By September, the nights have grown perceptibly longer and cooler. The less-dense, dry high-altitude air promotes nighttime cooling, and as the nights grow longer as we head towards the Autumnal Equinox, early morning temperatures are cooler. These are perfect nights for sleeping with the windows open. September is also the time gardeners worry about frosts and freezes. They are not they same thing. A frost can occur when the air temperature lowers to 35 F or less. While the air may be slightly above freezing, grassy surfaces, roofs, and car windshields may in fact be at or below freezing. If there is enough moisture in the air and the winds are nearly calm, then a thin layer of ice, i.e. frost, will form on these surfaces. A freeze is when the air temperature drops below 32 F. Freezes are much more damaging to vegetation, especially garden vegetables and flowers. A freeze marks the end of the growing season.
Average frost and freeze dates vary considerably around Nederland. Locations in valleys and along streams such as Cold Springs can be several degrees colder at night then locations higher up on the surrounding ridges. It is a matter of location, how well you are sheltered from wind, and if you are a bit lower, then cold air will tend to settle into your location.
Based on my records, the first freeze tends to occur around the second to third week of September. It is probably a week or two earlier in the sheltered locations along North Boulder Creek.