A weather resistant tumbler lock has an elongated body, and defines a longitudinal axis. The lock is interchangeable with another lock having a corresponding asymmetrical perimeter profile with an opening in a top surface sealed with a cap, the other lock having a different rotatable core and a different configuration of tumblers. A drainway provides a channel for water to flow outwardly from the interior of a shell housing, a rotatable core and a keyway within the core, to a drain below the lock. The drain connects to a channel through a lower edge. The core provides an arcuate gap in a lower quadrant adjacent the interior surface of the shell for fluid communication with the drainway. A method includes identifying a substitute tumbler lock with an impermeable outer wall for replacing another lock having an identical outer perimeter profile and a permeable closed opening on its top surface.