when climbing in an aircraft your ears will suddenly pop due to that sudden drop of pressure and the pressure changes on the earth's surface are a huge driving factor in the weather that we experience so what is pressure anyway how do we use surface pressure patterns to help predict what the weather is going to be let's find out [Music] hi i'm grant and welcome to the third class in the meteorology series in this class we're going to be continuing our breakdown of the atmosphere and take a deeper dive into the pressure element pressure is given by the formula that p equals f over a force over area if we consider a column of air maybe i don't know one kilometer high something like that it's not really important with a fixed um surface area as we travel up then as we get higher up the column the weight of the air which is the mass times the acceleration due to gravity reduces and reduces as we travel up the column there's also fewer particles higher up and more particles lower down because they're basically being pulled down by gravity so if we look at this section of air down near the surface it has a larger amount of air particles pushing down on it when you compare that to this section of air up near the top this means that the force element the weight element of our pressure equation reduces as we get higher up and that in turn makes the pressure reduce and then down near the bottom there's more weight so that's more force and that means there's more pressure so we can say that as altitude increases the pressure decreases the standard rate at the which this happens is we reduce in pressure by one heck to pascal for every 27 or 30 feet for quick calculations we measure pressure in hectopascals which is exactly the same thing as a millibar so one hectopascal is one millibar and in the night the united states they measure things and in some other countries i'm sure they measure things in inches of mercury as well and in the international standard atmosphere the sea level pressure is equal to 1013.25 hits pascals or 101 3.25 millibars which is the equivalent of 29.92 inches of mercury as well altimeters work by sensing the difference in pressure between a datum point and the current pressure and then you multiply the difference by that 27 foot per hectare pascal lapse rate we can set a few things as our datum we can have the qfe the qnh or the standard setting i've made a video previously on altimetry in the general navigation series where i go into a bit more detail i'll link that below and if you're happy with pressure settings and stuff like that what q and h are what qfe are in standard then continue watching this video but if you're unhappy i'd recommend you pause this go and watch that other one first to explain all of these terms okay so we have the date and pressure settings of q and h qfe and standard and meteorologists we use one more which is called the qff this is the pressure setting measured at the airfield or weather station corrected down to the sea level pressure setting for the actual day's conditions this is different to qnh because qnh uses the standard 1 hecta pascal every 27 feet whereas the qff is giving the adjusted lapse rate you can think of it as that because temperature variations have an influence on this lapse rate temperature corrections are very important when flying usually in cold air because the air levels the pressure levels become compressed together and your true altitude is lower than your indicated altitude so the qff factors in this temperature correction and the correction that we normally apply is four feet for every thousand feet for every degree of iso deviation or as a good rule of thumb you can use a one percent altitude error for every 2.5 degrees of iso deviation so that's where the main difference is between qff and qna is q h is using 27 feet per hectare pascal and the qff is factoring in this temperature correction to in order to give us a different lapse rate so you'll get the more accurate sea level pressure for that day when using qff which is why meteorologists like to use it because it's better for predicting weather whereas us aviators we use q h because it's not really that important um from the majority of things it's only when it gets really cold that you see significant changes in your altitude so i'm just going to do a quick example of the temperature error calculation for more information as i said before go back and watch that video on altimetry that was in the gnab series so this example will assume that you're comfortable with a few of the definitions vertical distances et cetera and yeah so anyway an aircraft is at flight level 200 where the temperature is minus 40 degrees c the q h at nearby airfield is 998 hectopascals there's an obstacle on route at 800 feet what obstacle clearance does the aircraft have so draw the effing picture flight level 200 equals 20 000 feet in pressure altitude and that's based off of standard which is one zero one three so i'll draw a line here one zero one three and twenty thousand feet above that is our aircraft we then have the q and h which is going to be a lower pressure of 998 and that's going to be higher up so it's going to be somewhere up here and then we can find out the distance in here to find out our indicated altitude which is above the q and h so this distance is very easy to calculate 1013 minus 998 times 27 which is equal to 405 feet so this distance in here 405 feet which means our indicated altitude our height above the q and h is going to be 20 000 minus 405 so our indicated out i don't know why i've done that i out um equals 20 000 minus 405 which is equal to one nine five nine five feet so that's our indicated altitude we apply temperature corrections to this to get our true altitude so we have to figure out the iso deviation so normally at 20 000 feet the temperature would be 15 degrees and then 2 degrees per thousand feet so 15 minus 40 is going to be minus 25 so iso at 20 000 feet so iso temperature at 20 000 feet is minus 25 degrees celsius and it today is minus 40 so the iso deviation is 15 degrees colder equals minus 15 degrees and then we apply the temperature correction so it's four feet for every thousand feet that we're above so let's call that 20. or more accurately 19.6 i suppose it should be that's our indicated altitude 0.6 and then we per also multiply that and multiply this by the 15 degrees 4 times the 19.6 for in thousand of feet above the q h and times that by our iso deviation 15. and that's one one seven six one one seven six feet of altitude correction to make and we're gonna be lower because it is colder than i said everything's getting squished together so it's gonna be this answer take away this number so our true out is equal to uh let's just do that on the calculator 19595 minus one one seven six that's going to be eighteen thousand four hundred and nineteen feet and we're asking for the obstacle clearance and so how far above this eight thousand through oscar just take away the eight thousand and our obstacle clearance is equal to 10 419 feet or and you don't have to do the temperature correction this way you can do that um one percent per 2.5 degrees of iso deviation i was talking about so we'll do a quick calculation of that so the iso deviation is minus 15. so 15 divided by 2.5 is gonna be six right so we're doing a six percent change um in altitude so 19595 times 0.06 that's six percent and we're looking at one one seven point and the actual difference is one one seven six so it's really really close it's really quite a good estimation and then we would take that from the 19595 and that is our uh answer in here and then you do this obstacle clearance the same way so the one percent per 2.5 degrees is just as good if not maybe a bit quicker than the temperature correction of four feet per thousand feet per degree of iso deviation so this is probably a diagram you've seen somewhere before or something a bit more colorful and a bit better than this but it's showing what we call isobars these lines are all lines of equal pressure and it's the calculated qff from an airfield or the actual measured pressure at sea level and every point along this line has exactly the same pressure and the difference between the lines is normally either two or four hectopascal so this would be one thousand this would be one thousand and two as with this one or this could be the other way around so this could drop down to or they're both going to highs but this one here would be nine nine eight this one here would be nine nine eight and so on and then you drop down these eyes of our charts can be named and labeled in various ways we give the highest pressure um on the chart in h and the lowest in l but there can be obviously secondary high points with the same high pressure as this and secondary low points and ice charts are very useful as pressure systems and areas have fairly predictable weather and which is something we'll learn about more in future classes but just a quick example um for you it would be if the isobars are close together it means that the wind is going to be stronger than if they're quite far apart like this so this is going to be a very windy area this will be very calm over here there are many more patterns and predictions that can be made using isobar as well as i said we'll look at some of them in future classes to summarize then pressure is the force over the area and because we have fewer particles above us that means our weight is lower and our force is lower as we climb up through the atmosphere so that means our pressure reduces so as altitude increases the pressure decreases the rate at which this happens is one hecta pascal drop for every 27 feet increase in altitude or 30 feet for easy calculations we measure pressure in hectopascals one hit pascal is equivalent to one millibar and in the international standard atmosphere at sea level we have a 101 3.25 hectopascal pressure or 101 3.25 millibar pressure or if you're measuring it in somewhere that uses inches of mercury it's 29.92 in higgs inches of mercury hgs being the chemical symbol for mercury so in terms of altimetry as i said there's a good class well i think it's quite good um i did in the gnab series explaining about this a bit more but basically if you're setting q h you're reading indicated altitude if you're setting standard which is one zero one three then you're reading a pressure altitude or a flight level if you round it up and take off the last two zeros and if you're setting qfe you're reading height above the ground um and the height of the ground from the sea level to the highest point of the ground is known as elevation so qnh is calculated by sensing the pressure at the airfield or the weather station then using 27 feet per every hecta pascal with your known elevation to calculate an equivalent sea level pressure the qff does this but doesn't use a standard 27 feet per every hit pascal it factors in temperature corrections so it uses the daily lapse rate in a sense um so it's the 27 feet adjusted for temperature and that temperature correction is four feet for every thousand feet um for every degree of iso deviation or a good estimation well very accurate decimal estimation is one percent of altitude for every 2.5 degrees of iso deviation and then we have isobar charts which use the qff which is the equivalent sea level pressure or the actual measured sea level pressure and the isobars all have equal pressure iso meaning uh the same i believe so iso bar same bar lines i don't know um but yeah they're all the same pressure and there's usually about two or a four hectopascal difference between them all so that'll be one thousand this would be one thousand and two 1004 and this would be 1006 which is actually not a very high pressure but yeah and we use the ice bar charts to help predict weather which we'll look at in future but just a quick sneak peek if the isobars are really close together it means it's gonna be a lot more windy than if they're quite spread apart