On the Metar for KSEA I see the altimeter as A3010 (air pressure) then I check KDEN Metar and see A3018 (air pressure). This makes no sense since KDEN is 5,400’ above KSEA how is the air pressure more in denver when it should be less due to altitude?

From what I understand, altimeters are calibrated with the following equation: . c is a constant, T is temperature, Po is the pressure at sea level, and P is the pressure at the altitude. My guess is the difference is probably due to temperature, but I’m not a physics major. I found this document which might be of use in terms of explaining things more in-depth.

In short: it’s probably the weather that’s causing things. The weather is not constant and will vary place-to-place.

still quite confused.

It might be what the “equivalent” altitude is compared to MSL under standard conditions. Like maybe the temperature is lower than usual in KDEN so its lowers the equivalent altitude. Or maybe at MSL the temp is higher than usual so it’s not that much different from 5000 ft, but MSL is the frame of reference so MSL has no ‘equiv altitude’. Everything else is adjusted to it.

But I may be totally wrong

ahhhh that makes alot of sense.

These are altimeter settings, not a representation of the actual pressure of the air at that airport. Altimeter settings are used to calibrate altimeters in aircraft.

Altimeter setting shows the pressure at sea level under the given conditions. So if you were sitting at some point sea level under the temperature and pressure conditions at DEN, you would need to set your altimeter to 30.18 to indicate 0 feet MSL. Similarly, at DEN today you would need to set your altimeter to 30.18 in order for it to indicate 5,400 (or whatever the correct altitude is there).

The actual pressure of the air at DEN may not be higher than the pressure at SEA (calculating pressure altitude would help you determine this), but adjusted to sea level DEN is higher pressure.

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