An hour after sunrise, just as the Sun was about to crest a forested ridge, Tom Bean (Member 41,135) saw the shadows of the pine trees fall across a thin layer of fog, with blue sky above, to form these crepuscular rays.
 
This was near Canyon Vista Campground, Flagstaff, in the US state of Arizona, which Tom says ‘is reported to be the least foggy of the 50 states in the US’. The claim, which is based on the number of dense-fog days in the year, is likely due to the extent of the state’s arid conditions. There must, however, have been enough moisture in the low air on this morning to condense into fog droplets as that air cooled overnight. Such overnight cooling of the low air happens on clear nights with little more than a gentle breeze. Warmth from the Earth’s surface radiates into Space beneath a cloudless, starry sky. The cold ground chills the low air, which can cause it to form a layer of fog – but only if it’s carrying enough moisture.

Thin fog like this wouldn’t have been classed as a dense-fog day. The warmth of the rising Sun would soon evaporate the tiny droplets and clear the air. Tom’s morning fog and the sunbeams it revealed would vanish into the day.