How Temperature Effects Altimeter Readings & Different Altitudes - For Student Pilots

[@GreedoTheDog]

Best damn explanation so far. Loved this. You really helped clear up my frustrations with understanding Pressure/Density altitude and what happens with the temperature changes. Thank you for posting this

[@PartTimePilot]

Appreciate the comment and glad to help!

[@StoneStraiff]

This is by far the best explanation I've found on youtube.

[@PartTimePilot]

Thanks! and thanks for watching!

[@jeremiahsokaim7528]

Thank you so much Nick. You’ve just answered all the question I had about Altimeter

[@PartTimePilot]

Anytime! Thanks for watching!

[@macemanVII]

This helped out a ton especially towards the end when you spoke about the air molecules contracting according to temperature.

[@PartTimePilot]

Glad to hear! Thank you for watching

[@aarthikrishnan4379]

I have to present about these front for this weekend class . I don't know about it priorly .. it will help my presentation as well. Thank you!

[@PartTimePilot]

Appreciate the comment and good luck!

[@y.h8383]

your video should have much more likes. and thanks for responding to questions in comments. very useful info

[@PartTimePilot]

I appreciate that and appreciate you watching!

[@lindat9531]

Thank you so much! Your explanation helps me a lot !

[@PartTimePilot]

Glad to hear it! Thanks for the comment : )

[@seattleraf]

Okay I was having a real hard time understanding why/how indicated altitude increases with lower temp since I thought about it as lower temp = denser air which I thought would make altimeter decrease since air density decreases with increased altitude. But your explanation made sense. Now what I’m a bit confused about is that if pressure and temperature always go up and down together how it’s possible to have a day with standard temperature but non-standard pressure or vice versa

[@PartTimePilot]

So in an ideal case pressure and temperature would always move together. In science they call this the ideal gas law that basically says when you have no outside heat exchange or forces on a volume of air with an ideal gas inside of it, the relationship or pressure, temperature and density is P = dRT where R is a gas constant. So from this equation we can see if P goes up, T goes up, if P goes down, T goes down and vice versa. Air is not 100% an ideal gas. It’s made up of mostly nitrogen, some oxygen and other stuff. Plus, the atmosphere is not a perfectly closed system. There is heat exchange all over the place from cold air masses mixing with warm air masses, warm land heating up cold air, water condensing and vaporizing and changing the air density. So it’s not a perfect system. I am saying all of this to say that the aneroid wafers which are like balloons inside the altimeter are more of a perfect system than the atmosphere so they stick to this P and T relationship. You can do this experiment yourself. When you have a full balloon at room temp, put it in a freezer and watch it deflate as the pressure lowers. This is because the mass of air isnt changing and the kinetic energy (temperature) is going down which causes the pressure against the walls of the balloon to go down. I know this doesn’t make it clearer/cleaner, but hopefully it makes sense.

[@seattleraf]

​@@PartTimePilot sounds good! Between your video and this video (kzbin.info/www/bejne/nJ6rqJewdtuFq6s) I'm understanding this much better now. Thanks for replying!!

[@PartTimePilot]

@@seattleraf anytime! Thanks for watching

[@darielgarcia7817]

Video released on my birthday and it helped me a ton, thanks man lol

[@PartTimePilot]

Awesome! No prob

[@dmacnet]

At 4:16 you say "Pressure altitude is not used while you're in the aircraft," but that's what's used in flight levels (above the transition level, 18,000 feet MSL in the USA). So you must just be referring to VFR pilots.

[@PartTimePilot]

Ya this video is specific to private pilot students who can only fly VFR. You need ifr to fly above 18,000

[@y.h8383]

excellent explanation. had to watch twice and it made sense

[@Kristian.87]

Wow this helped so much. Thank you

[@PartTimePilot]

Thanks for watching!

[@GOMD77]

Perfect explanation, thank you.

[@natalieea10]

for AN HOUR ive been trying to figure out how i keep getting these questions wrong in my head... this whole time ive been thinking that density=pressure. I have a lot of unlearning to do...

[@PartTimePilot]

It’s a confusing topic for sure! Takes a bit to get it to stick

[@deawhite6132]

Natalie, I've been reading posts and watching videos about this stuff and couldn't fully get it. Then I red your comment and it all clicked!!!!!!! So THANK YOU

[@noplatesnodates679]

@@PartTimePilotso density doesn’t equal more pressure? And you say near the end of your video that Higher temperature equals higher pressure. But I thought higher temperature equals lower pressure because warm air is less dense than cold air?

[@nonDescriptAviation]

@5:12 - "Back to the Future" reference or just happenstance?

[@PartTimePilot]

happenstance but I know the scene you're talking about!! haha

[@mxrriderr]

This still confuses me since high pressure areas, on a surface analyses chart for example, are associated with cooler air, and low pressure areas are associated with warmer air. If we take a compressed air tank with a fixed volume for example, the concepts in this video make sense to me. As air pressure increases, so would the temperature. But is the atmosphere not different since the volume is not fixed? As the air cools, more molecules are able to fit in the vertical column of air between the surface and the edge of the atmosphere, increasing the pressure.

[@PartTimePilot]

This is a difficult concept to grasp as well as explain lol. First off for any exam you’ll want to make sure you have the relationships down. Pressure is directly related to temperature in a closed system. Check. The atmosphere like you said is not a closed system so when you have a drop in pressure you increase in density and when you have a rise in pressure you have a decrease in density. The density increasing, increases the amount of air molecules in a column of air and increases the weight of that column (pressure). The same is true in reverse. But in a closed system like the aneroid wafer inside your altimeter (like a balloon)... when the balloon warms up the kinetic energy of the air inside increases and the force exerted on the walls of the balloon increase and that increases the pressure and the balloon expands. The opposite is true. You can do an experiment with a balloon full at room temperature. When you put it in the freezer it will deflate (lower pressure inside). This is because the mass has stayed the same inside. In the atmosphere we dont have the closed system so it is different. Did that help? Lol. This is probably the toughest concept to understand/explain.

[@mxrriderr]

@@PartTimePilot So in other words, let's say you have two hypothetical columns of air - one that is warm and the other that is cold. At the surface, the colder column might be a higher pressure than the warmer one, but as you climbed vertically through the columns, the colder one would "taper off" quicker than the warmer one until the pressure in the warm column was actually greater than the cold one at a given altitude. Or the rate of change of unit of pressure per unit of altitude would decrease faster in the colder column than in the warmer column.

[@PartTimePilot]

@@mxrriderr if I understand what you are saying which I’m 90% sure I do then, yes!

[@Sunufrate]

​@@PartTimePilotwhat in this comment is actually the most important concept to explain, because this thing that in a closed system t's and P's are proportional and in an open one they are opposite is tricky and needs to be discussed further.

[@JadePope-zz3ho]

This is, in general a reasonably good explanation, however there are some errors in the details, the biggest one being this: WE DO USE PRESSURE ALTITUDE WHEN IN THE AIRCRAFT. All higher altitude flights rely on setting the barometric pressure at 29.92" of mercury (1013.2 mb) in the Kollsman window, to give a Pressure Altitude reading, this is what Flight Levels (FL) are based on. I'm using the U.S. as an example. Flight safety in Class A airspace (i.e. >18000' MSL < FL600) relies on the fact that when at altitudes of 18000' and above all aircraft reset their altimeters to a sea level barometric pressure setting of 29.92" rather than the sea level barometric pressure setting obtained at their departure airport. Doing this allows vertical separation to be maintained over long distances with varied actual barometric pressure. For example, if two aircraft were flying the exact same ground track in opposite directions at the same time between New York (NY barometer setting 28.86) and Seattle (Seattle barometer setting 29.92) with the west bound flight supposed to be at 26000' and the east bound flight supposed to be at 27000', due to the different barometer settings they would both be at the same true altitude (i.e. height above mean sea level) when they meet in the middle even though the altimeters in the aircraft show 1000' difference. NOT GOOD! But when they both use the standard pressure setting of 29.92 their true altitude may vary with variations in local barometric pressure but as long as each aircraft maintains their correct indicated altitude their vertical separation relative to each other will be maintained. As such when the diagram indicates 20500' MSL and a barometer setting of 30.00 that is WRONG. When in the FL at 18000' MSL and above the barometric pressure setting in the Kollsman window of the altimeter should be reset to 29.92

[@PartTimePilot]

Appreciate the comment. Video is meant for student and private pilots who aren’t allowed above 18k. But you’re right!

[@soundfreak2309]

@@PartTimePilot It might be worth pinning a comment about this because, the transition altitude elsewhere in the world can be much lower than what I imagine you're used to. For example, in the UK it can be as low as 4000ft. Private pilots certainly do use standard pressure and this video could confuse.

[@tanytansters]

Why at 6:50, do you say, "The hotter you get, the less dense the air". But then at 9:56 you say, " So the temperature went up, so, pressure goes with temperature. So that means the pressure went up. In each instance, are you not saying completely opposite things? I assume I'm just not understanding clearly, but this is very confusing. I have read through some of the comments where you're trying to explain this, but why in the video do you make (seemingly) opposing statements?

[@PartTimePilot]

Pressure is different than density. Pressure is a measure of the force a substance makes on another substance Density is a measure of the amount of molecules per unit volume

[@tanytansters]

@@PartTimePilot Gotcha. Okay. That helps, thank you.

[@PartTimePilot]

@@tanytansters but there still effect one another. As temperature goes up the energy of the molecules goes up and they move and bounce more off of each other. This spaces them out more and they fill a larger volume (density decreases). Then… to my column analogy in the video… the spread out molecules increase the size of the column and the amount of molecules above the aircraft which increases the pressure

[@tanytansters]

Right! I watched a video about taking a helium balloon from inside a house to outside winter conditions, and the balloon quickly lost pressure. So that makes sense now. I'm still having trouble understanding the interplay between aircraft performance, altimeter reading, colder temperatures, lower pressures and how exactly that relates to density altitude. But for now, I'm understanding that the indicated altitude reads lower when temperatures go higher because there is more kinetic energy between air molecules, thus resulting in higher pressure. I hope this is right. My ppl is going to take years if every concept is this hard. Lol

[@PartTimePilot]

@@tanytansters you are getting it! Just keep at it. This is easily one of, if not the most difficult concept for students. I struggled with it too. For a pilot tho… just remember pressure goes with temperature for altimeter stuff AND density is opposite temperature for performance stuff. Boom

[@aloha-rob]

Thanks!

[@yamidgaray9074]

Thanks for sharing ✔

[@PartTimePilot]

Anytime!

[@SorenHume]

small note, but above 18K ft, everyone is flying Flight Levels and not Pressure Levels- so not a good to use 20,500ft as a kollsman window setting scenerio

[@SorenHume]

above 18K (Class A) everyone is set to 29.92

[@PartTimePilot]

I know thank you no realized after it was a bad example lol.

[@bartendingpretty]

Love that 🧢?! Do you sell merchandise?

[@PartTimePilot]

Yes! shop.parttimepilot.com

[@claynog101]

Thank you

[@rgfuller2]

This doesn't make any sense. Increased temperature means less pressure because warmer air is less dense.

[@PartTimePilot]

Not true for a closed system like the aneroid wafer inside your altimeter. Temperature and pressure are directly proportional in the atmosphere or in gasses that can be assumed “ideal”. The ideal gas law then applies which says Pressure = Density*Ideal Gas Constant*Temperature. When temperature goes up, pressure has to go up. Say Density is 2, ideal gas constant is 2 and temperature is 2 then we get 2*2*2 = 8 for pressure. If the temperature increases to 3 we get 2*2*3 = 12 (pressure went up proportionally) if temperature decreases to 1 we get 2*2*1 = 4 (pressure decreased proportionally)

[@blakemeservy4011]

@@PartTimePilot I believe this is wrong. The altimeter isn't a closed system. The aneroid wafer is set at standard pressure but everything outside of it in the canister is subject to static pressure. Warmer temperature = less density = less pressure

[@PartTimePilot]

@@blakemeservy4011 agreed about the aneroid wafer and agreed that nothing is really a closed system but for me this is how I remembered the relationship of pressure and temperature with altitudes in an aircraft. In a closed, adiabatic system the ideal gas law can be used which is more or less P = nRT so if T goes up, P goes up. If T goes down, P goes down. When you say warmer temperature = less density you are correct but when you say it equals less pressure I believe you are incorrect. In our atmosphere when temperature goes up so does the pressure and when temperature goes down so does the pressure. This is where the mnemonic from high to low look out below comes from. When you fly from high temp to low temp without adjusting or correcting for it on your altimeter your altimeter will sense a lower pressure which it associates with a higher altitude. So the pilot thinks they are higher than they actually are and hence "look out below"

[@XU23]

Yeah I got confused too, but I think the direct relationship between pressure and temperature is only true for a given volume, such as a “closed system” like the altimeter.

[@edwardperez5638]

Gay lussacs law only applies to closed containers not the atmosphere. As temperature goes up in a non closed container WARMER air is less dense and is spaced farther apart, this equates to LESS pressure because there are less molecules pushing down on each other in a given parcel of space. But because the altimeter is a closed container the molecules hit eachother at a faster rate causing the altimeter to falsely indicate that the plane is at a area of higher pressure closer to earth

[@PartTimePilot]

Yes but our altimeter has an aneroid wafer which is a closed system.. sorta. Warmer air has more energy and the molecules have higher velocity and impact the things around them with greater force. This is a good article that explains it. Also, air is commonly treated as an ideal gas in Thermo/aerodynamics for ease of calculation. As you say, it is not an ideal, adiabatic closed volume but the assumption still generally holds. The ideal gas law shows a direct relationship with temperature and pressure PV = dRT. sciencing.com/temperature-affect-barometric-pressure-5013070.html

[@Jmnp08]

Not very clear.

[@PartTimePilot]

What are you confused about?

[@Jmnp08]

@@PartTimePilot I understand DA and PA. What always confuses me is how in cold air true altitude is lower than indicated. Can you simplify this? It seems counter to the general function of altimeter. I can see how air column is "more dense" but that seems like your true altitude would be higher than indicated because air is more dense the cooler it is.

[@PartTimePilot]

Good question and I think I know what you are wanting but let me know if this doesn't solve it. So first off in a perfect world our altimeter would always give us the True altitude. It would automatically account for errors in pressure and temperature changes. Unfortunately, this is not the case and I think you understand this point but want to make that clear. The Altimeter computes an altitude based solely on reading a pressure and comparing that to another pressure that it considers is the pressure at sea level. When the pressure at sea level changes, we can change our altimeter setting in order to tell our Altimeter "hey, our sea level pressure that you are using to compare pressures to is wrong, here is the updated one". When we do that we are keeping our altimeter reading as close to true altitude as we possibly can. We have a video on how altimeters work and it might give you a good visual of this: kzbin.info/www/bejne/bmaQd3meeM6jgK8 If you do not change your altimeter setting when the sea level pressure has changed you are going to get indicated altitudes that are different from true altitude... which leads us to your question. When you fly into an area of colder temperature without changing your altimeter setting you are going to have an indicated altitude that is different than true altitude. This is because when the temperature changes inside the aneroid wafer of your altimeter so does the pressure. When the temperature gets colder the pressure gets lower in a closed system (they go the same direction). When you have a lower pressure your altimeter sees this as a rise in altitude. Why does it see this as a rise? That is because it is reading a drop in pressure and it associates low pressures with high altitudes (it knows that as you increase with altitude you decrease in pressure and the whole instrument works based off this fact). So now you are in a situation where your true altitude is actually lower than what is indicated on your altimeter. This situation could have been avoided if you changed the altimeter setting for the new and lower sea level pressure giving your altimeter the most up to date "reference" for which it will compare pressures to. To boil it all down is as few as words as possible I would say: The altimeter calculates an indicated altitude based off the difference between the pressure its reading from the static ports on your fuselage and the pressure input into the altimeter setting as sea level pressure. If the temperature changes so does the pressure and if the altimeter setting is not changed your indicated altitude is wrong (not true altitude). When going from warmer to colder air the pressure drops and your indicated altitude will be higher than true. When going from colder to warmer air the pressure rises and your indicated altitude will be colder than true.

[@Jmnp08]

@@PartTimePilot yes. Perfect. Excellent explanation.

[@PartTimePilot]

@@Jmnp08 oh good. Let me know if you have more questions!

[@laurenadams6704]

Horrible Audio

[@PartTimePilot]

Thanks!