temperature, pressure, altimeter error

[energie]

I have some question that revolves around these matter and I haven't been able to figure out. I am hoping to get some help here (again).


1. For the experienced pilot, is there a general rule of thumb that link the pressure, temperature, and weather condition together for you?

I.e. if you are flying into an area with altimeter setting of say 30.00 ", or 1050 millibar etc, is it true that you'll associate that with good weather? why? If that's not enough infomation, what else do you gather/reference to paint a picture of the upcoming weather?

2. I am trying to understand the effect temperature has on the altimeter. Specifically about how extreme cold temperature would cause the altimeter to have an indicated altitude higher than the true altitude. I am aware that in the pressure difference is more pronoucned in a cold air mass than a relatively warmer airmass. But that's only by memorization. Does anyone mind explaining the reason behind this?

cheers, and thanks in advance!

E.

[bwicker]

in regards to the second question.

"imagine the atmosphere to be very cold, well below the temperature for which the altimeter has been calibrated. The air molecules have lost energy and therefore gravity is able pull them closer to the Earth. Under these conditions the air's density and pressure fall more rapidly with height. Now imagine the atmosphere to be warmed to a very high temperature, well above that for which the altimeter has been calibrated. The molecules have gained energy and can counter the force of gravity, so the change in density and pressure with height is less. If the altimeter is used under the two conditions it will experience, for the same true height change, a greater pressure change under the cold conditions than it will under the hot conditions. Because it converts pressure change into height change, the altimeter will register a greater height under the cold conditions than it will under the hot conditions even though the actual height ascended is the same." --from: http://bubl.ac.uk/org/tacit/tac/tac43/theuseof.htm

Seems as though that should be right though. I'd love to hear some other comments on the subject though!

[sir.pratt]

can't say i've ever heard of gravity affecting an altimeter.
unless you run out of the stuff.

it's reasonably simple. cold air is more dense than warm air. example: flying at 10000ft with qnh set at a 1010hpa. oat is zero. in the absence of any other changes (eg the qnh remaining the same, actual altitude remaining the same), an increase in temperature of 1 degree c will result in the altimeter reading higher - as the air is slightly less dense, as it is warmer, so the altimeter thinks it is slightly higher than it actually is. i can't remember what the feet per degree rate is - 130ft/degree rings a bell, but i could be wrong

[bwicker]

If it can affect light i'm sure it has an effect on air molecules :confused:

[sir.pratt]

have a read through the 'similar threads' at the bottom of this page. written by some much more intelligent guys than I

[HotDog]

MSA
Altimeter setting -- Lowest Useable Flt. level -- Increment add
29.92 or lower -- 180 -- 0
29.91 - 29.42 -- 185 -- 500 ft
29.41 - 28.92 -- 190 -- 1000 ft
28.91 - 28.42 -- 195 -- 1500 ft
28.41 - 27.92 -- 200 -- 2000 ft
27.91 - 27.42 -- 205 -- 2500 ft
27.41 - 26.92 -- 210 -- 3000 ft
During flight within areas of HIGH temps or HIGH pressure, your aircraft will be HIGHER than indicated.
During flight within areas of LOW temps and Low pressure, your aircraft will be LOWER than indicated.

[Jaguar Pilot]

For what it's worth:

The altimeter is no more than an aneroid barometer with readings in feet.
It is calibrated to all the standard ISA atmosphere constants.
It can only measure from a given pressure datum (subscale setting).

In colder air, this pressure datum will be at a lower altitude/height because at any given level the air density is increased. It will thus indicate a value higher than actual. The reverse is true in warm air.

Remember the two Chinese restaurant owners, Mr. HI LO HI and Mr. LO HI LO. Their father was Mr. WUN HUNG LO, but that's another story.

By the way, gravity is not a force - it is an acceleration.
There is no such thing as g-force.
Weight is a force.

JP

[Paris Hilton]

Energie.

High pressure is usually associated with fine weather.
Low pressure is usually associated with bad (stormy convective) wx.

Check WiKi 'anticyclone' and 'cyclone' for good explanations.

[Paris Hilton]

JP.

Gravity is an attraction between masses, and is one of the four fundamental forces of nature.

It does indeed cause us to have weight.

Like any force, when unbalanced, it causes an acceleration (F=ma).

When balanced, like me sitting in my chair, there is no acceleration. If my chair were to be removed (rudely) from under me, I would accelerate downwards. Just like the apple from the tree.

[energie]

thanks Paris

[Paris Hilton]

energie.

Look it up. GooGoo?

[SB4200]

in the absence of any other changes (eg the qnh remaining the same, actual altitude remaining the same), an increase in temperature of 1 degree c will result in the altimeter reading higher - as the air is slightly less dense, as it is warmer, so the altimeter thinks it is slightly higher than it actually is.

sir.pratt doesn't this imply that following an increase in temperature, altimeter reads higher so you drop down to maintain your planned level, actually ending up lower than what you should be - contradictory to what actually happens?

[Robert Woodhouse]

You could check PANS-OPS Volume I. This is the ICAO document that covers information for pilots and operators and conntains a section on altimetry. Note that for instrument approaches the effects of wind should also be considered.