Northern lights in Colorado: tonight’s chance

Why only big storms reach Colorado

Colorado lies far from the magnetic pole on the aurora's scale. Even Fort Collins, near the Wyoming line, sits at 48.9 degrees magnetic latitude, well south of where the oval usually ends. Seeing the aurora from here takes a storm big enough to push the oval down over the northern states, and then the light reaches Colorado from a few hundred kilometers away. Two things follow. The display sits low in the north, and it is often red: the top of the aurora, above 300 km, is where oxygen glows red, and at a distance it is the part that clears the horizon, as the colors guide explains.

Plains, mountains and the Western Slope

The eastern plains are the best ground for a storm night: a flat northern horizon, small towns, dry air. The Pawnee National Grassland, northeast of Fort Collins, is a federal grassland with almost no lights and long views over the prairie. The mountains offer some of the darkest skies in the state, but their valleys face in every direction and their ridges hide the low northern sky. A high pass or an open park, such as North Park around Walden, combines altitude with a wider view. West of the Continental Divide, Grand Junction sits lower on the magnetic scale and needs about Kp 7.6.

Reading the watch

For Colorado the useful forecast is an SWPC watch for G3 or G4 conditions, issued a day or two ahead when a fast coronal mass ejection leaves the Sun toward Earth. The watch says the storm is possible, not that it will reach its peak during a Colorado night. On the evening itself, the Kp estimate and the OVATION line of the tool tell you whether the storm has arrived and how far south the oval has moved. If the current Kp stays below 7, the display is very unlikely to show above the Front Range horizon.

Dry air, but light domes

Colorado's semi-arid climate gives clear skies more often than most of the country: Denver's cloud amount ranges from 38% in September to 57% in February in NASA POWER averages. The Front Range cities, though, form an almost continuous band of light from Fort Collins to Pueblo, and on a storm night the drive east or north out of it is what makes the difference. By clear dark hours, November, October and September lead in Denver. The Denver page gives the month-by-month figures.