Under what conditions can a runway be considered effectively dry for preflight planning calculations?

A runway can be considered effectively dry for preflight planning calculations primarily under conditions where it is either grooved or treated, and the weather indicates less than moderate rain. This is because grooved or treated runways are designed to facilitate water drainage and maintain a high surface friction, allowing for effective performance under variable weather conditions. When rain levels are less than moderate, the effects on runway surface conditions remain manageable, enabling pilots to evaluate takeoff and landing distances accurately without significant concerns about decreased traction or increased stopping distances due to water accumulation.

In contrast, a runway being level or well marked does not directly impact its dryness or the coefficient of friction important for landing and takeoff calculations. While these characteristics are beneficial for overall safety and operational compliance, they do not inherently relate to the moisture level of the runway surface. The active use of the runway also does not guarantee its effective dryness, as it may still be affected by weather conditions or other factors impacting surface traction.