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Cold air is heavier than hot air because of its greater density, so the QNH of an airport inside a cold front would be higher. Flying towards the cold front would be, "low to high, hello sky".

But I also know that for altimetry, cold air generates lower pressure inside the pitot-static system than warm air, and it would increase the altimeter reading, so "hot to cold, look out below".

I'm confused because of the 2 contradictory factors. If we fly towards a cold front without adjusting the altimeter setting, would there be an increase or a decrease in the indicated altitude?


There's an ATP question:

Event after an aircraft passes through a front into the colder air.

Answer:

Atmospheric pressure increases.

Well, yes, that's why QNH would be higher, but our altimeter would sense the temperature and "feel" a decrease in atmospheric pressure.

So, what would be the real indication of the altimeter in this situation?

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"For altimetry, colder air generates lower pressure inside the pitot-static system than warm air" may be the source of your confusion.

We are taught the gas law PV= nRT. The application that lower temperature lowers pressure is valid for a CLOSED container, not for the atmosphere we fly in.

If you reduce the temperature (as weather over a region), you increase the air density, all other things (such as altitude) being equal. This results in a lower altimeter reading for the same altitude when passing INTO a cold front.

A study of barometer trends as cold fronts pass at the same altitude (the ground you are standing on) should confirm these thoughts.

Regarding the reverse case, flying into warmer air, it's "high to low, look out below!". That is the one we need to look out for (gives higher altitude reading than where the aircraft actually is).

This is why it is never a bad idea to radio in for the barometric pressure at your landing site if weather changes significantly.

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The truth of it is always the simplest.....

Since you are flying and registering your height by your Altimeter there will be no change in your altitude at all.

However, if you wish to know if your height above the ground after flying through a cold front that too could be a poser....

You would need to know how you are crossing the isobars....

Put it most simply then pressure generally would increase along the flight path unless you were flying on a course towards the low centre ( in this regard you would need to draw a hypothetical set of isobars and plot a hypothetical course across the cold front ).

You will see that you can maintain the similar pressure or also actually have it decrease or increase depending on your track.

Flying from high to lower pressure whilst maintaining altimeter reading and local altimeter setting would result in a descending path...

Mnemonic....High to Low....Look out BELOW....

Ouch.....

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Well, I think you have a wrong point of view. If you fly from warm air mass into colder air your altimeter will read the same. It indicates pressure altitude so it does not matter what the density of the surrounding air is. Your reference is the assigned pressure altitude and that is remains constant.

But the thing is that the cold air is more dense than warm so, as you suggested, your true altitude is lower in the cold air mass. Should there be any obstacles you should increase your indicated altitude so that acceptable obstacle clearance remains.

If we turn the situation around and you actually would maintain your true, physical altitude constant some how, the indicated pressure altitude would be higher in the cold air mass.

The QNH is not directly affected by the cold air. The QNH is always referenced to airport altitude so on the ground altimeter always displays airport elevation regardless of the air temperature.

Another question is the weather phenomena that cause extremely cold temperatures. When temperatures go below -20C it is often caused by local high pressure accompanied by arctic air mass from the polar region. I’ve seen QNH over 1050 with very cold temperatures.

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  • $\begingroup$ you have it right. The third paragraph isn't "turning it around" though, it is the actual case, as the pilot would (generally) maintain heading. So the altimeter would show a decrease in altitude (greater pressure) even though the plane did not dive. $\endgroup$ – Robert DiGiovanni Sep 19 at 11:18
  • $\begingroup$ "If we turn the situation around and you actually would maintain your true, physical altitude constant some how, the indicated pressure altitude would be higher in the cold air mass." - This flatly contradicts the "ATP question" quoted in the question without giving any explanation. Can you explain why the ATP question is wrong? $\endgroup$ – Terran Swett Sep 19 at 12:37
  • $\begingroup$ Also: "If you fly from warm air mass into colder air your altimeter will read the same." - Just to clarify, you're saying that your altimeter will read the same if you're flying at a constant indicated altitude, right? $\endgroup$ – Terran Swett Sep 19 at 12:50
  • $\begingroup$ Yeah, flying constant indicated altitude would be the “real life” case. There is no practical means to maintain constant physical altitude when temperature of the surrounding air changes. But yes, that is the case asked in the question. $\endgroup$ – busdriver Sep 19 at 15:04

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