I'm also in the Denver area and I typically get 1.5 - 2.5 gallons per hour evaporated during the boil. Your boil off rate is probably not noticably impacted by humidity, ambient temperature, air pressure, or liquid surface area, except where those parameters might impact the efficiency of transferring heat from your heat source to your wort. However, all of these parameters can be in play during the times you're not actually boiling, like when you are heating to a boil, or when you're chilling. If you're getting a lot of undesired loss at these times, you may want to cover your pot. But watch out for boilovers!
Hello DMS, IMO, covering your boil may lead to bad flavors. Here in BFE, WYO, I experience around a gallon
per hour in my boils and my elevation is at 1 mile above sea. The time I had water issued like you describe,
I attributed it up to error....now that I am dialed in with my system, Water quantities are apparent.