300 days of sunshine per year in Denver? Casting a shadow...
The current Men's Journal casts a shadow on Denver's claim to have 300 days of sunshine annually, pegging our sunny total at 245. Turns out, boosters are counting not days, but hours of the bright stuff.
How many days of sunshine does Colorado enjoy a year? Here's the answer from Colorado State University's Nolan Doesken:
This is a question that comes up several times per year. You will find in many Chamber of Commerce publications from all areas of Colorado that we get at least 300 days of sunshine each year. The only problem is, there is no official definition of "days of sunshine" so there is no data set that you can easily turn to.
Have you ever wondered if anyone actually keeps track of stuff like this? It turns out that for many years, three locations in Colorado have operated an instrument called a "sunshine switch" -- Pueblo, Denver and Colorado Springs. If this instrument is cleaned and perfectly calibrated (which it rarely is), it can tell you minute by minute each day when the sun was shining. We did a study over ten years ago based on these three stations and found that for Denver if you count every day when the sun came out for at least one hour, that then you could come up with an average of around 300 "days of sunshine" each year.
But my assumption is that most people, if they heard "day of sunshine" would assume that meant it was a sunny day. The National Weather Service did establish a criterion for determining clear, cloudy and partly cloudy days based on sky cover. Any day, with an average skycover of 30 percent or less was considered a clear day, while if the sky cover was 80 percent or more, (averaged from hourly sky condition reports between sunrise and sunset), it was considered a cloudy day. Anything in between counts as "partly cloudy". Based on this definition, there are 115 clear days, 130 partly cloudy ones and 120 cloudy days, on average, each year. Over in Grand Junction the number of clear days is great (137) but the number of cloudy days is almost the same (121).
But the fact is, here in Colorado and much of the Rocky Mountain region, there are relatively few totally clear days but a whole lot of days when the sun peeks out at least a little. Therefore, we tend to brag about our sunshine -- but mislead folks along the way.
I am circling around your questions. Of course the answer will differ from one location to another in Colorado with the most sunshine occurring down around Alamosa with the least around Boulder and in the northern mountains of the state. In the Denver area there are probably only 30-40 totally overcast days per year, and some of them are even fairly bright -- about 300 days would have at least one hour of sunshine sometime during the day, but only about 115 days per year fit the classic definition of "clear."