ROCKY MOUNTAIN STATES

Fall aspens, Grand Teton National Park, Jackson, Wyoming

As it does at the spring equinox, the sun "crosses" the equator again at the fall (or autumnal) equinox, on September 21 or 22, this time heading south. At high elevations the first frost occurs in August, but throughout the eastern plains and in southern Idaho it occurs typically in September or October. Cold polar air masses dropping down from the north clash with warm tropical air, creating the cold season's first organized snowstorms.

PHOTO: Ken K. Ringer