Fog has a way of transforming a familiar landscape into something hushed and mysterious, and for all its everyday-ness it’s a genuinely interesting bit of atmospheric science. At its simplest, fog is just a cloud that has formed at ground level — countless tiny water droplets suspended in the air, scattering light and reducing visibility. But the different ways fog comes into being explain why some mornings are blanketed while others stay clear, and why the fog burns off when it does.
The common thread in all fog is saturation. Air can only hold so much water vapour, and that capacity depends on temperature — warmer air holds more, cooler air less. When air is cooled to the point where it can no longer hold all its moisture, the excess condenses into droplets. The temperature at which this happens is the dew point, and fog forms when the air temperature falls to meet it. So every type of fog is really a different route to that same destination: getting the air cool and moist enough to saturate right down at the surface.
The most familiar variety is radiation fog, the kind that pools in valleys on calm, clear nights. After sunset, with no cloud blanket to trap the day’s warmth, the ground radiates its heat away to space and cools rapidly. The air in contact with that cooling ground chills too, and if it’s moist enough and the night is calm, it reaches its dew point and fog forms — settling first and thickest in low-lying hollows where the coldest, densest air drains and collects. This is why river valleys and dips are so often fog-filled at dawn while the hilltops above stay clear. A breeze or a layer of cloud will usually prevent it, because both interfere with the rapid surface cooling it depends on.
Another major type is advection fog, which forms not from cooling overnight but from movement. When a mass of warm, moist air drifts horizontally over a cold surface — classically warm air flowing over cold sea or over snow-covered ground — the surface chills the air from below until it saturates. Coastal regions know this fog well, as mild sea air slides over cooler waters and rolls inland as a grey bank. Unlike radiation fog, advection fog can form in windy conditions and can be stubborn, persisting through the day as long as the warm air keeps flowing over the cold surface.
There are subtler varieties too. Steam fog appears when cold air moves over much warmer water, and you can see it wisping off lakes and rivers on a frosty morning as the water evaporates into the cold air and immediately recondenses. Upslope fog forms as air is forced to rise along terrain, cooling as it climbs until it saturates against the hillside. Each is a variation on the same theme of reaching the dew point, just by a different mechanism.
As for why fog lifts, the answer is usually the sun. Once daylight returns and begins warming the ground, that warmth heats the lowest air, raising its capacity to hold moisture above what’s present — and the droplets evaporate back into invisible vapour. This is the "burning off" we see through the morning, often from the edges inward and from the top down. Wind can disperse fog too, by mixing the saturated surface air with drier air above. So the next time a thick dawn fog clears to a brilliant day, you’re watching the simple physics of warming air pulling back below saturation — a cloud, quite literally, dissolving back into the sky it came from.