What effect does a varying pressure lapse rate have on indicated altitude?

A varying pressure lapse rate directly influences the relationship between indicated altitude and true altitude. The indicated altitude is derived from the altimeter setting, which assumes a standard atmosphere where pressure decreases uniformly with altitude at a specific rate, known as the standard lapse rate.

When the pressure lapse rate is not standard, the indicated altitude can differ from the true altitude. For example, if the actual atmospheric pressure is higher than standard at a particular altitude, the altimeter will indicate a lower altitude than the aircraft's true altitude. Conversely, if the pressure is lower than standard, the indicated altitude will be higher than the true altitude. This discrepancy is important for pilots to understand because it can affect terrain separation, navigation, and overall flight safety.

Thus, the correct answer highlights how variations in the pressure lapse rate lead to differences between indicated and true altitude, emphasizing the need for pilots to be aware of the local atmospheric conditions when interpreting altimeter readings.