True v Indicated Altitude

[SkyGirl]

Why is indicated altitude higher than true altitude when the temperature is below standard?

[MidlifeFlyer]

I used to always get this wrong. Using "high to low look out below", I would get it right, but if I thought it out...

Finally heard and ended up with this visual:

Your airplane is riding on top of a column of air that is enclosed on the sides, but can expand up and down.

Temperature rises, the air, like any gas, expands, so the column rises. Temperature lowers, density increases, and the column of sir shrinks. Same for pressure changes. Lower pressure moves upward, low pressure moves downward.

That just covers the airplane. It moves higher with increases in temperature or pressure and moves lower with decreases. Problem is the altimeter doesn't change unless you adjust it (for pressure anyway)

So, you're altimeter reads 2000'. The pressure setting is correct, but it's an unusually hot day. You're riding higher than the standard day that the altimeter is set for. Unusually cold, the air is more compressed and you're riding lower.

[braidkid]

That's an excellent analogy MidlifeFlyer. I'll have to remember that one.

[flyguy]

[ QUOTE ]
Why is indicated altitude higher than true altitude when the temperature is below standard?

[/ QUOTE ]
This assumes that you do not reset your altimeter. Remember that higher temperature air is less dense. Air density also decreases as you go higher in the atmosphere. Basically what is happening is you are flying from a high pressure area to a lower pressure area. The altimeter does not know that the temperature is higher and only knows the pressure is lower and indicates the altitude for that lower pressure which is higher than you really are. Always make sure you have a current altimer setting, and this problem is aleviated.

[MidlifeFlyer]

[ QUOTE ]
Always make sure you have a current altimeter setting, and this problem is alleviated.

[/ QUOTE ]No it's not. The altimeter setting is based on sea level pressure - what the pressure reading would be if the airplane were sitting on the ground at a sea level airport directly below.

The accuracy of the altimeter in telling you true altitude assumes a "standard day" and a standard lapse rate. When its not a standard day (almost always) you can almost guarantee that indicated altitude will not equal true altitude due to both temperature and pressure errors. Pressure changes can be compensated for to some degree, but not temperature changes.

[flyguy]

What is the cause of variations in sea level pressure?

[MidlifeFlyer]

[ QUOTE ]
What is the cause of variations in sea level pressure?

[/ QUOTE ]At a specific station? Widespread high and low pressure weather systems.

[flyguy]

What causes high and low pressure systems?

[av8rmsu]

unequal heating of the earths surface

I think...don't hold me on this...i'm still learning

[E_Dawg]

You get semipermanant high pressure belts around 30*N lat and 90*N; low pressure at the equator and 60*N due to atmospheric circulation patterns. Also, temp differences due to uneven heating may cause local areas of high and low pressure. These pressure systems are formed and my drift over a given area, taking on the properties of that area and becoming airmasses with fronts.

[flyguy]

That's what I though. Unequal heating of the earth's surface. So if heat is responsible for high and low pressue weather systems, and high and low pressure systems are responsilbe for variations in sea level pressure, and sea level pressure is responsible for altimeter settings, how is it that altimeter settings are independant of temperature?

[E_Dawg]

Because temperature is only part of the picture. Not that I know much about this... but I'll give it a try

As said, you have those rising and descending air currents every 30* lat or so... This is due to corrolis force and the thinness of the atmosphere... air heated at the equator rises and then sinks at 30*, and so on.

This is why you typically get the high pressure belt around 30* north latitude: you have constantly descending cool air from the upper atmosphere, this also causes the deserts that stretch around the Earth at 30*N; such as the Middle East and the deserts of western USA.

So you have dense air over deserts heated by the sun, and the air takes on the properties of the land mass, which means you can have high temperature and high pressure at the same time - the pressure levels expand but you still have the same weight of air above you, which means it will exert the same pressure regardless of temp.

[MidlifeFlyer]

[ QUOTE ]
That's what I though. Unequal heating of the earth's surface. So if heat is responsible for high and low pressue weather systems, and high and low pressure systems are responsilbe for variations in sea level pressure, and sea level pressure is responsible for altimeter settings, how is it that altimeter settings are independent of temperature?

[/ QUOTE ]You may be trying to take a global phenomenon and apply it too locally. That temperature changes across the earth's surface cause weather systems to form doesn't necessarily mean that there is a one-to-one correspondence between temperature and pressure. And it doesn't tell us what that weather system's effect will be at one specific location on the earth.

This explanation is still too simplistic, but, with apologies to any meteorologists around, look at it this way. A low pressure system is created over the Pacific. Because of other systems being created in other locations and the prevailing winds aloft , the system moves on shore in the San Francisco area. Which do you expect will have more effect on the local altimeter setting? The low pressure system that settled over the area or the temperature fluctuations that normally take place throughout the day?

There's a cool website that will give you historical METAR for any weather reporting station. Here's the last 200 reports for Dan Francisco.
http://www.uswx.com/us/stn/?code=c&a...0&stn=ksfo

If local temperature correlated with pressure, you would expect that given the same reporting station, higher temperatures would be associated with higher pressures and vice versa, right? But if you look through the table, you'll see that it's not.

For example, the 1356Z report on January 21 had the temperature a 7°C and the altimeter setting at 30.20. But on the 18th at 2356Z, the temperature was 13°C (6° warmer) but the altimeter setting was 30.15 (lower).

I pulled up a fun one yesterday, but didn't have time to post it. At 2253Z Bethel Alaska, at 123 msl, was reporting -11°C and 30.20. Brooksville, FL at 77 msl was also reporting a sea level pressure reading of 30.20 but at a temperature of +12. Obviously altitude and temperature aren't the only factors.

But you don't have to go across days or the continent. The last three observations at for KSFO were

METAR KSFO 240956Z 12007KT 1 1/2SM RA BR SCT006 BKN012 OVC021 09/09 A3002
METAR KSFO 241056Z 12007KT 2SM BR SCT004 BKN012 OVC020 09/09 A3000
METAR KSFO 241156Z 13008KT 2SM -RA BR SCT004 OVC011 09/09 A2999

Looks like they're in for a bad day. Over the past three hours, stable air, no change in temperature, but the pressure steadily decreasing.

[flyguy]

Well, I'm not trying to argue your point, however if air temperature does not effect altimeter settings, it must not have a very pronounced effect on indicated altitude. Because I've never had my altimeter be more than about 10ft off of field elevation when I set it to the local setting.

[E_Dawg]

That's because the local setting is supposed to give you true altitude at field elevation.

If you set it to the local setting and fly to an airport 5000' above where you were, when you land your altimeter will read much lower on a hot day.

[flyguy]

Isn't that why you're supposed to change the altimeter setting to the local altimeter setting at the destination airport?

[ananoman]

[ QUOTE ]
Isn't that why you're supposed to change the altimeter setting to the local altimeter setting at the destination airport?

[/ QUOTE ]

If you set the correct altimeter setting, your altimeter will read the correct altitude when on the airport surface. This is not the problem. The problem is that if non-standard temperature exist, at any altitude above the surface your altimeter will be in error.

We have all heard that atmospheric pressure decreases by about 1" Hg per 1,000' increase in altitude. This is only true in a 'standard' atmosphere. If it is very hot, then the decrease in pressure will be less. It may for example only decrease .9" per 1,000'. This will not cause much of a problem, you will be slightly higher than you are supposed to be. The real danger is when it is extremely cold. If it is cold, the decrease in pressure will be more than 1" per thousand feet, and you will be low. (Remember your altimeter relies on the 'standard' lapse rate to gauge your altitude.) Enroute this is not a problem. Everyone else will have the same altimeter error, and separation will be maintained. During an instrument approach it has the potential to kill you.

In the AIM, section 7-2-3, there is a table "ICAO Cold Temperature Error Table", which tells you how much to increase your MDA over what the approach plates indicate in very cold weather. Using an example from the chart we can see that even if we set our altimeter to the correct altimeter setting, we can experience potentially fatal errors. If we are planning to fly a circling approach with a circling MDA of 1000' AGL and it is -50 C at the surface, then we will have to add 300' to our MDA. If we do not do this, we will be 300' low and could hit an obstacle or terrain. (Remember that the guaranteed obstacle clearance during a circling approach is 300'.)

VFR this would not usually be a problem, but it could be of interest if you plan on flying over mountainous terrain. Looking at the chart, if you set in the correct altimeter setting, then take off and climb to an indicated altitude of 5,000' AGL over the airport when the reported temperature is -50 C, then you will be 1500' low. This error decreases with an increase in temperature, and at +10 C, the error is only 90'.

[av8rmsu]

WOW

That is a great chart. I had no idea that non-standard temperature had that much effect on altitude.

[MidlifeFlyer]

[ QUOTE ]
If you set the correct altimeter setting, your altimeter will read the correct altitude when on the airport surface. This is not the problem. The problem is that if non-standard temperature exist, at any altitude above the surface your altimeter will be in error.

We have all heard that atmospheric pressure decreases by about 1" Hg per 1,000' increase in altitude.

[/ QUOTE ]Great answer. That's the whole point. Let's separate temperature from pressure for a moment.

An altimeter is set to correspond with an altitude on the ground. In the air, it =assumes= that the standard pressure lapse rate is correct. The needles move at the rate of 1000' for each 1" of static pressure change. The problem with the assumption is that the lapse rate that day might =not= be 1"/1000'.

The altimeter that reads 3000' =isn't= saying "you're 3000' above sea level. " It's saying, "Based on the sea level pressure setting you put into the Kollsman window, the static pressure here is 3" lower."

As you pointed out, the problem with the scenario is that the pressure lapse rate may not be standard. If the lapse rate is smaller, the pressure changes coming closer together, that 3" change that makes the altimeter say "3000" will happen at an lower true altitude.

[flyguy]

Okay I think I get it now. Thanks guys.