why high to low, look out below?

wrote in message
ups.com...
[...]
SO WHATS THE DEAL?

The deal is that this is a great example of why one needs to be very careful
about using mnemonics. They are memory devices, not physics lessons.

The phrase "high to low, look out below" applies to both temperature and
pressure, for different reasons. You have mistakenly applied the physics of
the pressure situation to a change in temperature, combining the two
situations when in fact the mnemonic really assumes that you are holding
temperature constant when you change pressure, or vice a versa.

In both cases, the issue is easier to understand if you can think about
flying at a constant pressure, rather than a constant altitude. This is, of
course, what we actually do when we fly, assuming the altimeter setting
isn't changed. All the altimeter knows is pressure, so assuming we fly at a
constant indication on the altimeter, we are flying at a constant pressure.

What this means is that as the actual pressure levels change in height, so
does the airplane. For a given barometric pressure, a higher altimeter
setting results in a greater actual height above ground. The same thing is
true for higher temperature.

You've already shown why it's true for a higher altimeter setting, but just
to review...

Your actual altitude is proportional to the difference between the
barometric pressure at your current position and the barometric pressure at
the ground below you (always higher than your current position's pressure).
If the altimeter is set with too high a setting, your indicated altitude
will be too high, and if the altimeter is set too low, the indicated
altitude will be too low. This means that if you fly from high pressure to
low, the altimeter setting will be too high, resulting in an indicated
altitude higher than actual. You can see why thinking you are higher than
you are might be bad.

In the temperature case, it's because temperature affects how quickly the
pressure actually changes with altitude. The higher the temperature, the
farther apart two given barometric pressures will be vertically. It's as if
the atmosphere is made up of layers of pressures, and higher temperature
makes those layers expand away from each other. Conversely, as the
temperature goes down, those layer get closer and more compact. Those same
two given barometric pressures wind up nearer each other vertically.

The altimeter only indicates correctly at a specific temperature. When the
temperature is higher than that, the altimeter indicates too low (because
the pressure layers have expanded apart), and when the temperature is lower
than that, the altimeter indicates too high. And of course again, an
altimeter that indicates too high is not good.

The mnemonic applies best to the pressure case, as it's easy to see how
flying from an area of high pressure to an area of low pressure might be an
issue. In addition, in the pressure case it applies mainly to the situation
when you actually change position without resetting the altimeter (so it
fits the wording of the mnemonic reasonably well).

In the case of temperature, the error exists regardless. So in that
respect, the mnemonic is misleading. It might be a useful way to remember
the issue, but it might mislead someone into thinking the hazard exists only
when one starts in an area of higher temperature and flies to an area of
lower temperature. That's not actually the case though. The real problem
is simply that the altimeter is calibrated assuming a standard atmosphere,
with a constant mapping from pressure change to altitude change when in fact
a real atmosphere varies this mapping according to temperature.

If you are flying in an area of cold air, the indicated altitude (except
right on the ground) will always be higher than your actual altitude,
assuming you're using the correct altimeter setting. It's not the act of
flying from a warmer area to a colder area, it's the act of flying in a
colder-than-standard area, period.

Hope that helps.

Pete