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  1. let's say we are at x-airfield which is at 0 MSL on the ground. We set the Kollsman window to 29.92 on a standard day, whatever the read is that would be the Pressure altitude, correct? (ideally zero too?). The next day is a very hot day and I set it to 29.92 would that give the same indication from the previous day?

if the pressure altitude changed from the previous day, I would say it was because of temperature, correct? however, when reading about it, it says that:

"Because Pressure altitude is a measure of the weight of the air above you. Temperature does not affect that. Heating the air just makes it expand"

it seems contradictory to me, saying PA wont change due to temp but if in the airfield the temp is different, it would read a diff PA, right?

I would appreciate the explanation. Thank you

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First of all, the pressure at sea level varies from day to day. Pressure systems are moving over the earth, driven by interactions of temperature differentials in the atmosphere, temperature differences between the atmosphere and water within oceans and lakes, the influence of upper-level disturbances, as well as the amount of solar heating or radiationized cooling an area receives.

The ICAO Standard Atmosphere (ISA) assumes a sea level pressure of 1013.25 hPa (equal to 29.92 inches of Mercury or inHg). ISA also assumes a temperature of 15°C at sea level, with a lapse rate of -6.5° C/ 1000 m of geopotential height. This is what the altimeter is calibrated to.

If you are at sea level on a ISA standard day and altimeter is set to 29.92 inHg (1013.25 hPa) then the altimeter will read 0 ft.

If the temperature increases, but the sea level pressure remains standard (1013.25 hPa) then the altimeter will still indicate 0ft.

Only if the sea level pressure is not standard, then an altimeter (set to the standard pressure of 29.92 inHg/ 1013.25 hPa) will not read 0 ft at sea level. For every hPa of deviation, the altitude will change by approximately 27 ft.

Now it gets more interesting when the airfield is on top of 1000ft cliff next to the sea.

On the ISA standard day, the airport will report a QNH of 29.92 inHg / 1013.25 hPa. The altimeter set to that value will read 1000 ft at the airport.

On the hot day, with sea level pressure at 1013.25 hPa, but a sea level temperature of 35° Celsius (ISA +20° C), the same altimeter setting would result in an altimeter reading of approx 935 ft. This because the high temperature causes the atmosphere to expand, and the pressure at which 1000ft would be found in the standard atmosphere, is now elevated to approximately 1065 ft.

The meteorological station at the airport, will assume that the atmosphere is standard. Based on the actual pressure measured at 1000ft, they will calculate what the sea level pressure is, based on the assumption that the temperature is standard. And thus the QNH will be reported as 29.99 inHg / 1015.65 hPa, and at this setting the altimeter will read 1000 ft again.

When we now land at the sea with this setting of 1015.65, the altimeter would read approximately 65 ft, because in reality the pressure at sea level was 1013.25.

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  • $\begingroup$ thank you for your answer. it seems like I go back to my question, why is it that pressure can stay constant at sea level (1013.25) but temperature can change? ... so are you saying pressure is always 1013.25 at sea level? $\endgroup$ Jun 5, 2023 at 21:40
  • $\begingroup$ also in your last example, how is it possible to have 1013.25 at sea level and 1015.65 at 1000 ft? when you say "..now land at the sea with this setting, the altimeter would read 65 ft approx" which setting are you referring to 29.99 or 29.92? $\endgroup$ Jun 5, 2023 at 21:42
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    $\begingroup$ @YamchaAviator, the answer starts by clearly stating that pressure varies due to weather (due to wind patterns), even at sea level. And its lapse rate also varies with weather (with temperature). In the last example, the actual pressure at 1000 ft is 979.5 hPa, not 1015.6 hPa. The 1015.6 hPa is the actual pressure, plus lapse rate over 1000 ft the altimeter is supposed to show. But the altimeter does not have a temperature setting, so it uses the higher lapse rate for standard temperature. Since it's warmer, the lapse rate is lower. That's the point of the image you quoted in the question. $\endgroup$
    – Jan Hudec
    Jun 7, 2023 at 5:33
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"Because Pressure altitude is a measure of the weight of the air above you. Temperature does not affect that. Heating the air just makes it expand"

The above statements are true but extremely misleading in my opinion. I cringe every time I see pressure altitude described like that and I understand why one might get confused.

Pressure altitude is just a way to express a pressure as an altitude. When you see a pressure altitude think of it as a pressure because that's all it is.

Imagine that instead of saying that your height is 6ft, you say that your "height frequency" is 0.37 Hz, because that's the frequency a simple 6ft pendulum would have. It would just be an equivalent way to express your height. That's what pressure altitude is.

So in a way it's true that temperature does not affect pressure altitude because you don't factor in temperature to compute pressure altitude. But temperature does affect pressure so you can see why it's confusing.

In the same analogy it's like saying that age doesn't affect height frequency because you just need your height to compute your height frequency... But your height frequency was not the same 40 years ago (assuming you're older than 40yo) so in a way, age does affect it.

So to answer your question you could get the same reading on a cold day and a hot day but very likely you won't. It's like asking "is the pressure the same on a cold day and on a hot day?" It could be... But most likely it isn't.

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I think the confusion lies in the altimeter setting or kollsman window.

The airport altimeter setting is designed to ensure the airplane's altimeter will read field elevation (within 75 feet).

That airport altimeter setting is affected by both pressure and temperature.

The aircraft altimeter is designed to mimic the lapse rate of a standard day. Roughly 1" mmHg and 2­° c per thousand feet.

This is the key point. If the aircraft altimeter is at a higher elevation than where the airport altimeter setting was read, the altimeter will show an error as in the chart you posted. If the aircraft is at the airport field elevation, temperature has no effect on the altimeter as temperature has already been calculated in the airport altimeter setting.

This might make you think that temperature effects are increased with deviation from the airport elevation... and I would agree. The FAA has a discussion of cold temperatures on approaches and the effects it has on the altimeter. There is a correction chart for temperature on the altimeter. The correction is based on the temperature and the altitude above the airport altimeter setting.

AIM Chapter 7 Section 3. https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap7_section_3.html

You can see the effects of temperature on the altimeter setting by finding close geographic weather stations at quite different elevations. It is quite pronounced by examining the Colorado airports and comparing them to the weather stations on top of the mountains. There is a very short distance between the stations but quite different altimeter settings.

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