Strictly speaking, the business that the Kollsman window shows ‘the barometric pressure adjusted to sea level’ is not correct. What it is, is the setting that allows a standard altimeter to correctly display the altitude of the reporting station. The difference is that it includes a correction for non-standard temperatures. For example look up the current METAR for KTVL, elevation 6000’. You’ll see that the altimeter setting is noticeably higher than that for KSMF, just a hundred miles away. This higher setting is needed to make the altimeter read 6000’ on the ground, because of the much higher than standard temperature at TVL today. Also, on the same METAR, you’ll find ‘sea level pressure’ in millibars. This is the uncorrected number for temperature. If you look up the conversion of millibars to inches of Hg you’ll see the two numbers are not the same. You have to be up pretty high to see this effect.
