm going to pull up that same picture again and you're going to see that not only does the temperature or not only is the temperature the average kinetic energy of all the particles or the rate in which they move and cold air has particles that are moving slower than warm air it's also going to be that when you have colder overall temperatures you're going to have more densely packed particles so the actual particle density the spacing between one molecule of nitrogen and a molecule of oxygen and another molecule of nitrogen it's going to be much much more closely spaced when the air is cold relative to when it's warm so you might want to kind of equate this to how closely you want to stand next to other people when temperatures are really warm outside when it's really really hot outside you typically want to have sort of a bigger personal space bubble and you want to stand a little bit further away from everybody else if it's cold outside you might feel a little bit more snuggly you might want to cuddle up next to somebody when it's cold and again the particles are essentially kind of doing the same thing so another thing that I want you to associate with cold versus warm air and I'm keeping all of this in relative terms cold air is going to mean more closely spaced particles and therefore particles that are going to exert a higher density and a higher pressure on the surface of the Earth than warm air so when the temperatures are cold when temperatures are low that means that the atmospheric pressure is actually higher and the reverse is true when temperatures are warmer when temperatures are warmer those particles are not spaced as closely together and your atmospheric pressure is going to be lower so it's due to this that I always say that when you're talking about what fuels storms and various events it's going to be heat that's going to fuel all of these systems whether it's a fairly minor Center of low pressure existing on the continents in the form of these upper level lows that we're going to talk about a little bit later or hurricanes the same holds true that their fuel source in a sense is going to be warm moist air that is what's going to be dropping the center of low pressure and allowing these things to strengthen over time so now that I'm on the subject of temperature before I go on to talk about humidity the other major control and air pressure I want to take a minute and talk about the different types of heat transfer that exists in the Earth's atmosphere so heat is transferred in the Earth's atmosphere by the same basic ways that heat is transferred in anything in the Earth's mantle near its core and an oven in a pot of boiling water the same basic types of heat transfer are going to sort of follow you around some are going to be more important in some environments though than others take for instance heat transfer by convection was really really important in the first earth science class when we were talking about how plate tectonics happens it's the convection or overturning of material due to a transfer of heat in the mantle that causes the plates to shift on the surface of the planet and the atmosphere convection is still going to be important but you're going to have other modes of heat transfer as well so the first one that I want to start with is conduction heat transfer by conduction is going to take place as you have particles that are very very densely packed so that right off the bat makes air a really terrible conductor of heat when you're talking about a material in a gaseous state so all the nitrogen and the oxygen in the room right now when I'm having this conversation you're talking about all of these individual particles that are so widely spaced that the Heat or the motion of all of those particles can't transfer very well from one particle to the next so the air is a really bad conductor of heat but dense solids are a great conductor of heat that's why you can stick your hand in a 400 degree oven but you wouldn't want to touch the plate that was at 400 degrees or the cookie sheet without a glove on your hand it's the difference in how the air conducts heat versus how that metal plate would conduct heat so take you know basically of this for now that the air is a really bad conductor of heat and we'll also see a little bit later on that air is a really terrible conductor of electricity it's essentially why lightning happens in thunderstorms it's due to the Air's inability to conduct electricity okay so that's heat transfer by conduction it's going to be less important in the atmosphere than in more dense material the next one is going to be a little bit more important convection is a different type of heat transfer where you actually have an overturning of the material whenever you heat something up it becomes less dense so whenever you heat up air that's certainly true the particles become more widely spaced and because of that that air wants to rise up off of the surface of the planet cold denser air in the upper atmosphere always wants to sink back down and so you end up having huge convective cells in a sense the heat rising up off of the surface and the cold air in the upper atmosphere sinking back down so this is going to control both the global circulation of the atmosphere and the circulation of smaller centers of low pressure even something as small as the development of a single thunderstorm on a summer afternoon it's all going to relate back to convection of heat now the difference between conduction and convection is that in convection you're overturning the material whereas in conduction you're transferring the heat from one particle to the next in a sense the last form of heat transfer is going to take place in terms of wavelengths of light we'll talk about this a little bit more towards the end of the semester when we get into the nature of light and how we understand you know Stars how we understand distant galaxies and so on but I'll just say for now that there are many many different wavelengths of light but only a few that can actually travel into the Earth's atmosphere we can actually get at the surface of our planet parts of the radio wave spectrum the visible light spectrum and just maybe a little bit of the UV spectrum and some infrared heat and so all of these are just different parts of the electromagnetic spectrum and only some of those again can actually make their way through the Earth's atmosphere and come to the planet come to the surface so that's the way that the surface of the planet and the atmosphere is heated to begin with is wavelengths of light all of these different wavelengths of light traveling from the Sun into the Earth's atmosphere then what happens when it gets close to the surface is that those wavelengths are reflected or scattered off of the surface of the Earth and some of those wavelengths are absorbed by the surface that heats up the surface and it allows the surface to emit thermal radiation basically infrared heat and different part of the spectrum and then that heat is reflected up off the surface so when you take into account heat traveling in a sense by radiation by wavelengths of light and to all of those other types of ways that heat is transferred into the Earth's atmosphere you get a decent understanding of the total Global heat budget of particularly the lower part of the Earth's atmosphere the troposphere so there is one part of this that we haven't discussed yet and that I'm saving for last it's the least intuitive so it takes a little bit of time and it's a little tricky the last thing that we're going to talk about is the effect of latent heat on the surface of the planet and the one thing that I want you to really remember about the subject of latent heat is the fact that the whole idea of latent heat is going to come into play because the majority of the earth's surface is covered in water so the big takeaway message is going to be that because most of the surface the planet over two-thirds of the surface of the planet are is covered in water we have the tendency of that water to transfer in between different forms solids liquids gases and along the way it's possible for that molecule to either retain or emit Heat so having water at covering a big part of the surface of the planet is going to be a really important factor in terms of the global heat budget so before I take this any further I want to get the the basic terminology under our belts so when we go from something being in a solid form to a liquid form we of course call that melting there's nothing more complicated to it than that and if you go in the reversion you go from a liquid to a solid form we call that freezing and so the big molecule that this is going to apply to on the surface of planet Earth is going to be of course water water being particularly abundant and our planet having the kind of range of temperatures that allows water to exist in a solid liquid and gas on other planets in the outer solar system Waters always locked up in an icy form as a solid but other compounds like methane and ethane on Saturn's moon Titan those exist as liquids on the surface of the planet or even sometimes solids so it's sort of a very very different overall water cycle on a on a planet like that I digress a little bit here so going from a solid to a liquid and from a liquid to a solid that's pretty easy that's melting and freezing going from a liquid to a gas so liquid water to water vapor for instance is called evaporation and going from a water vapor back into a liquid again that's called condensation it's also possible to go directly from a solid to a gas that's called sublimation and from a gas back to a solid again and that's called deposition we don't talk about those too much it's going to be mainly all of the smaller intermediary steps and not the one where we skipped a liquid phase entirely now the reason I cover this is besides just covering those basic uh you know terminology and and reminding everybody again what evaporation and condensation means is because whenever you go from one state of matter to another it's possible for the heat required to make that transition to either be released from the molecule or absorbed into the molecule and that's what latent heat is latent heat is stored heat and in a sense it's stored inside the water molecule so whenever you go in a series of steps that requires an increase in temperatures on the surface of the planet so to let's say melt ice or to evaporate seawater in order for that to happen you have to raise temperatures whenever you do that you're applying that temperature to the water molecule and that energy some of it is stored within the water molecule and so that is going to lead to heat actually being taken out of the environment if you go in the opposite direction and you go from let's say a gas to a liquid a water vapor to liquid water that requires a drop in temperatures and that's why clouds typically form up away from the Earth's surface a mile off the surface because it's colder up there and you have a solid surface in the form of Ash and dust for the water vapor in the atmosphere to condense onto and whenever that happens billions and billions of times that's how you form a cloud a cloud is essentially nothing more than billions and billions of solid little particles of ice or more often times little balls of liquid water condensed onto some very small piece of material and all of those little droplets are too small to fall down to the surface of the Earth so they stay suspended up in the upper atmosphere now the process of forming a cloud the process of undergoing condensation from going from a water vapor to a liquid water State actually releases heat back into the environment again so you have heat constantly being stored into the water molecule and released back into the environment as conditions like evaporation and condensation take place and so that's what latent heat is all about it's the heat energy required to take to change the state of the substance being stored within that molecule and anytime you're going in one particular direction going from solid to liquid or liquid to gas you're taking heat out of the environment those are all cooling processes so the melting of ice and the evaporation of water those are all cooling processes as you're going in the opposite direction you're releasing the heat stored within the molecule the latent heat back out into the environment again and so those are warming processes these become really important evaporation is a cooling process and condensation is a warming process these become really important when you're talking about the development of big storms like hurricanes part of the story of how hurricanes form and how they Thrive is the fact that as those thunderstorm clouds are forming in a hurricane that delivers heat back into the system again that can continue to fuel the storm in a sense continue to strengthen that system so that is our last form of heat transfer in a sense is heat transfer in terms of latent heat so just remember that it's a little counter-intuitive the heat energy required to change the state of the substance actually gets stored so even though evaporation for instance takes warm temperatures to happen it requires that a little bit of that heat get absorbed into the molecule retained by the molecule all right so now the next subject that we're going to cover is the second factor that controls air pressure the first was temperature and temperature is definitely the most important but it's not the only Factor the composition of the Earth's atmosphere in a sense is part of what is going to control air pressure and some of the molecules in the Earth's atmosphere are going to vary from one point on the Earth's surface to another the one compound in the Earth's atmosphere that's going to vary the most is going to be the total amount of water vapor and so we say humidity is going to be the second factor that controls air pressure humidity in a sense of how much of the atmosphere is actually made up of water vapor and it's usually going to be in absolute terms relatively small so air humidity is going to control air pressure by controlling the overall weight of a massive air on one part of the surface of the Earth and it's going to control it in the opposite way that you really want to think on a really really humid day and we have lots and lots of those in south Louisiana if you go outside and it's extremely humid maybe in the morning when relative humidity is the greatest you walk outside and the air feels heavy on you and the only real reason why it does actually feel heavy is because you're sweating as a result of having warmer overall temperatures and a lot of humidity in the Earth's atmosphere it makes you sweat so and it makes that sweat cling to you the sweat doesn't just evaporate off your body so it makes it feel heavy but the actual way that humidity controls air pressure is exactly the opposite of that humidity is going to make air actually way less because water vapor compared to all the dry components of the Earth's atmosphere is actually going to weigh less so you increase the amount of water vapor and you make that massive air actually lighter in terms of weight than a similar dry mass of air so the more humidity the more water vapor you have in the Earth's atmosphere the lower the air pressure is as a result so in order to understand this I'd like to first kind of break down what absolute humidity means and what relative humidity means if you're used to watching weather forecasts and you get an idea of what the relative humidity might be on a particular day it's not at all uncommon in south Louisiana for us to have relative humidity of 70 80 90 percent and so whenever you have those High relative humidities that doesn't mean that 90 percent of the Earth's atmosphere is composed of water vapor relative humidity is all about the amount of water vapor that the Earth's atmosphere can actually hold and that is going to be dependent on temperature if the temperature is higher that means that that the atmosphere can actually hold more water vapor so relative humidity is all about the actual amount of water vapor in the Earth's atmosphere versus how the capacity of the Earth's atmosphere to hold on to that water vapor so it's not uncommon in terms of relative humidity to see on a weather forecast in south Louisiana a relative humidity in the 70 or 80 percent or even maybe 90 percent on a particular day so that means essentially that you're maxing out the ability of the Earth's atmosphere to hold onto that water vapor with a particularly a high temperature that also means that you're going to have just a greater overall amount of the Earth's atmosphere composed of the water vapor now in terms of absolute humidity that's a real measure of how much of the Earth's atmosphere is actually composed of water vapor and that's always going to be extremely low so it's not very likely that you're going to be able to really mix up these numbers at all the actual amount of water vapor compared to all the dry components of nearest atmosphere is less than four percent so that means the other 96 percent of the Earth's atmosphere is composed of things like nitrogen oxygen argon and carbon dioxide the first three of those are non-greenhouse gases and they're going to stay relatively constant unless you consider the fact that with an increase in absolute humidity you're changing the amount of dry components as a result in terms of at least a percentage so in terms of how the humidity is actually affecting air pressure you kind of want to think about it in terms of absolute humidity just because it's a little bit more simple in terms of relative humidity you can use that as a measurement of how much the atmosphere can actually hold in terms of water vapor so in order to kind of get this any further we need a basic breakdown of the composition of the Earth's atmosphere and this pie chart does a pretty good job of showing that the vast majority of the Earth's atmosphere is actually composed of nitrogen so oxygen is going to be something that is much less abundant molecule but it's certainly far more abundant than argon carbon dioxide water vapor and anything else that you can think of that exists in the Earth's atmosphere so essentially more than three quarters of the Earth's atmosphere are composed of nitrogen a little small slice maybe about 20 percent of the Earth's atmosphere is composed of oxygen one person to the Earth's atmosphere is composed of argon and then you have very very very small components of everything else and a lot of these additional molecules that make up the Earth's atmosphere that are abundant and very very small percentages those are most often greenhouse gases so here's another way of looking at the breakdown on all of these gases the permanent gases are in the left of this table and the variable gases are over to the right and you can see that nitrogen oxygen argon and water vapor are composing 99.9 percent of the Earth's atmosphere so much so that we can basically ignore all of the other permanent gases like neon and helium and hydrogen the variable gases on this list are entirely greenhouse gases including water vapor they all have a more complicated structure and that means the heat that is released off the surface of the Earth and feared back out into the Earth's atmosphere is then sort of locked up in those molecules they have to vibrate for a longer amount of time to transfer that heat onwards and so that's why we call them greenhouse gases they insulate the lower part of the atmosphere so water vapor is the most abundant greenhouse gas and the most abundant of the variable gases carbon dioxide makes up less than one percent of the Earth's atmosphere so .038 percent of the Earth's atmosphere with methane and nitrous oxide and ozone and so on following this breakdown of the Earth's atmosphere wasn't always this way if you go back throughout the entire history of the earth four and a half billion years it's only been for the last approximately 2 billion years that Earth's atmosphere has actually been composed of a large amount of oxygen oxygen is just not part of the original building block of the atmosphere of a planet stuff like nitrogen and carbon dioxide and ammonia and methane those are typically original components of a nurse of a planet's atmosphere like Earth so it was blue-green algae coming into existence around two and a half billion years ago that started to take the CO2 out of the Earth's atmosphere and replace it with oxygen cooling down the planet and giving a component of the Earth's atmosphere that we would later be able to breathe and all organisms that undergo respiration would be able to breathe okay so going back to the way that humidity controls air pressure which is the main reason I'm covering humidity the water vapor is going to be much much less massive than all of the other dry components of the atmosphere like the nitrogen the oxygen and the Argon and even CO2 all of those other compounds are going to be more massive than water vapor is so I've found a little video that should explain the difference but pretty pretty well of the weight of a single atom or a single molecule I should say of water vapor versus a single molecule of nitrogen nitrogen or oxygen and you're going to see how the water vapor actually makes the atmosphere way less so I hope you enjoy this so I've got two jars here and what I'm going to do is I'm going to take the jar on the right and I'm going to make this moist air following Avogadro's Law so I'm going to try to add water vapor but I have to keep the same total number of molecules in here before this experiment as I had after it so for example if I want to add some water vapor to the jar on the on the right I have to get rid of some other molecules now this is a little hard to explain how this would happen inside of a sealed jar in the free atmosphere this would involve the air expanding and stuff like that but um in just a simple little jar experiment we're just going to think of it as we're going to replace a couple of these molecules like nitrogen and oxygen with water molecules H2O so now my jar on my uh right is moist air and the jar on the left is completely dry air well now let's actually get real information in here and let's replace these nitrogen and oxygen and water molecules with their actual molecular weight what do these molecules weigh for example nitrogen N2 has a molecular weight of 28. with the units aren't relevant to our current discussion or anything so I could take all those little nitrogen molecules out and replace them with the number 28 and that is a measure of how much each of those molecules weigh in the same sort of way I could take these oxygen molecules out here and replace them with their molecular weight o2 molecules have a molecular weight of about 32 so I could take those oxygen molecules out and replace them with the number 32. finally the molecular weight for a height for water is 18. not surprises people that's an awfully low number but keep in mind it's H2O two hydrogens one oxygen hydrogens only weigh one the oxygen weighs 16 together they weigh 18. so let's replace the oxygen uh the water molecules in here with uh the number 18. well now check out what's happened the dry air jar on the left and the humid air on the right still contain the same number of molecules but notice that the humid air molecules overall weigh less that's because we have replaced a couple of heavy dry air molecules like nitrogen and oxygen with light water vapor molecules overall for the whole jar the weight of the air or the mass of the air inside of the jar has gone down once we have made the air humid so hopefully that showed you a little bit about how air pressure is going to be controlled by humidity and then of course the other part of that is going to be that air pressure is also controlled by temperature to temperature and humidity are the two major controls on air pressure and then air pressure in turn controls the overall cyclicity the overall circulation of the Earth's atmosphere so in an attempt to just describe that to kind of finalize this entire discussion on air pressure I wanted to give you one big sort of sum up thought here drier colder air the drier it is the more lacking in water vapor and the colder the lower the temperatures that means that the air is going to be higher density and higher pressure it exerts a higher pressure on the surface of the Earth and if that is the case then the air always wants to move away from high pressure so in high pressure systems these are called anti-cyclones you always have air being pushed outwards from a center of high pressure and it always has to go somewhere it has to flow to another location and that's always going to be wherever you have low pressure like water flowing downhill because of a change in elevation air is going to move because of a change in pressure always from high to low so the more humid the wetter the air is and the warmer it is so the higher the humidity and the lower the temp I mean the higher the humidity and the higher the temperatures the lower the particle density the lower the weight of the air and the lower the pressure is as a result and that in a sense actually draws air inwards so I have this great picture over here on the right of the slide that shows you a high pressure system and a low pressure system existing on the surface of the Earth like opposite ends of a magnet where you have high pressure air moving outwards being drawn down from the upper atmosphere and flowing out across the surface of the Earth and low pressure air being drawn in then moving up into the upper atmosphere so what the entire system is is really just one big convective cell and the surface circulation is driven by differences in air pressure so in terms of air pressure I want to leave you with one final thought that might be a little bit more conducive to having you really remember this and all of these the all of this jargon all of these pictures think about going into one of those really big department stores or big box stores going into a Walmart or JC Penney's or something some huge store and the middle of summer let's say it's August 1st you get out of your car you walk across the parking lot and it's really really hot outside it's maybe 92 degrees with 80 percent humidity that means that the relative air pressure outside that store is very low now whenever you open those doors swish open those automatic doors swish open in the store that air-conditioned air that dry cold air from inside the store blasts out the door to meet you and it's always one of the most refreshing things in the middle of the summer to open the door and feel that Ace there that air conditioning just blast into you that's because the air pressure inside the store the colder drier air is higher pressure it's pushing essentially at the schemes it's pushing to escape the building and whenever the doors open does that and so that it's just a you know something hopefully to kind of help you remember how temperature and humidity are affecting air pressure cold dry air is high pressure warm humid air is low pressure okay so the last thing that we're going to cover is part of this lecture is the basic structure of the Earth's atmosphere and we're going to move on from the subject of talking about air pressure to talk about how the Earth's atmosphere is structured how we designate these different zones in the Earth's atmosphere for the most part for the entire rest of this particular unit on meteorology we're going to be focused on the first seven miles of the Earth's atmosphere that we call the troposphere the rest of the Earth's atmosphere is further broken down into Stratosphere mesosphere and thermosphere the important thing to remember though more than the names of these individual layers and what might be happening in these layers is why we zone out the Earth's atmosphere the way we do certainly air pressure gets less and less and less and less as you move away from the surface of the at of the of the planet but that's not how we structure the Earth's atmosphere that's not how we Define the end of one layer in the beginning of the next there are some slight changes in composition but that's also not the way that we separate these layers these layers are defined by what is happening with temperature in each one of these layers so the picture over here on the left is of the temperatures changing in each of these layers of the Earth's atmosphere and that first little tiny layer the troposphere down at the bottom temperatures decline so much so that you would really think from our perspective on the surface of the Earth if you've ever traveled up to the top of a mountain experience colder temperatures you'd think that temperatures just continue to decrease all the way out until you get to the vacuum of outer space but that's not the case temperatures decrease from your standard temperatures on the surface of the planet to well below zero as you go up the first seven miles of the Earth's atmosphere then at the top of the troposphere in the beginning of the stratosphere they turn around and they increase and it's that change in the track of what's going on with the temperatures that defines the difference between one layer and the next so this is called the thermal structure of the Earth's atmosphere for a reason it's based on what's going on thermally with temperature so the troposphere the first seven miles of the Earth's atmosphere is defined by this decrease in temperature and it's because temperatures are decreasing this first layer of the Earth's atmosphere that we get clouds forming where they do we typically don't get clouds forming at the surface unless you have a lot of warm humid air moving across a cold ground surface in which case you might get some fog that's essentially clouds on ground level but most of the time you only see clouds at least around a mile away from the Earth's surface that's because temperatures are colder up there it forces the warm humid air that wants to rise up off the Earth's surface to cool and condense in the upper atmosphere and form clouds and often precipitation so it's called the weather Sphere for a reason the troposphere because this is essentially where the clouds form the top of the troposphere is essentially the top of where you typically get cloud formation in the atmosphere of our planet as we get up into the stratosphere and all of these overlying layers you essentially don't have cloud formation up there there's no mechanism to get the water vapor up there and away from the surface of the planet so the next layer is called the stratosphere this is the next approximately 23 miles of the Earth's atmosphere and the one big claim to fame besides the fact that temperatures are increasing throughout this layer is that this is where the ozone maximum is ozone is this really interesting little molecule made up of instead of two atoms of oxygen like the oxygen that we breathe and the oxygen that makes up so much of the um the atmosphere of our planet ozone is oxygen in a little cluster of Threes so it's three atoms instead of two this structure allows UV rays that come into the Earth's atmosphere to be a kind of Bamboozled by all of these individual little ozone molecules and they protect they Shield the surface of the planet so that ozone maximum is really important it's not even so much in a sense an ozone layer it's the part of the Earth's atmosphere where ozone concentrations are at the highest that exists Midway through the stratosphere the next layer the mesosphere is where temperatures are going to turn around again and at the very top of the message here that's where you have the coldest temperatures in the Earth's atmosphere hundreds of degrees below zero so this is typically where you have enough air pressure in the Earth's atmosphere so that incoming objects are going to actually experience a frictional drag so let's say you have a meteor meteorite coming into the Earth's atmosphere that gets a certain way through the Earth's atmosphere before it starts to experience this frictional drag with the pressure of the atmosphere within the mesosphere and that causes a lot of these particles to break up so whenever you see a meteor shower what's actually happening is that you're seeing the destruction of some little piece of rock and metal coming in from outer space it heats up and it leaves a bright ionized Trail that's visible from the surface of the planet so not exactly a falling star although they might look like it this is debris breaking up in orbit the last layer of the Earth's atmosphere is called the thermosphere and the exact end of the thermosphere is somewhat up to speculation it's somewhat indecisive because with the end of the thermosphere is essentially the point where you go from having some particles of gas remaining in the air solder atmosphere to essentially nothing so in this outermost part of the Earth's atmosphere you transition from having a few stray particles of nitrogen and oxygen to the vacuum or The Emptiness of outer space it's devoid of air pressure entirely so in the thermosphere temperatures are increasing again to increase to Far Far higher temperatures than we ever experience on the surface of the planet and the very very outer part of the thermosphere and points on the surface of the planet can actually be outside of the Earth's magnetic field the Earth's magnetic field is essentially this sort of invisible force field that protects the surface of the planet and it's generated in our core and charged particles from the Sun can take that gas of the Earth's outer atmosphere and ionize the particles what that creates for us on the earth's surface is just a really really cool light show called the northern and southern lights the northern lights are called the Aurora Borealis Boreal being the god of the North Sea and the Southern Lights being Aurora Australis so that is going to be about it for us today the next lecture is all about the circulation of the Earth's atmosphere in a global perspective of why the air wants to move on the surface of the Earth a hint going into it is going to be very dependent on air pressure again everything that we talked about today is going to be very relevant to that so that's all we have time for today um until next time keep looking up