gestrich1311

I guess this explains why the lowest region of the atmosphere is shorter at the poles comapared to the equator? Would we then say that a pilot flying in the very Northern/ Southern Hemisphere almost always need to be aware that indicated altitude is usually higher than actual since it is usually colder than standard?

shdw

Don't know about the first one. As for the second question, only if you have the wrong altimeter setting in. You are only lower when you fly from hot to cold if, big if here, you do not change your altimeter setting.

Consider this, I hope your ok with numbers.

Starting point:

Pressure at the surface (also known as the altimeter setting from ATIS): 30.00
Your altimeter is set to: 30.00 (correctly set)
Current altitude: 5000' AGL
Indicated altitude: 5000'
Current pressure at altitude: 25.00

Ending point: MUCH COLDER

Pressure at the surface (also known as the altimeter setting from ATIS): 29.00
Your altimeter is set to: 30.00 (incorrectly set)
Current altitude: 4000' AGL
Indicated altitude: 5000'
Current pressure at altitude: 25.00


See, the altimeter chased the pressure down as the pressure went down. That kept the indicated at 5000' even though you were in fact descending. Read over those numbers and explain your understanding please. It seems you missed something based on the quoted reply.

gestrich1311

Your example shows a change in pressure from high to low. You also show a change in temperature hot to cold. Why is the appropriate altimeter setting decreasing with a decrease in temperature? The previously posted picture shows a constant 29.92 altimeter setting.

shdw

A constant 29.92 altimeter setting. Just like my example has a constant altimeter setting of 30.00. This isn't working, give me a bit and let me see if I can't come up with a link to a real world scenario, something you already know. These other guys have posed a rising and falling parcel of air, but I think your problem is you don't really understand how the altimeter works.

Hmm maybe there was some confusion in my example too, I edited it. Reread it and see if that helps.

gestrich19

***Changed gestrich1311 to gestrich19 due to account access issue*****

I understand the operation of an altimeter but I think you are mixing two different concepts into one.

From "hot to cold, look out below" is regarding temperatures lower than standard, not incorrect altimeter settings. Remember "High to Low pressure" has its own little rhyme.

If the altimeter setting were the real issue, cold weather correction charts which are used to make corrections for really cold weather during approaches would be useless as you would just set your altimeter from ASOS/ AWOS and that would solve the problem.

I think the FAA's website explains it better than I can. As you can see, it doesn't mention this having anything to do with the wrong altimeter setting.

****
Temperature also has an effect on the accuracy of altimeters and your altitude. The crucial values to consider are standard temperature versus the ambient (at altitude) temperature. It is this "difference" that causes the error in indicated altitude. When the air is warmer than standard, you are higher than your altimeter indicates. Subsequently, when the air is colder than standard you are lower than indicated. It is the magnitude of this "difference" that determines the magnitude of the error. When flying into a cooler air mass while maintaining a constant indicated altitude, you are losing true altitude.
http://www.faa.gov/air_traffic/publi...7/aim0702.html

shdw

Interesting stuff bud. I was always taught high to low/hot to cold are the same thing since they both reduce pressure, which scientifically made sense to me. Nobody ever showed it to me like this, a very informative video that I think everyone should take a few minutes to watch: Cold Weather Altimetry - Cold Weather Flying | Robin's Roost

To add one more thing, there are a couple rules/things to think about with regards to this:

1) As with any IFR flying, you may request different altitudes. (Talked about in the video) If able, give them a reason.
2) Altitude assignments should never be corrected for cold/warm weather ops. If they say maintain 5000', you do it.
3) Radar vectoring altitudes are already corrected for cold weather ops. (Unconfirmed)
4) If applying altitude changes to any procedure between the IAF and DH/MDA you should advise ATC. (One source says this, the video contradicts it. You be the judge.)

gestrich19

Nice video. I wish there was more of that kind of aviation media online.

I only found a little information in the AIM regarding cold weather correction:

"The possible result of the above example should be obvious, particularly if operating at the minimum altitude or when conducting an instrument approach. When operating in extremely cold temperatures, pilots may wish to compensate for the reduction in terrain clearance by adding a cold temperature correction"

> Followed by a cold weather correction chart.

I can't find any hard rules regarding when to correct and when to notify ATC for the civilian however the USAF training program has a nice PowerPoint explaining what their procedures are for adjustments. The adjustments are applied during approach procedures in certain cases.

www.terps.com/cold/USAF%20AIS%20Cold%20WX%20Altimeter.ppt

Here are the operational concerns that I can sum up that the pilot needs to be aware of with non-standard temperature... Feel free to add or critique:

1. Approach altitudes in extremely cold conditions.

2. Staying clear of overlying/underlying airspace on hot/cold days. It is important not to fly to the vertical edges of the airspace even if your altimeter reported the correct field elevation before departure since non-standard temperature error won't be observed until you are fly above the reporting station due to non-standard pressure lapse rate.

3. Clearing mountaintops in extremely hot conditions (compared to standard). This one is probably the most concerning for the FAA when establishing procedures because altitude variations due to non-standard temperature increase with the vertical distance from the reporting station. In mountainous areas where the reporting station is much lower than the mountaintop, a much greater altitude error will occur when temperatures are less than standard. IFR minimums probably provide plenty of separation but this can help explain another reason why those higher minimums over mountains are in place. (MVA's of 2,000 instead of 1,000)