On Mon, 13 Aug 2007 17:26:59 -0500, Dallas
wrote:

On Mon, 6 Aug 2007 12:05:17 -0500, Dallas wrote:

An increase in temperature will result in a higher atmospheric pressure - a
higher temperature speeds up the movement of the air molecules, thereby
raising the pressure they exert on the surrounding atmosphere.

Most respondents discounted this statement as incorrect or a flawed
problem. And yet it appears very simply stated in this form as a test
question on the private pilot written:

How do variations in temperature affect the altimeter?

A. Higher temperatures expand the pressure levels and the indicated
altitude is higher than true altitude.
B. Lower temperatures lower the pressure levels and the indicated altitude
is lower than true altitude.
C. Pressure levels are raised on warm days and the indicated altitude is
lower than true altitude.


Answer (C) is Correct - On warm days, the atmospheric pressure levels are
higher than on cold days. Your altimeter will indicate a lower than true
altitude. Remember, "Low to high, clear the sky."

:-/

"Everything else being equal". But it rearely is.

Consider the following example. The numbers are temperatures and
pressures as reported by six airports in San Francisco Bay area about
an hour ago. Flying from SFO to RHV one flies over (or close to) SQL,
PAO, NUQ, and SJC, in that order. The total distance from SFO to RHV
is a little over 30 nm.
Check the numbers:

SFO 21 30.01
SQL 25 29.99
PAO 24 29.99
NUQ 23 30.00
SJC 26 29.98
RHV 28 29.99

The temperature *increases* significantly, white the pressure
*decreases" (or stays the same if we assume that 0.1% variation is
within measurement tolerances).

We can also look at temperatures and pressures in one place changing
over time, for example at SFO over the last eight hours:

15Z 14 30.07
16Z 16 30.08
17Z 17 30.08
18Z 18 30.07
19Z 19 30.06
20Z 21 30.05
21Z 21 30.03
22Z 21 30.02
23Z 21 30.01

The numbers wouldn't make FAA question designers proud.

- Tom