MTN News- Most Montanans in the Mountain West Region are accustomed to seeing Valley Fog especially in the Fall and Winter months.

Fog typically forms at night when the skies are clear and the winds a lite.  Clear skies allow for temperatures to drop much faster than if there were clouds, which can act like a blanket.  The lack of wind helps fog to stay thick and undisturbed, but have you ever wondered why this fog really only forms in the valleys?

One big reason is the way the terrain cools at night.  We all know warm air rises and cold air sinks, but after the sun sets in the sky the peaks of mountains cool off much faster than the valleys.  Cold air from the mountain peaks then sink down into the valley. This is called cold air draining into the valley.

This cold air causes moisture near the ground to condense into a thick cloud which we call fog.  If we are below the freezing temp though, the fog can actually turn into freezing fog, which can also leave it picturesque hoarfrost in its wake.

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In addition on days in which we have a temperature inversion in place we are much more likely to see that fog sticks around longer during the day.


Reported by: Mikenzie Frost