Ask a simple question — is Britain getting foggier, or less foggy? — and the honest answer is that it depends on which fog you mean and where you are standing. Fog forms when air cools to its dew point, or when mild, moist air drifts across a cold surface. Anything that shifts temperature, moisture or the tiny particles that seed droplets will shift fog too. A warming climate tugs on all three at once, and not always in the same direction. That is why a century of rising temperatures has thinned out fog in some regions, barely touched it in others, and in a few places may have made it worse.

What Fog Actually Needs

Fog is cloud at ground level, and it is fussier than most weather. Four ingredients have to line up: moisture, cooling, condensation nuclei, and — for the classic kind — a bit of calm. Wind that is too strong mixes the air and stops the surface layer chilling. Wind that is too light leaves you with dew or frost instead.

  • Radiation fog forms on clear, calm nights, when the ground radiates heat away and the air above it cools to its dew point. This is the autumn and winter fog of river valleys and inland airfields.
  • Advection fog arrives when warm, moist air travels over a cold surface — a chilled sea, a snow-covered field, a cool city centre.
  • Sea fog, or haar, is the coastal version, familiar on the North Sea coast of Scotland and north-east England in spring and early summer.
  • Steam fog runs the opposite way: cold air over warm water, as seen over a lake on a frosty morning.
  • Upslope and valley fog forms when air is lifted over a hillside or pools in a hollow overnight.

Each of these responds to a warming climate differently. Treating "fog" as a single phenomenon is where most arguments about it go wrong.

Where Warming Pushes Fog Away

Radiation fog needs the temperature to fall all the way to the dew point. Warmer nights, fewer frosts, more cloud cover and stronger urban heat islands all shorten the window in which that can happen. Long-term records from Europe and North America generally show fog becoming less frequent across the twentieth century. Temperature is only part of that story — cleaner air matters too — but the direction of travel fits with warmer, less sharply cooled nights.

Cloud is an underrated factor here. A blanket of low cloud at night acts like a lid on the cooling process, and in places where winters have turned cloudier, that alone can remove fog mornings without any dramatic change in temperature.

Where More Moisture Pushes Back

Warm air holds more water vapour — roughly 7 per cent more for each degree Celsius of warming. That raises dew points, and in some regions it raises them faster than the night-time minimum temperature climbs. When that happens, a night can still reach saturation and produce fog, with more water available to condense when it does.

Two consequences follow. In places where fog is limited by moisture rather than by cold — humid coastlines, monsoon regions, damp highland valleys — a wetter atmosphere can mean more fog rather than less. And heavier rainfall leaves soils, rivers and reservoirs wetter, so the air above them carries extra moisture into the evening. Anyone who has watched mist rise off a soaked field after a downpour has seen the mechanism at work.

Cold regions show the clearest version of it. As Arctic sea ice retreats, more open water is exposed in autumn, and cold air flowing over that water produces steam fog — the sea smoke that once appeared mainly in the depths of winter.

Pollution: The Ingredient People Forget

Fog droplets need something to condense onto. Sulphate particles, nitrates, soot, sea salt and dust all serve. That makes air quality a first-order control on fog, as anyone who remembers London's pea-soupers knows. Those were not simply the product of cold weather; they were coal smoke and stagnant weather working together. When the Clean Air Act era cut smoke and sulphur dioxide, London's fog days fell steeply, and they kept falling for decades afterwards.

The relationship is not linear, though. Adding particles makes droplets smaller and far more numerous, which changes how thick the fog looks and how long it lingers. Heavy aerosol loads can hold back the drizzle that would otherwise clear a fog layer away, so a polluted fog can outlast a clean one. Recent cuts to sulphur in shipping fuel have altered the particle mix over busy sea lanes, and what that means for fog near ports and coastlines is still an open question.

What This Means for the UK

Inland radiation fog

The classic recipe is a slack winter anticyclone, clear skies, light winds, a long night and moist ground after recent rain. Milder, cloudier winters have reduced the number of textbook fog mornings, particularly in southern England and the Midlands. But when a winter anticyclone does settle over the country, the fog that forms can still be thick and stubborn, and a wetter autumn gives it more moisture to work with.

The east coast haar

Haar depends

Photo: carloyuen / Pixabay