What occurs at FL310 if the ambient temperature is colder than standard?

At FL310, if the ambient temperature is colder than the standard atmosphere, the true altitude will be lower than the indicated altitude, which is 31,000 feet. This occurs because atmospheric pressure decreases with altitude, and in colder air, the density is greater than what is defined by standard conditions. As a result, the aircraft is higher above the earth than what is represented by the pressure altitude, which is based on the standard atmosphere.

When ambient temperatures drop below the standard, the actual altitude where certain pressure levels occur (true altitude) is less than what is indicated by the altimeter calibrated to standard temperature conditions. Thus, the correct conclusion is that true altitude is lower than 31,000 feet when conditions are colder than standard at that flight level. This concept is critical for accurate navigation and safety in aviation, as flying at a lower true altitude than what is indicated could lead to inadvertent terrain clearance issues.