Mountain Fog

The morning after a recent rain I was hoping to see snow on the New River Mountains. Overnight lows here at elevation 2200 feet were in the mid-30s (°F), and the mountains top out around 5000 ft where temperatures are even lower. My immediate first-glance impression was “Wow, that’s more snow than I’ve ever seen!” But then I noticed the fuzzy edges — and saw it move.

[My apologies for not shooting a time lapse. Will do next time I see this. Just got my camera back from sensor cleaning yesterday.]

This is not snow — it’s dense fog hugging the contours of the mountains while drifting from west to east (left to right). It was fascinating to watch new fog “appear out of nowhere” on the west slopes and slowly climb up over the summits. The motion was almost too slow for the eye, but would have made a great time lapse.

I took this photo around 7 am, when the temperature at my elevation was up to the low 40s. NWS reported a dew point of 20 °F (-7 °C). Since temperature decreases with altitude, there will be some elevation where temperature equals the dew point (yellow line around 3500 ft). At this elevation invisible water vapor condenses into micro-droplets and forms visible clouds. Although not visible in this photo the sky overhead had a cloud deck at about that same altitude.

After a rain (and we got over an inch) air near the ground picks up a lot of extra water vapor from wet vegetation and damp soil. There’s some mixing of this humid air with the air above, but humid air is denser (heavier) so as it flows with the wind it clings to the ground like a laminar sheet. That’s why it looked so much like snow capping the mountains.

Since this cloud is in contact with the ground it’s more accurately referred to as fog. And it’s opacity shows that it’s dense enough to create near-whiteout conditions. There’s some rugged 4WD trails in those mountains where off-roaders would have had extremely limited visibility — even with fog lights. 4WD weather tip: If you ever get into one of these mountain fog banks and want out, head down instead of up.

Next Week in Sky Lights ⇒ Southern Summers vs. Northern Summers

Q&A: How to Watch a Meteor Shower