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Mountain 101: How are 'sunny days' measured?
Don't believe everything you hear
John Fayhee

Though there is generally a shortage of attribution when the statements are made, Coloradans are justifiably proud of the fact that the state receives “300 days of sunshine per year.” Though the actual number varies, to hear people tell it, Colorado gets about as much sun as the Sahara.

Well … not so fast.

The problem is that there’s no official definition of “days of sunshine,” which is something otherwise rational people might consider somewhat important when they are considering, and bragging about, the number of “days of sunshine” the state receives.

Here’s the pertinent skinny.

For many years, the National Weather Service has operated instruments called “sunshine switches” in three Colorado locations — Denver, Pueblo and Colorado Springs. These instruments measure and record minute by minute each day when the sun is shining. Based upon a study conducted in the late-1990s at the Denver “sunshine switch,” it was determined that, if you count every day that the sun came out for at least one hour, then you could come up with an average of around 300 “days of sunshine” each year in Denver.

The problem is that your average person probably equates a “day of sunshine” with “a sunny day.” Few would contend that having the sun come out for an hour during an otherwise cloudy day is the same as having a day completely devoid of cloud cover. Yet, the “300 days of sunshine per year” claim still maintains traction in Colorado, especially among marketing and real estate people looking to entice people living in dismally dreary parts of the country to bring their money to the Centennial State.

A decade ago, the National Weather Service established a criterion for determining “clear,” “cloudy” and “partly cloudy” days based upon sky cover (read: clouds). Any day with an average sky cover of 30 percent or less is considered a “clear” day. If the sky cover is 80 percent or more, it’s considered a “cloudy” day. Anything in between is a “partly cloudy” day. (These measurements are averaged from hourly sky-condition reports taken between sunrise and sunset.)

Based upon these definitions, Denver receives 115 “clear” days, 130 “partly cloudy” days and 120 “cloudy” days on average per year. In Grand Junction, the number of “clear” days is high — 137 on average per year — but the number of cloudy days (121) is almost the same.

According to the NWS, the most total sunshine per year in Colorado occurs around Alamosa, while the least occurs around Boulder and in the northern mountains.

RFWJ Editor M. John Fayhee is the author of 10 books, including “The Colorado Mountain Companion,” from which this column is excerpted.