During the longer autumn and winter nights, the ground has more time cool than at other times of the year. This is particularly the case when it is a starry night, devoid of cloud cover, for the ground loses (or ‘radiates’) its heat out to space much more readily without any blanket of clouds to keep it in. When such a situation coincides with no more than the slightest breeze of moist air, conditions are ideal for the formation of ‘radiation fog’.
This is when the low air is cooled by the plunging ground temperatures to such an extent that some of its moisture condenses into droplets. It is a ground-level Stratus cloud that consists of chilled air, and so has a tendency to sink into and run down valleys, in much the same way that rainwater runs down them as rivers. If fog gathers at low ground like this, it is known also as ‘valley fog’.
And that is what they call the beautiful fog shown above, flooding down the course of the Potomac River early in the morning, near Washington, DC, in the US. Forming further upriver, towards the the Blue Ridge Mountains, the low-level cloud creeps silently through the night to find its level in the lower reaches of the river valley. It is little wonder, therefore, that this city district, past which the river flows, has the nebulous name of Foggy Bottom.