From: Gary LaPook
Date: 2020 Sep 26, 02:27 -0700
You're right, Wikipedia does a good job but it left out one point.
On days when the QNH is lower than 29.92 you cannot use the lowest flight levels beause they would result in you flying below the transition altitude and possibly bumping into an airplane flying using QNH. For example FL180 may be as low as 16,000 feet. Also, at the low altitudes at which you fly an instrument appoarch, you may need to correct you altitude when the temperature is extremely cold. This is because the assummed pressure lapse rate used in setting these rules is not valid in very cold air.because a cold column of air, which is denser, has a faster presure lapse rate than does the standard atmosphere.