# Do airliners adjust altitude for latitude?

According to this NASA article, the troposphere extends from about 5 miles at the poles to about 9 miles at the equator. Though the height of the troposphere is defined by the height of the tropopause as per this Aviation SE question.

I'd expect that the air pressure at the top of the troposphere would be similar at 5 miles above the poles as it would be at 9 miles above the equator.

As the altitude of an airliner increases, the angle of attack necessary to produce the same amount of lift also increases. Eventually an altitude is reached where the angle of attack necessary to produce sufficient lift is also the stall angle of attack. This pretty much defines an airplane's service ceiling. (Certainly, there are other factors, but this is a pretty big one.)

This suggests that the service ceiling would be lower at the poles than at the equator. Does this mean that an airliner flying a polar route would fly at a lower altitude than one staying in the temperate or tropical regions?

• Aircraft at high altitudes fly based on pressure altitude. Also, pressure/temperature at the tropopause is not the same at the equator and the poles. – fooot Jun 21 '16 at 18:10
• You are confusing altitude with height – expeditedescent Jun 21 '16 at 18:23
• This figure, which its isobars used for flying at high altitude illustrates both @J.Hougaard 's idea and the answer to your question. It's part of the Wikipedia article on altitude. – mins Jun 21 '16 at 18:55

(aviationweather.gov) Significant weather chart.

Note the numbers in boxes. They decrease as the latitude increases: 450 → 400 → 350 → 250.

Note: those values change day to day and season to season, they're not fixed.

Those are the tropopause heights. 450 is FL450, i.e., 45,000 feet at a standard altimeter reading.

This figure is very important in flight planning. Jets typically fly around this figure for optimal aircraft/engine performance.

So yes, you're right, the farther you fly from the equator, the lower you should fly. However, the following two statements are incorrect:

• I'd expect that the air pressure at the top of the troposphere [be the same regardless of latitude.]

The pressure gradient is still the same. See: Why is the troposphere 8km higher at the equator than the poles?

• As the altitude of an airliner increases, the angle of attack [also increases.]

As the altitude increases, so does the speed, to counter the thinner air. Until you end up in a coffin corner.

Also, for high latitude polar flights, an airline can do away with the optimal performance, because the rise in temperature helps keep the fuel from freezing, however I may stand corrected on that.

• "As the altitude increases, so does the speed, to counter the thinner air." Just to add some color.. While true airspeed increases keeping lift constant, the indicated airspeed would also appear to remain constant since the dynamic pressure is unchanged. However, once the aircraft begins to approach the speed of sound, the airspeed indicator becomes unreliable due to the changing compressibility of air. A machmeter, which takes into account the speed of sound, is used to keep the aircraft well below its critical Mach number. – hemp Jun 21 '16 at 19:46
• But notice that the tropopause height depends much more on whether you are in the high or low pressure system than on the latitude. You can have 280 and 400 on the same latitude. Despite generally correct conclusion ("the farther you fly from the equator, the lower you should fly"), the reasoning is not what OP believes: the pressure may not necessarily be higher at the equator (on the same height), but it just becomes (usually) pointless to climb further if temperature doesn't fall. – Zeus Jun 22 '16 at 4:11
• On the second thought, you are right. What confused me (and I believe the OP) is that you should have mentioned pressure and so I expected it, because it was the core of the question (" I'd expect that the air pressure at the top of the troposphere would be similar...") and it is fundamentally wrong. – Zeus Jun 23 '16 at 1:07
• “You should fly at or below [tropopause] for optimal aircraft/engine performance.” No; the actual rule is that efficiency increases with altitude up to tropopause, is mostly constant for some height above it and then starts to decline again. So you want to climb at least to tropopause if your performance (which always decreases with altitude) permits, but not much above. There is no point in descending back if you move to region with lower tropopause though. – Jan Hudec May 18 '17 at 15:37

Does this mean that an airliner flying a polar route would fly at a lower altitude than one staying in the temperate or tropical regions?

Yes but not "this".

Your understanding of the effects of altitude is correct, except

• It is air density rather than air pressure that plays major role aerodynamically. Density falls with altitude similarly to pressure but a bit slower.
• Airplanes can compensate lower density with higher speed to avoid increasing the angle of attack, until they hit another limit.

Generally, distribution of pressure and density with altitude does not depend significantly on latitude, especially if we use (as we should) the correct geopotential altitude. As the diagram in the ymb1's answer shows, pressure depends much more on weather than on latitude.

At the same time, tropopause is defined exclusively by temperature. Unlike pressure and density, which have smooth distribution, it has "special points" where distribution "breaks". One of such points defines the troposphere/tropopause border. And temperature does depend on latitude.

Nevertheless, if you are flying a polar route, indeed you'll want to fly lower. But this is simply because flying higher when temperature doesn't fall anymore doesn't make much sense, primarily because engines efficiency won't increase. You may want to climb for other reasons though, but that's another topic.