Altitudes, temps, and pressures.

[flying dog]

Ok, I'm showing my lack of knowledge asking this question, but it isn't that I don't know it: It is more I don't remember the details.

 

(That's my excuse, and I'm sticking to it)

 

Ok, ISA is 1013.2 @ 15 deg (or close to that). As pressure and temp vary the effect on the plane and how it flys has to be considered.

 

I have been told that temperature has more bearing on the altitude than pressure, but just to cover all bases, I shall ask for that too.

 

So, what are the changes to pressure / thousand feet

 

what is the change in temperature / thousand feet (or what ever).

 

I am wanting to make up a "cheat sheet" which people could print and when at their airport, get the temperature and calculate what the plane thinks it is at.

 

Anyone mind helping me?

 

 

[facthunter]

Density altitude

 

Density altitude is what relates to aircraft performance.

 

Your standard is a theoretical atmosphere upon which the instruments are Based. (ICAO.standard atmosphere) As you say it is 15 degrees C and 1013.2 Millibars at Mean sea level.(or 29.92 inches of mercury)

 

1 millibar equals approx 30 feet of altitude.

 

The lapse rate (change of temp with height change) is 2 degrees /1,000 ft, decreasing with increase of height. This is the "standard" lapse rate. The reality may be quite different.

 

On a standard day at say 5000' your temp would be 15 - (5x 2) = +5 degrees, and the pressure would be 1013.2- (5000'/30 which is 166 mb therefore pressure = 847.2 mb..

 

Variations to actual pressure and temps cause the density altitude to vary and high temps and lower pressures make the density altitude higher and the aircraft performance deteriorates. Most pilots calculate the density altitude on the usual pilots calculator, but it can be done with a rule of thumb for the temp. I think it is 200' per degree variation from standard. Check it up Nev

 

 

[flying dog]

Nev',

 

Thanks.

 

I am kind of doing that now.

 

Though I am distracted with "learning" how to use the "wizz wheel" again to satisfy the curiosity. (See other post)

 

Would you like to take me through that temp calculation again? It doesn't look right.

 

"On a standard day at say 5000' your temp would be 15 - (5x 2) = +5 degrees"

 

2 degrees / 1000 feet. 5000 feet = 10 degrees.

 

Just checking.

 

 

[facthunter]

temps.

 

The change is 5 x 2 = 10. subtract from the MSL of 15 = +5. It is worth spending a bit of time on this as it has big safety ramifications when you operate at altitude with high temps. Your plane may have an absolute ceiling of say 6000' at a certain weight, but you can get a density altitude above that at an aerodrome which has a much lower height above sea level.

 

on a hot day and the QNH may be a bit lower than 1913. at the same time. The temp is the biggest factor. When a gas expands Ie when it heats up it expands has more volume but the number of molecules remains the same so the density reduces. Everything an aeroplane does relies on the relationship Lift etc. = constant x density x V squared..Nev

 

 

[frank marriott]

30ft ft/mb yes

 

Try 120ft per degree from ISA not 200

 

 

[farri]

https://www.recreationalflying.com/xf2/uploads/emoticons/002_wave.gif.62d5c7a07e46b2ae47f4cd2e61a0c301.gif[/img]

 

 

[Yenn]

I couldn't remember the theory of this so looked it up in my text books. I was surprised to see that the Dry Adiabatic Lapse rate is quoted as 3 deg C per 1000' and the saturated rate is about 1.5 deg C. My memory told me the figures were 2 and 1 degrees. Looking at my whizz wheel the 2 deg per 1000' looks correct, so I am wondering about the text book, could it be degrees F, but even that looks wrong.

 

The Dry lapse rate DALR is the rate at which a parcel of air cools as it rises. The Saturated rate SALR is the rate of cooling of air with water vapour as it rises and condensation starts, or where clouds start forming, so we should not be there. The 1mb per 30' sounds right to me.

 

I shall have to do more study as this has me worried.

 

 

[facthunter]

120' /degree.

 

Yes I wasn't sure of that. That is why I said check it up. I couldn't lay my hand on the info at the time. Thanks.

 

Regarding the behaviour of gases you can use the universal gas laws provided you use the right units. Temp has to be in degrees absolute, (Kelvin) where zero is -273 ? degrees C. The gas equation is P1V1 over T1 equals P2V2 over T2 = K (a constant) for any parcel of gas. Adiabatic expansion (cooling) occurs when pressure is taken off a gas, and it expands, as it does when it rises. When pressure is applied to a gas (as in a tyre pump) the temp increases, as when a parcel of air descends.. The Standard Atmosphere is not what occurs in reality. It is an agreed approximation. It is used to calibrate pressure sensitive instruments so they will all indicate the same figure under the same conditions .Nev

 

 

[flying dog]

(Big blush)

 

Yeah, it is confusing.

 

So every 1,000 feet I go up, I lose 2 deg C.

 

ISA says at sea level it is 15 deg C.

 

So, in the model, at 3,000 feet, the temp should be 9 deg C.

 

If the temp is 13 deg, there is a 4 deg difference. 4 deg = 2,000 feet.

 

So, do I add or subtract that from the actual height? Is the plane thinking it is at 5,000 or 1,000 feet?

 

Sorry folks, I *AM* listening, but the data just doesn't seem to be getting in.

 

 

[Guest ozzie]

I remember the lapse rates as 2 and 1.5 deg c. never know it may have been refigured from all the climate change stuff going on. wow an extra 1 degree on the rate both wet and dry would probably give a whole new weather system.

 

 

[facthunter]

Correction.

 

IF the conditions are different from ISA, (The STANDARD Atmosphere,) THEN you make a correction. . You ADD/ SUBTRACT the effect of the difference as appropriate. IF it is HOTTER than ISA then the air is less dense and your DENSITY altitude is higher than your pressure altitude. eg, if you are flying at a pressure height of 5000' where the ISA temp should be +15 C minus (5x2) = 10 The ISA temp should be 15-10 = +5 C IF it is actually = +8 degrees, then you would apply a correction of 3x 120 feet to get the DENSITY altitude, adding it because the temp is ABOVE the ISA temp for 5000', so your density altitude is NOW, 5360' OK. Practice a few examples and you will get the hang of it. There must be some of this on the RAAus site. It may explain it better than I have. Hope this helps Nev

 

 

[flying dog]

Facthunter,

 

Could you explain/resolve my example?

 

I do remember the 1 deg / 120 feet part as well, but ....... as I said, I am not remembering enough and really need a definitive walk through.

 

Pressure height well: That's another story.

 

 

[facthunter]

Correction.

 

Flying dog I had to go and eat and in my haste (She who must be obeyed, must be obeyed,) I made an error which I have now corrected, in my example above. Now to your example, (and I have got to keep scrolling to it,) the actual temp was 4 degrees hotter. NOW apply the correction of 4 degrees to get the Density altitude and the correction (call it an adjustment if that helps), is 120 FEET per degree equals 480 feet. It is hotter than standard so ADD it . Answer 5480 feet DENSITY height.. Remember hotter air kills performance so your plane performs as if it is flying at 5480' instead of 5000'. The opposite happens if it is colder than the standard atmosphere Nev.

 

 

[flying dog]

Answer 5480 feet DENSITY height

 

DENSITY? Isn't it Temperature height?

 

 

[flying dog]

Oh, and isn't that 3480 feet?

 

Sorright, we all make mistakes.

 

;)

 

 

[flying dog]

Ok, sticking my neck out further:

 

In the model, for every 1,000 feet you go up, you lose 2 deg temp.

 

Yeah, there is this Laps rate, etc.... But indulge me.

 

So if the air cools down 2 degrees every 1,000 feet we go up, why is it, that when we determin that there is a 4 degree difference between the world and the model, you only adjust it 120 feet per degree?

 

(Runs and hides under a rock.)

 

 

[flying dog]

I've learnt something!

 

ONE: To find the first value (pressure height?) while flying, you simply turn the altimeter to 1013, after noting the original value. Then turn it back.

 

TWO: Get the OAT. Compare that temp to the altitude you saw when the altimeter was on 1013.

 

Every degree is 120 feet difference.

 

So if it is 2 degrees warmer, add 2 x 120 (240) feet to the height. If it is 2 degrees colder, subtract.

 

THAT easy.

 

This is the track on which I was thinking but just hadn't realised it until someone else explained it in those words.

 

I'm happy.

 

 

[frank marriott]

The lapse rate of 1.5deg and 3deg is correct. For private met it is acceptable to use 2deg / 1000ft as an average.

 

When I did my cpl met it was required to use 1.5 in dry air and 3deg in wet (ie inside cloud) from the forecast.

 

Perhaps that might help you understand the three different lapse rates

 

Frank

 

Sorry other way around Dry 3deg and Wet 1.5deg

 

 

[facthunter]

Meteorology.

 

Altimetry is different from Meteorology. The ACTUAL lapse rate is dependent on water content , absolute humidity or relative humidity, which gives the water content as an expression of actual water /over saturated for those conditions. When water goes from liquid to gas it takes in heat. Bad way of putting it I know, just like when you sweat you cool by the evaporation of water. This affects the stability of the atmosphere and the water and high temps cause the high energy content of tropical cyclones . When a parcel of gas with a high moisture content rises it condenses some of the water vapour (forming cloud) The heat released may be sufficient to keep that parcel of gas hotter than the surrounding air mass. It has a lower LAPSE rate therefore and continues to rise and form Cumuliform cloud. Ie turret cumulus and Cumulonimbus Cb.

 

ALL this has nothing to do with Pressure height and Density Height, which really relates to navigation, Cruising height and separation (vertically) from other aircraft, and Performance of the aircraft for the purposes of take-off weight calculations, cruise height selection etc.

 

Try to get the concepts clear. There is no such thing as temperature height, excepting there is a reference to freezing level for obvious reasons. Nev

 

 

[flying dog]

Nev,

 

Thanks. I admit I am probably using these "incorrect" terms. This is partly because I don't know the correct ones anyway and I get them confused.

 

Though a "dog" my "cat" qualities come out now and then and the insatiable curiosity takes over on what is happening etc.... Thus my questions.

 

I am an RAA pilot who tried to get into GA back in '88 - and faild misserably.

 

That aside, I am just interested in this just now for reasons unknown to myself. I am trying to get the picture in my head correct to how it all fits together.

 

---------

 

I hope to not stop learning until the day I check out.

 

 

[facthunter]

Never stop learning.

 

That is what appears at the bottom of my posts. None of us Know so much that we don't get something wrong. Often a little bit of extra research will make the matter clearer, but if you are not using some things all the time then it may get a bit stale. THe RAAus website has a LOT of technical flying stuff there, so it should be a good reference. It may go into it a bit more than you need occasionally but it's there. I cannot recommend a book that will have it all either . It hasn't been written yet, and a lot of terms and rules keep getting changed, just to make the matter more difficult and confusing. Good luck FD. Nev.

 

 

[farri]

(Big blush)Yeah, it is confusing.

 

So every 1,000 feet I go up, I lose 2 deg C.

 

ISA says at sea level it is 15 deg C.

 

So, in the model, at 3,000 feet, the temp should be 9 deg C.

 

If the temp is 13 deg, there is a 4 deg difference. 4 deg = 2,000 feet.

 

So, do I add or subtract that from the actual height? Is the plane thinking it is at 5,000 or 1,000 feet?

 

Sorry folks, I *AM* listening, but the data just doesn't seem to be getting in.

Sorry skydog,wish it were that simple but barometric pressure comes into the equation also.

 

Looking at temperature on it`s own though,if the temperature at the point in question is above the ISA standard the air is less dense and the difference in temp height is added to that point, if temp is lower,the air is more dense and the difference is subtracted from that point.

 

Keep in mind to always refer to the temperature as + or - degrees.

 

Frank.

 

 

[farri]

IF the conditions are different from ISA, (The STANDARD Atmosphere,) THEN you make a correction. . You ADD/ SUBTRACT the effect of the difference as appropriate.

Good explanation on that post,Nev.

 

Silly me, got home for lunch and being in a hurry,I never got past the first page and didn`t see your post.

 

Cheers,

 

Frank.

 

 

[Guest ozzie]

I couldn't remember the theory of this so looked it up in my text books. I was surprised to see that the Dry Adiabatic Lapse rate is quoted as 3 deg C per 1000' and the saturated rate is about 1.5 deg C. My memory told me the figures were 2 and 1 degrees. Looking at my whizz wheel the 2 deg per 1000' looks correct, so I am wondering about the text book, could it be degrees F, but even that looks wrong.The Dry lapse rate DALR is the rate at which a parcel of air cools as it rises. The Saturated rate SALR is the rate of cooling of air with water vapour as it rises and condensation starts, or where clouds start forming, so we should not be there. The 1mb per 30' sounds right to me.

I shall have to do more study as this has me worried.[/quote

 

Yenn

 

i checked my text book and it too says 3deg dry. Hmm why does the 2 deg pops up in the head? maybe from the TAFE class i sat for Met.? maybe cause i have never had to put this amazing bit of science into practise since i closed the text book. either way maybe i better join you back in class Yenn

 

ozzie

[Yenn]

Ozzie. Was that back in class or back of the class?