Radiation fog sounds like something that might occur during a nuclear winter, but it’s not that kind of radiation.

The radiation here is heat radiation—infrared wavelengths radiated by the ground during the night, particularly when the skies are clear.

Usually, the air temperature gets lower as you get higher—a rising packet of air expands and cools as it moves into lower pressure at higher altitude; conversely, a falling packet of air contracts and warms. This is called the lapse rate.

But if the ground radiates away its heat into a clear sky on a long night, it can get colder than the overlying air. By conduction, the chill of the ground spreads to the air immediately above it. If there’s a wind, this cooled air is moved away and stirred into the general atmosphere, but in the absence of wind it can form a puddle of cold air, underlying the warmer atmosphere above. This is called a temperature inversion, because it reverses the normal progression from warm air low down to cold air higher up.

If this puddle of cold air cools below its dew point, water droplets will condense out—some can settle on the ground surface as dew or (if the ground is cold enough) even frost.

But some can also condense on particles of dust in the air, forming fog. And that’s radiation fog.

Ideal conditions for its formation are long, cold nights, clear skies and little wind—the sort of conditions that accompany high pressure in autumn and winter.

Although it’s called fog, it may actually be mist—the technical difference being that fog reduces visibility to less than one kilometre, whereas mist is thinner and permits visibility beyond a kilometre. In either case, it’s really just cloud at ground level.

And when the sun rises, we see wisps of radiation fog drifting past, gradually thinning and dissipating as the sun warms the air and ground.

Radiation fog often happens on low ground between hills, where it’s called valley fog. The sheltered valley protects the fog from the wind, and air that cools on the high valley sides  flows downhill to the valley bottom, where it enhances the puddle of cold air. Valley fog can persist for days during stable periods of high pressure—the roof of reflective fog prevents the sun warming the ground below by day, and the cold valley sides recharge the supply of cold air by night.

These sheltered puddles of cold air can form on a remarkably small scale—here’s a little patch of persistent frost I found, reflecting a pool of cold air that had accumulated overnight in a small dip in the lee of the trees:

It used to be that we could easily see the temperature inversion conditions even without the presence of fog. In the days when more people burned coal and wood for warmth, warm packets of smoke-filled air would rise from their chimneys, climb through the cold air, and then suddenly stall and stop rising when they encountered the layer of warmer air above the inversion. Winter valleys would be roofed with a thin pall of smoke, trapped at the top of the inversion. And, of course, if you were unlucky enough to live in an industrial town at the foot of a valley, a temperature inversion could trap pollution from factory chimneys for days, causing lung disease and deaths.

That’s a rarer sight, nowadays, but I managed to capture the tell-tale sign of a temperature inversion above the smoke from heather-burning in the Angus glens, early this spring:

The other main type of fog is advection fog, when moist air is pushed over colder land or water by the wind. It’s a common occurrence in these parts, with damp air from the North Sea pushing up the estuary on an easterly wind. The Scots word for the result fog is haar. So to finish with, here’s some haar photographed by The Boon Companion recently. I’ll perhaps write more on that topic in another post.