An altimeter setting used at an airport for landings,
instrument approaches, etc is taken at a known position and
true altitude. It is corrected for all know errors in
"standard atmosphere" and that leaves the aircraft altimeter
errors as the primary error to be concerned about.

When you are flying in the mountains, the altimeter setting
may be from a location 100 miles away, on the other side of
a range and in a different air mass.
The greater the difference between the reporting station
altitude and the aircraft indicated altitude and the
pressure altitude, the greater the error between indicated
and true altitude.

True altitude is what gets you across a mountain with a
known elevation. Indicated altitude keeps you from hitting
other airplanes.
At a certain altitude, 18,000 feet in the USA and as low as
3,000-5,000 feet in parts of the world, they go Flight
Levels, which are pressure altitudes. The lowest useable
Flight Level is adjusted for pressures below 29.92 [1013.2
mB] so that aircraft at a Flight Level will really be above
the indicated MSL altitude.



"Andrew Sarangan" wrote in message
ps.com...
| On Oct 21, 9:17 pm, Terence Wilson wrote:
| On Sun, 21 Oct 2007 20:06:30 -0400, "An Aviator"
|
| wrote:
| Current temperature is not used to convert
| pressure to altimeter settings.
|
| Can you please elucidate? Isn't current temperature
implicitly used if
| we use a barometer to measure local pressure and then
extrapolate the
| pressure at sea level (to be used in the Kollsman
window)?
|
| The interpretation of the altimeter setting (Kollsman
window) as the
| sea level pressure is what causing this confusion. The
altimeter
| setting is just a number. It is not the sea level
pressure. If you set
| your altimeter correctly for your airport elevation, and
go down a
| hole to sea level, your altimeter will not read 0 ft. What
it might
| read will be greatly influenced by temperature, though. On
a very cold
| day it might read a negative elevation, and on a hot day
will be read
| a positive elevation. On rare occasions it might read 0
ft.
|
| Therefore, temperature only affects what the altimeter
will read at
| altitudes other than the elevation to which it is
corrected for. The
| altimeter setting is only affected by the atmospheric
pressure at your
| airport elevation, and this is not affected by
temperature.
|
|