why high to low, look out below?

i'm studying for my private license and i am a bit confused about this
high to low, look out below. the way I see it:

imagine an altimeter sitting at sea level. lets say the atomspheric
pressure at the location of the altimeter is "10" (no units, just for
simplicity). now lets say you set the altimeter to "10". so now the
altimeter reads "0 feet"

now decrease the temperature of the air surrounding the altimeter, and
since the colder air gets the more dense it gets, the atmospheric
pressure goes up. the altimeter doesnt know this. all it sees is an
increase in pressure, which it thinks means an decrease in altitude, so
it indicates say "-10 feet". this is directly contrary to the "high to
low look out below" because the altimeter is telling you you are 10
feet lower than you are, which is not what that saying is implying.

SO WHATS THE DEAL?

wrote:
i'm studying for my private license and i am a bit confused about this
high to low, look out below. the way I see it:

imagine an altimeter sitting at sea level. lets say the atomspheric
pressure at the location of the altimeter is "10" (no units, just for
simplicity). now lets say you set the altimeter to "10". so now the
altimeter reads "0 feet"

now decrease the temperature of the air surrounding the altimeter, and
since the colder air gets the more dense it gets, the atmospheric
pressure goes up. the altimeter doesnt know this. all it sees is an
increase in pressure, which it thinks means an decrease in altitude, so
it indicates say "-10 feet". this is directly contrary to the "high to
low look out below" because the altimeter is telling you you are 10
feet lower than you are, which is not what that saying is implying.

SO WHATS THE DEAL?


If I'm not mistaken, the phrase "high to low" means you are moving
from a region of higher barometric pressure to one with a lower
barometric pressure. Your altitude does not change, but because
your altimeter is seeing a lower pressure it thinks your altitude
is higher than it really is. You "think" you're at 100 ft, but you've
actually just landed.

The situation you described is more accurately referred to as "low
to high".

wrote:

now decrease the temperature of the air surrounding the altimeter, and
since the colder air gets the more dense it gets, the atmospheric
pressure goes up. the altimeter doesnt know this. all it sees is an
increase in pressure, which it thinks means an decrease in altitude, so
it indicates say "-10 feet". this is directly contrary to the "high to
low look out below" because the altimeter is telling you you are 10
feet lower than you are, which is not what that saying is implying.

No, it's not contrary. The "high to low" means a flight from an area of high
pressure to low pressure, not high temperature to low temperature. When you
decreased the temperature, you increased the pressure, which means you're going
from low to high.

George Patterson
Drink is the curse of the land. It makes you quarrel with your neighbor.
It makes you shoot at your landlord. And it makes you miss him.

Hight-to-low refers BOTH to pressure and temperature, actually. An increase
in temperature causes an increase in pressure as air molecules now move
faster.

Pressure and temperature act in the same direction in terms of alimeter
settings, but work in the opposite directions in terms of aircraft
performance.

-- City Dweller

"George Patterson" wrote in message
news:rXUcf.5372$P.3083@trndny03...
wrote:

now decrease the temperature of the air surrounding the altimeter, and
since the colder air gets the more dense it gets, the atmospheric
pressure goes up. the altimeter doesnt know this. all it sees is an
increase in pressure, which it thinks means an decrease in altitude, so
it indicates say "-10 feet". this is directly contrary to the "high to
low look out below" because the altimeter is telling you you are 10
feet lower than you are, which is not what that saying is implying.

No, it's not contrary. The "high to low" means a flight from an area of
high pressure to low pressure, not high temperature to low temperature.
When you decreased the temperature, you increased the pressure, which
means you're going from low to high.

George Patterson
Drink is the curse of the land. It makes you quarrel with your
neighbor.
It makes you shoot at your landlord. And it makes you miss him.

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

On Fri, 11 Nov 2005 04:17:27 GMT, George Patterson
wrote:

No, it's not contrary. The "high to low" means a flight from an area of high
pressure to low pressure, not high temperature to low temperature. When you
decreased the temperature, you increased the pressure, which means you're going
from low to high.

I thought at the time, and I am even more convinced today, that the
solution to all this stuff is fly VFR in daylight!

(I appreciate that this isn't a great deal of help on the written exam
or the checkflight, but the poster might keep it in mind for the
future


-- all the best, Dan Ford

email: usenet AT danford DOT net

Warbird's Forum: www.warbirdforum.com
Piper Cub Forum: www.pipercubforum.com
the blog: www.danford.net
In Search of Lost Time: www.readingproust.com

George Patterson wrote:

No, it's not contrary. The "high to low" means a flight from an area of
high pressure to low pressure, not high temperature to low temperature.
When you decreased the temperature, you increased the pressure, which
means you're going from low to high.

Actually Geoerge, it works for temperature as well as pressure.
Going to a colder than standard pressure causes your altimeter
to read high as well.

wrote:
i'm studying for my private license and i am a bit confused about this
high to low, look out below. the way I see it:

imagine an altimeter sitting at sea level. lets say the atomspheric
pressure at the location of the altimeter is "10" (no units, just for
simplicity). now lets say you set the altimeter to "10". so now the
altimeter reads "0 feet"

now decrease the temperature of the air surrounding the altimeter, and
since the colder air gets the more dense it gets, the atmospheric
pressure goes up. the altimeter doesnt know this. all it sees is an
increase in pressure, which it thinks means an decrease in altitude,
so it indicates say "-10 feet". this is directly contrary to the
"high to low look out below" because the altimeter is telling you you
are 10 feet lower than you are, which is not what that saying is
implying.

SO WHATS THE DEAL?

You are arbitrarily changing the pressure due to decreased temperature. IF
that actually happens you are correct. But temperature can decrease without
increased pressure. That's why "cold or low, look out below" is correct.
If everything else stays the same except temperature drops, you will be
lower than indicated. If only the pressure drops, you will be lower than
indicted. If both decrease you will be much lower than indicated. If one
drops and the other increases, ??????,

You are overthinking the problem.

--

Darrell R. Schmidt
B-58 Hustler History: http://members.cox.net/dschmidt1/
-

Next time a big low pressure system moves through, go sit in a plane
and check the altimeter. It should read HIGHER than the field
elevation. The altimeter is reacting to a decrease in pressure, which
it registers as an increase in altitude. Now instead of sitting on the
ground, imagine that you had *flown* from a higher pressure region to a
lower pressure region while trying to maintain a constant altitude.
You would have compensated for the (false) indication of increased
altitude (actually you were just moving from high pressure to low
pressure) by lowering the nose ever so slightly and actually *losing*
altitude. Thus "high to low, look out below".

And forget all the stuff others have posted about temperature. That
does nothing but confuse the issue.

Jim Rosinski

wrote:

now decrease the temperature of the air surrounding the altimeter, and
since the colder air gets the more dense it gets, the atmospheric
pressure goes up. the altimeter doesnt know this. all it sees is an
increase in pressure, which it thinks means an decrease in altitude, so
it indicates say "-10 feet". this is directly contrary to the "high to
low look out below" because the altimeter is telling you you are 10
feet lower than you are, which is not what that saying is implying.

SO WHATS THE DEAL?

"jim rosinski" wrote in message
oups.com...
And forget all the stuff others have posted about temperature. That
does nothing but confuse the issue.

The issue is already confused. People like you who want to pretend it's not
only serve to confuse it even more.

Forget "all the stuff others have posted about temperature" at your own
peril. It's every bit as important an altimeter error as that caused by
changes in pressure.