How Does Fog Form? A Cloud That Touches the Ground

Air holds water as invisible vapour, and how much it can hold depends on temperature, roughly doubling for every ten degrees of warming. Cool a parcel of air and its capacity falls until it reaches the temperature at which the vapour it holds is all it can hold, the dew point, and any further cooling forces water out as liquid. In free air the water condenses on particles, salt, dust, smoke, pollen, into droplets a few hundredths of a millimetre across, so small that they fall at a few centimetres a second and are held up by the slightest air movement. A cubic metre of thick fog contains about half a gram of water, a few hundred million droplets, and it is their number rather than their mass that scatters light and cuts visibility, by definition to under a kilometre.

The air reaches its dew point by being cooled, by being given more moisture, or by mixing with air of a different temperature, and each route makes a named fog:

• •Radiation fog: on a clear, calm night the ground loses heat to space and chills the air lying on it; forms in valleys and over fields in autumn and winter, is thickest at dawn, and burns off as the sun warms the ground
• •Advection fog: warm, moist air moves over a cold surface and is chilled from below; the fog of San Francisco, where Pacific air crosses cold upwelled water, and of the Grand Banks of Newfoundland, where the Gulf Stream meets the Labrador Current, the foggiest place on Earth
• •Upslope fog: air pushed up a hillside cools as it expands, and hill tops sit in cloud
• •Evaporation or steam fog: cold air over warmer water, so that vapour rising from the surface condenses at once, the mist on a lake on a frosty morning and the sea smoke of Arctic harbours
• •Frontal fog: rain falling from warm air aloft into cold air near the ground evaporates and saturates it

Fog needs moisture, a cooling surface and still air, and the places that have all three regularly are its capitals. Coasts where cold currents run beside warm land, such as California, Peru, Namibia and the Grand Banks, get advection fog on most summer days, and in the Namib and Atacama deserts it is the only water there is, harvested by beetles that stand on dunes to let it condense on their backs and by villages with nets that drip into tanks. River valleys and basins trap cold air on clear nights and fill with radiation fog for days in winter, as the Po valley, the Central Valley of California and the Fraser valley do. Cities used to make their own: the London fogs of Dickens and Sherlock Holmes were smoke fogs, coal soot providing the particles and the sulphur making the droplets acid, and the Great Smog of December 1952, which lasted five days and is now thought to have killed around 12,000 people, led to the Clean Air Act and the end of the pea-souper. London's fog today is no thicker than anywhere else's.

Fog goes when the air is warmed above its dew point, which the sun does from the top down on a radiation fog, thinning it to mist and then lifting it into a layer of low cloud before it dissolves; when wind mixes it with drier air from above; or when the moisture is removed by the droplets growing and falling as drizzle. Advection fog over cold sea does not clear by day, because the sun cannot warm the water, and it lasts until the wind changes. Freezing fog, at temperatures below zero, is supercooled droplets that turn to ice on contact with anything solid, coating trees, wires and aircraft wings in rime, and ice fog, at minus 30 and below, is ice crystals rather than water and hangs over Arctic towns all winter.

Fog is the weather that stops transport. Airports measure runway visibility every few minutes and hold or divert flights below a few hundred metres, and the instrument landing systems and autoland that let a jet touch down in visibility of 75 metres exist because of fog; before them, fog closed Heathrow for days each winter. Motorway pile-ups in fog, where drivers follow tail lights into the back of a crash, kill dozens a year in Europe, and shipping developed the foghorn, the bell buoy and, eventually, radar for the same reason. Forecasting it is hard because it depends on the last degree of cooling in the last metre of air, and the forecasters' rule that fog is likely when the dew point and the air temperature are within two degrees at dusk on a calm clear night is still the best guide there is.

The sky is blue because air molecules, which are far smaller than a wavelength of light, scatter blue light much more strongly than red. Fog droplets are the opposite case. They are many times larger than a wavelength, so they scatter all colours almost equally, and the mixture of all colours looks white or grey. The same physics explains why clouds are white on top and why a glass of milk is white. It also explains why the edges of a fog bank seem to glow on a sunny morning: every droplet is sending light off in all directions, including towards your eyes.

That scattering is what makes driving in fog so difficult. When you switch on full beam, the light hits the droplets in front of the car and a large share of it is scattered straight back at you, so you see a bright wall rather than the road. Dipped headlights aim lower and dazzle you less, and front fog lights are mounted low and spread wide for the same reason. Rear fog lights are bright so that drivers behind can pick you out, which is why the UK Highway Code says to use them only when visibility is seriously reduced, and to switch them off when it improves.

Whether a clear, cold night produces fog or only dew depends partly on the wind. In perfectly still air the chilled layer is very thin, and much of its moisture is deposited as dew on grass and cars before the air itself can fill with droplets, so the result may be a soaking lawn and only wisps of ground mist. A very light breeze, of a metre or two a second, spreads the cooling through a deeper layer and allows a thicker fog to form. A stronger wind mixes in drier, warmer air from above and stops either from forming, which is why windy nights stay clear.

Fog that has formed overnight often gets thicker for a while after sunrise, which surprises people who expect the first sunshine to clear it. The ground goes on losing heat until the sun is high enough to warm it, so the lowest temperature of the night often comes just after daybreak. The first weak sunshine then evaporates the dew that collected overnight, adding moisture to a layer that is already saturated, and gently stirs the air so that the fog deepens. Only once the sun has warmed the ground and the air above it by a few degrees does the fog begin to thin and lift.

Fog is a cloud at ground level, formed when air near the surface is cooled or moistened to its dew point and its vapour condenses on particles into droplets too small to fall. Radiation fog forms on clear calm nights and burns off by day, advection fog forms where warm moist air crosses cold sea or land and persists, and upslope, steam and frontal fog form on hills, over cold water and under rain; it clears by warming, mixing or drizzle, and London's famous fog was coal smoke.