In cold weather, at what point should pilots apply corrections for terrain avoidance when unable to display true altitude?

In cold weather conditions, it becomes crucial for pilots to adjust their altitude according to changes in outside air temperature to avoid terrain. The correct choice states that corrections should be applied when the outside air temperature is colder than ISA minus 10°C, which is essential because lower temperatures can lead to altimeters indicating a higher altitude than the actual true altitude. This discrepancy occurs due to the fact that the altimeter is calibrated based on the International Standard Atmosphere (ISA) model, which assumes a certain temperature gradient that may not hold true in colder climates.

When the temperature drops significantly, as indicated in the correct choice, the atmospheric pressure decreases more rapidly than what the standard model would predict. Therefore, without the proper adjustments, pilots may unintentionally fly at an altitude that is insufficient for safe terrain clearance. By applying corrective measures when temperatures drop beyond this threshold, pilots enhance their margins of safety and ensure they maintain adequate separation from the terrain below.

The other options do not appropriately address the necessity for altitude corrections in relation to cold weather; they either focus on atmospheric conditions that do not pertain to significant cold temperatures or suggest timings that do not relate specifically to the need for such adjustments related to true altitude display.