what causes the weather to change this is a question every pilot should know the answer to if we want to be safe when we fly so in this video i'm going to go over the basics of what you need to know as a private pilot our planet is surrounded by multiple layers of gases and these help support life on earth by absorbing heat and light from the sun they also allow water to be recycled on the planet these layers of gases are called the atmosphere our atmosphere is made of many gases but mostly nitrogen and oxygen and also a little bit of water vapor which we'll talk a little bit more about here in a little bit we divide our atmosphere into four distinct layers because they're made of different stuff and they have different characteristics but today i'm mainly going to focus on the troposphere because that's where the majority of weather takes place now at the poles the troposphere starts at the surface and goes up somewhere between 4 and 12 miles over the equator it only goes up to about 48 000 feet and above that there's a thin boundary layer between the troposphere and the stratosphere called the tropopause and this boundary layer traps most moisture and weather inside of the troposphere and that's why there's really not a need to study the other layers inside the troposphere for every thousand feet of altitude that you gain the temperature decreases about two degrees celsius now oddly enough even though they're invisible all these gases in our atmosphere have weight in fact our atmosphere is 350 miles high and if you were to weigh a one square inch column of air the whole height of our atmosphere it would weigh 14.7 pounds and as you increase your altitude there's less air on top of that other air so it would actually weigh less and this is why air pressure decreases as altitude increases to measure air pressure we used to use mercurial barometers and at sea level on the average day the air pressure would push the mercury up to 29.92 inches and this is where we get that standard pressure setting today we use aneroid barometers to do that instead they're not quite as accurate but they're a lot more readily available and they're probably a little bit safer now as i mentioned a second ago our standard air pressure at sea level is 29.92 inches of mercury but in some countries they measure pressure in millibars instead if you're lucky enough to be on the metric system the standard pressure in that case is one thousand thirteen point two millibars anyways for every thousand feet of altitude that you gain the air pressure decreases about one inch of mercury or roughly 34 millibars now let's go back and talk about the atmosphere for just a little bit as you probably already know the sun is responsible for the majority of our weather and it's mainly because the earth's surface is heated by the sun unevenly when the earth's surface is heated this causes the air near the surface to also be heated and when this happens the air molecules in this air spread apart and the warm air gets less dense that causes the warm air to rise and it gets replaced by cooler air as it does if you haven't already started flying this is one of the first things you're going to notice if you fly on a sunny afternoon you're going to feel those updrafts push your plane around quite a bit especially if you're somewhat close to the ground and some types of ground heat up more quickly than others and water heats up a lot less quickly than the ground and because of that those updrafts won't be nearly as bad over water so now what we get is a bunch of air circulating in our atmosphere that warm air rises and the cooler air rushes in to replace it and that's what causes our atmosphere to be in constant motion now as i just kind of touched on the sun heats the ground unevenly but it also heats the entire earth unevenly in fact the equator takes most of the direct sunlight and the poles receive very little because of that if the earth didn't rotate this is what the majority of that airflow might look like for my friends who believe in a globe of this shape this is probably what that circulation looks like for them sorry i can't help myself i just find flat earth stuff funny anyways we're going to focus on the baller theory because that's the way the faa explains it in the pilot's handbook of aeronautical knowledge at least for now anyways so as we mentioned if the earth didn't rotate the flow of the warm and cool air would look something like this but because the earth rotates the coriolis force forces these airflow patterns to look more like this let's spend just a minute talking about the coriolis force and i want to do that because to me it seems like the coriolis force works backwards and i feel like if i don't explain this very well this next part's not going to make very much sense to do that let's take a look at the earth from the top of the globe from up here the earth rotates counterclockwise on its axis as you can see from up here everything out here by the equator has to travel a longer distance than anything closer to the poles because of that everything out here closer to the equator travels just a little bit faster now let's say i'm standing here in mexico and i could throw a baseball all the way to the north pole not only would the ball be traveling north but it would also be traveling in the same direction as the rotation of the earth but as the ball moves north the earth beneath it actually moves slower and because of that as it travels north it also moves to the east and that's because it was already moving east but now it's moving east faster than the ground beneath it and that's the coriolis force in a nutshell and just like my baseball when that warm air at the equator rises it travels north and when it does it's slowly deflected to the east and it only makes it up about a third of the way before the air is completely moving east and this is actually why the upper level winds in the continental us are usually out of the west while all this is happening this air begins to cool down this causes the air to become more dense and when it does it sinks back down to the surface once it does it moves back to the south to take the place of the warm air in the southern hemisphere the air moves in the opposite direction and really similar cycles occur between the 30th and the 60th parallel and up at the poles now things do get a little bit more complicated when we start talking about the seasons and the fact that the continents heat up more quickly than the oceans do we'll talk a little bit more about those here in just a minute but you should be aware that friction caused by the topography of the earth also comes into play and as you can see from this picture this can be anything from mountains hills rocks and trees to man-made features like buildings or even hangars and all this stuff can cause changes in the wind direction wind speed and also cause gusty winds okay so now we know that when the sun heats the ground the air above it also gets heated and when the temperature changes so does the air pressure because those air molecules spread apart not only is the air pressure lower now but there's also more room for water vapor in fact for every 20 degrees increase in temperature the amount of water vapor that the air can hold actually doubles and water vapor is lighter than air so this can make the warm air rise even more quickly we'll talk about this more in just a minute but for now just realize that with all these areas of different temperature we also get areas of different air pressure and because of that now we have areas of high pressure and areas of low pressure with that in mind air always moves from areas of high pressure to areas of low pressure think of a glass of water if i pour it out on the ground it's going to move to the lowest spots in that area but oddly enough the air doesn't move in a straight line to these low pressure areas because of the coriolis force remember high pressure air is cooler air that is sinking back to the surface so in the northern hemisphere the northern parts of the sinking air are deflected to the east and any air that moves to the south is deflected to the west so because of that air actually moves around the high pressure system in a clockwise motion areas of low pressure are those areas where the warm air is rising so as that air rises it also deflects to the east and this causes a counterclockwise movement of that air but the closer the air is to the surface the straighter the air travels to that low pressure area because of the friction on the ground so now you're probably wondering why you need to know this as a pilot and the first reason is that this can be really helpful when you're planning a flight because it can help you pick a flight path where the winds are more favorable for example if i know that i'm flying near a high pressure system would i want to fly north or south of it well in this example we want to fly south of it because we're flying westbound this will give us a tailwind which will actually increase our ground speed and that's going to help us get to our destination even faster now we can get an idea of how strong these winds are one of two ways let's pull out a couple weather charts from aviationweather.gov now if you have four flight you can actually look at these charts in there as well for flight makes everything so easy these days because almost everything you need for flight is in one spot but if you don't have four flight this is where you should be checking the weather because this is probably the best weather source available for pilots first things first let's take a look at this prog chart this one is an analysis of the surface conditions as you can see we've got some high pressure areas here and some low pressure areas so we know roughly which direction the wind will be moving at altitude but notice these lines surrounding the pressure systems these lines are called isobars and these represent how much the pressure is changing in these areas this is important to you because the closer these isobars are together the windier it is in these areas because of that i know that if i'm flying here in north dakota i should have less wind than normal and it should be from the south down here in northeast oklahoma it's going to be super windy and the wind should be coming from the northeast the other way we can see the wind conditions are on these weather charts that depict wind flags this particular chart is a surface weather chart now to me these little wind flags work backwards the side with the flag is the side where the wind is coming from short bars represent five knots of wind long bars represent ten and the triangle represents fifty and you have to add all these up to get your total wind speed in this area so as you can see here we have south winds at 15 knots in north dakota as i mentioned before it looks like winds are almost straight out of the north at 20 knots in oklahoma and that's probably because there's a cold front that just passed to the southeast otherwise that wind should have been more out of the northeast we'll talk more about fronts in a minute but before we do i just want to mention something else about these pressure systems that i think is important when you think of high pressure areas remember these as areas of cooler drier air that drop back down to the surface this drier heavier air is much more stable so you're typically going to see good weather in these areas now think of those low pressure areas as that warm air that rises it can hold more moisture so it brings the water vapor up with it which causes clouds rains and all kinds of other bad weather and this is going to come back into play in just a minute but for now let's talk about fronts okay so we've already discussed how the sun heats the different types of ground unevenly and how water and land heat differently as well and this causes air circulation in a lot of areas across the globe but there are also a lot of areas on our planet where the surrounding ground in water is very similar and these stagnant areas occur in polar regions tropical oceans and dry deserts so these areas don't circulate that much these become stagnant areas of either cool air or warm air the air in these areas build over time and they eventually push their way into other regions these are what we call air masses as these air masses move forward there's a boundary layer that forms between the air mass and the air in front of them and this boundary of changing air is called a front the front at the beginning of a warm air mass is called a warm front and the boundary at the beginning of a cold air mass is called a cold front if we take a look at our weather charts we can see the different types of fronts moving into our area warm fronts are depicted as these red hot blisters while cold fronts are depicted as these cold blue icicles now on occasion you will see two other types of fronts these are occluded fronts and stationary fronts and we'll talk more about those here in just a minute let's start by talking about warm fronts first warm fronts are very slow and only move about 10 to 25 miles an hour the warm air tries to push into the cooler air but because it's lighter the warm air is also pushed up above the cooler air in the process now as i mentioned earlier warm air can hold more water vapor so warm fronts will typically bring a lot of humidity into the area as this warm air rises the temperature drops and when that happens you get condensation let's stop right here for a second and go into a little bit more detail about this because once you understand a few basic things you'll understand what kind of weather you can expect when a front passes now if you remember from earlier the air in our atmosphere actually has water vapor in it and the warmer it is the more water vapor the air can hold this ratio of water vapor in the air to how much it can hold is called relative humidity and dew point is a specific temperature that we can use to determine our relative humidity let's say the dew point is 14 degrees celsius when that air is cooled to a temperature of 14 degrees the air becomes completely saturated and it can't hold any more moisture and because of that moisture begins to form in the air and this can take the form of dew frost fog clouds rain or snow now as we already mentioned several times when air is heated it rises and in the beginning of the video we also mentioned that the temperature drops two degrees celsius for every thousand feet of altitude that you gain and as that warm air rises it also expands because that air at higher altitudes is less dense and as that warm air expands this also causes the temperature to drop and how quickly the temperature drops depends mainly on how much moisture is in the air as it rises if the air is completely dry it'll cool off about three degrees celsius every thousand feet of altitude that it goes up and if there's any moisture in the air it cools off even slower than that that means that as that warm air rises it cools and gets closer to its dew point that's why clouds typically form high in the sky with that in mind we can actually estimate the height of the clouds if we know our temperature and dew point so now we know that as that warm air is lifted it cools at a certain rate depending on how much moisture is in the air in addition to that as that warm air is cooled it can't hold as much water vapor either so the dew point decreases as well but it doesn't decrease quite as quickly as the temperature does so as altitude increases temperature and dew point get closer together or converge and the rate at which it does this is 2.5 degrees celsius or 4.4 degrees fahrenheit and remember anytime you see the temperature equal to dew point that's where you're going to see the clouds rain fog and all kinds of other moisture so now that we know that we can take the temperature and subtract the dew point and then you can actually divide that by the convergence rate of 2.5 degrees celsius then all we have to do is multiply that times a thousand feet and that's where the base of our clouds should be here's another example let's say our temperature is 22 degrees celsius and our dew point is 18. 22 minus 18 is 4 and 4 divided by our convergence rate of 2.5 is 1.6 and 1.6 times a thousand is 1 600. so in this example the base of our clouds start at 1600 agl and this can change slightly depending on how stable the area is in that area stability is the atmosphere's ability to resist vertical motion when the air is stable if there's a minor disturbance in temperature and pressure these small disturbances will get smaller and smaller until they completely disappear when the atmosphere is not stable these small disturbances will get bigger and bigger and that warm air will rise faster and faster temperature and moisture are the biggest contributors to atmosphere stability the hotter the area is close to the ground the faster it will rise and the hotter the air the more water vapor it can hold and because water vapor is actually lighter than air this can cause warm air to rise up even faster if it's got a lot of moisture in it and the faster that air moves upward the more violent the weather can get this is why thunderstorms and other convective weather is so common in hot moist areas and the more moisture that is added to the air the more unstable this air gets and with that in mind moisture can be added to the air in a few different ways now as you probably already know there are three forms of water in our atmosphere solid liquid and gas and they can easily change forms to any other form when a heat exchange takes place these are the different types of changes that can take place evaporation sublimation condensation deposition melting or freezing but to understand today's concept the only two you need to be familiar with is evaporation and sublimation these are the only two ways that water vapor can be added to the atmosphere sublimation is when the solid form of water changes directly into a vapor this process is a little less common than evaporation because it requires a drop in air pressure as well as an increase in temperature when i think of sublimation i think of dry ice now dry ice isn't really the solid form of water but the process that happens when you heat it up is really similar evaporation on the other hand is the primary process that water vapor is added to the air and the hotter it is the more quickly water evaporates and the more quickly it evaporates the more moisture is in the air and that makes the air even less stable in addition to all that sometimes the temperature doesn't always drop exactly 2 degrees celsius every thousand feet if the temperature drops faster than that the warm air can rise even more quickly and once again this would mean that air is even less stable in these areas if the air temperature changes more slowly than 2 degrees for every thousand feet this would make the atmosphere more resistant to upward or downward movement in fact it's also possible for the air to get warmer with altitude and this is called the temperature inversion when this happens any moisture in the air can get trapped close to the surface this can mean low clouds fog haze smoke and other types of poor visibility situations and this is where a lot of students get confused a stable atmosphere does not necessarily mean good weather it means that you don't have a lot of upward and downward movement of air in these areas and the stability of the air in conjunction with the temperature and moisture are what determines what types of clouds you see now clouds are primarily broken up into four categories low clouds middle clouds high clouds and clouds expand all of these because of vertical development low clouds can be anything from fog stratus stratocumulus and nimbo stratus these form near the surface and extend to about 6500 feet agl and these are a particular importance to you because these are going to affect your ability to fly under visual flight rules here's some examples of some low clouds stratus clouds are typically flat sheet like clouds cumulus clouds are the puffy friendly looking clouds so these stratocumulus are just a mix of the two and anytime you see nimbo or nimbus that just means it's a rain cloud or it has rain in it and once again it's not uncommon to see any of these low type clouds when the air is stable and that includes those nimbo stratus clouds that have light steady rain and that's because that stable air resists vertical motion which keeps those clouds close to the ground they're probably going to try to trip you up on the test with that question next we have middle clouds these typically form between 6 500 and 20 000 feet agl typically these come in two types altostratus and altocumulus in altostratus clouds you'll often find areas of turbulence and even areas of moderate icing altocumulus clouds may also contain areas of light turbulence and light icing and these things are important to remember when you start working on your instrument rating and you start flying through clouds altocumulus clouds are usually formed when altostratus clouds break up high clouds usually only form in stable air these form above twenty thousand feet agl up here you'll find serostratus cereal cumulus and plain old cirrus these are these wispy ones then last but not least we have clouds with extensive vertical development the bases of these typically form in the low to middle range and these contain lots of warm moist unstable air which rises very quickly as we said before this is what we call convection in addition to these updrafts air that cools is brought back down along with air that's cool by precipitation this can cause downdrafts in excess of 3000 feet per minute and this is one of the most dangerous parts of a thunderstorm but in addition to that thunderstorms can produce hail lightning tornadoes heavy rain and all kinds of scary stuff that i don't want to fly in so now that you have a good understanding of that let's go back to talking about fronts and we left off talking about warm fronts and how they bring a lot of warm moist air into the area and when any front moves into an area the weather you get depends on how stable the air in that area was before that front moved in remember warm air is less stable because warm air rises and it holds more moisture which makes it even less stable now in almost all atmospheric conditions this is typically the type of cloud conditions that you'll see with an approaching warm front before it hits you'll usually see these flat stratus type clouds which roughly follow the bottom edge of that warm air and the weather you get right here at the base of the warm front depends on the stability of the air and if the air is unstable which is typical of the summer months you could see thunderstorms develop in these areas with that in mind no matter how stable the air is you can still expect to see drizzle or steady light rain in these areas visibility might be bad right before the front passes but once it does it can often get really clear outside because that warm air can pick up a lot of contaminants and carry them away from the surface also as the front passes expect the wind to change directions and right before the front passes the air pressure can rise slightly and once it does the pressure should go back down next we have the cold front these are large air masses that are made of colder denser and more stable air these commonly come from large areas of cold air in the arctic and polar regions now even though the colder air is more stable than the warm air cold fronts move a lot faster than warm fronts and they typically move about 25 to 30 miles an hour okay so we already know the air inside of a cold front is more dense than the warm air in front of it and this causes the cold air to push that unstable warm air forward and up which causes that lifting motion and that means that air right here is less stable and once again the type of weather that you get when a cold front passes depends on the stability of the air that was already there so during the summer when the air is hotter and more moist it's very common to see thunderstorms along the leading edge of a cold front and cold fronts can be hundreds of miles long and this can cause huge lines of thunderstorms at the beginning of the cold front these are called squall lines right before the cold front passes you'll usually notice a drop in air pressure because that warm air is being pushed up you'll also find gusty winds coming from all different directions and poor visibility and right before that cold front passes is when you can expect the worst kind of weather there can be anything from heavy rain showers to severe thunderstorms with lightning hail and severe turbulence it's also not uncommon for cold fronts to produce tornadoes then once the front passes you'll usually see cooler temperatures higher air pressure and winds typically flowing the same direction the front is moving these winds usually help visibility improve as the front moves farther ahead now there is a possibility for a mixture of these two fronts to occur if two opposing air masses meet each other and they're somewhat equal in strength you'll get something called a stationary front stationary fronts can cause bad weather for days and the weather you get is usually a mix between the weather you see in a cold front and a warm front you can identify a stationary front on a weather chart by the opposing cold and warm front symbols an occluded front on the other hand is what you get when a cold front catches up with a warm front this is indicated on a weather chart by these purple lines with a mix of warm and cold front symbols with an occluded front in the beginning you can expect the same weather as a warm front and this weather is immediately followed by the same weather that you would expect from a cold front typically the cold front plows its way in and pushes that warm air from the warm front up and pushes the cool air that was there already straight ahead when it's able to push that cool air forward without any problems this is called a cold front occlusion but if for some reason that air that was already there is cooler than the air in the advancing cold front this can cause some extremely violent weather in this case the cold front air can no longer push the cooler air forward so instead it's pushed up over the warm front air this is what we call a warm front occlusion with this cooler air being pushed above the warm front air this can cause extremely unstable lapse rates and this air can be so unstable that the weather you get can be even worse than what you might see in a normal cold front so the big thing i want you to remember today is that stability is a key factor in what type of weather you'll have the hotter it is and the more moisture there is the less stable the air will be and that's why thunderstorms are a lot more common during the summer months and remember stability doesn't necessarily mean that you won't have clouds or precipitation in stable air it's not uncommon to have stratiform clouds and light steady precipitation you can even have poor visibility because that moisture is held closer to the surface it's that unstable air and those unstable lap rates that cause rapidly rising air which can cause all kinds of violent crazy weather and this is great information to know before you go fly that piper cherokee or cessna 172. i hope you learned something today but don't quit studying just because this video is over here's another great video to help keep you on the right track towards your goal as a private pilot