[dhs12345]

Altitude — definitely and it seems logical.

The disk head is like a fast moving jet aircraft that flies very close to the ground (the platter). The closer the better. Higher altitudes means less dense air and lower altitudes means denser air.

High altitude can also affect cooling — less dense air means less cooler matter blowing across heatsinks and IC cases to carry heat away.

[Paladin2]

I recognize those laws of physics (humidity is typically lower at most western USA locations), so what might me the size of the effect on HD life?

I'm sure some Denver located organizations know.