altimeter question,PPL level

[kb1]

Hello folks,

I'm hoping one of you could explain why an altimeter over reads in cold weather, in simple english. I understand why when flying from high pressure to low pressure the alt. over reads, so does that mean that in cold weather the pressure decreases? If so does it matter as you should really be flying with regional QNH set. HELP! I think I,m more confused than ever.

[Delta Wun-Wun]

Right here goes....(Get ready to correct me)..Altimeters measure pressure.So if you set a pressure setting on the altimeter say 900mb.and at that setting you are flying at 2000ft.If you then fly into colder air,that 900mb column of air will be lower,so you are not now at 2000ft but could be a lot lower.I think thats what the notes I have are getting at,but if I am wrong or anyone can put it simpler please do. http://www.pprune.org/ubb/NonCGI/eek.gif http://www.pprune.org/ubb/NonCGI/eek.gif

------------------
GET THE BLOODY NOSE DOWN!

[chicken6]

kb1

As Delta Wun-Wun said, the altimeter actually measures pressure, but the scale on the instrument we have in the cockpit is labelled "feet". So we have a problem to start with - we are flying pressure but our instrument tells us it's feet AMSL. No matter, so long as we know what we are dealing with, and pilots in the same area have the same reference (QNH).

This does mean, however, that instead of flying a constant "altitude" contour, we are actually flying constant "pressure" contours.

If you fly from a place where the pressure on the ground is high, and you fly to a place where the pressure at the surface is low and you are keeping a constant altimeter reading, you actually descend because the pressure contour you are following is descending. If your altimeter shows 2000' but you have descended under this, we say the altimeter is overreading (i.e. reading over what it should).

Part 2

Pressure decreases with height, but not at a constant rate. Delta Wun-Wun had it mostly right with the colder air being more dense.

Given the same surface pressure and warm air vs. cold air, the 900mb level will be lower in the cold air i.e. your altimeter overreads again.


Just a phrasing issue there, the column of cold air to 900mb is shorter, not lower. The 900 mb level is lower, and the column is shorter.

Any questions, ask again!

------------------
Confident, cocky, lazy, dead.

[kb1]

Thanks you guys.

I need to have a look at my notes again . Whatwas confusing me is that I thought since cold air was denser, then pressure would be higher, therefore you would be flying from low to high pressure so alt. would under read. Thanks for taking the time.

[Tinstaafl]

kb1, you're correct - cold air is denser.

The altimeter pressure-to-feet calibration presumes a certain density of air.

If air temp. changes then so does the vertical distance of the column of air in which you're flying, for any constant pressure reading (ie the constant altitude you're keeping).

Another way to look at it:

Take two columns of air, one at a low temp & the other at a warmer temp. Move up each column of air to a common pressure eg 900Hpa or whatever.

You will have travelled a greater distance (in feet) vertically in the warmer column than in the colder column because the air is less dense in the warmer column.

Hope this helps.

[aerostude]

pressure and temperature are related by the expression

P = ro R T

Where P is the pressure, ro is the density, R is the universal gas constant and T is the temp.

Clearly all things being equal, if the temp goes up, so will pressure.

[kb1]

Sorry but I'm a little slow. Is this correct.

I am flying ar 2000ft in warm airspace. Let's say 2000ft represents on that day flying at a pressure level of 900Hpa. As I travel towards cold airsplace, keeping 2000ft reading on the alt. I am in effect following the 900Hpa pressure level.Since the air is denser, a shorter column of air is required to exert 900Hpa, as opposed to a longer column of air to exert the same pressure in warm air. Therefore in order to keep a reading of 2000ft, I descend along with the 900Hpa pressure level, so I am now flying lower than 2000ft so my alt. overreads.

If this is right let me know please.

Ps. When at 2000ft(pressure 900Hpa) that represents the column of air above the aircraft?

[HugMonster]

Spot on! :)

------------------
Breeding Per Dementia Unto Something Jolly Big, Toodle-pip

[kb1]

Thanks everyone. Can I put it another way?
If you look at it in terms of pressure difference from say 900Hpa at altitude to 1010Hpa at ground level (110Hpa).
Then if cold air is denser, it takes a shorter column of air from the ground up to reach 900Hpa and make a pressure difference of 110Hpa. If the ambient temp is greater, the the air is less dense and it takes a longer column of air to reach 900Hpa and make a pressure difference of 110Hpa. If that is correct then I understand and I thank you all very much.

Ps. How's it going hugmonster, hope you are well and the flying id good.

Cheers kb1