Missoula, MT UV Index Today

Two things here push the UV index in opposite directions. Pushing it up: elevation. The Missoula valley floor sits near 3,200 feet, about 970 meters, and the standard estimate of 8 to 10 percent more UV per 1,000 meters puts this site something like 8 percent above a sea-level city at the same sun angle. Pushing it down: latitude. At 46.9 degrees north Missoula is north of Minneapolis, north of Toronto, north of Montreal, and only slightly south of Seattle. The noon sun tops out around 67 degrees at the June solstice, lower than the June maximum anywhere in the southern half of the country. The altitude gain partly offsets a sun angle that never gets steep.

Day length swings with that latitude. Around the June solstice Missoula gets close to sixteen hours between sunrise and sunset; around the December one, closer to eight and a half. The summer window over which any meaningful UV accumulates is roughly twice the winter window, on top of the difference in the angle itself.

Then there is smoke. Western Montana’s fire season runs through August, while the sun is still high, and a smoke layer thick enough to lift the particulate reading also scatters and absorbs part of the beam on its way down. The two numbers this site publishes can move opposite ways on the same afternoon: PM2.5 up, index down.