You step outside and the street has gone soft at the edges. Some mornings that softness is a thin veil that burns off by nine. Other mornings it swallows the far end of the road and the traffic on the bypass disappears altogether. Then there are the days when nothing looks wet at all, yet the hills have faded to a flat grey outline and the sun hangs in the sky as a dull orange disc.

Fog, mist and haze get used interchangeably in ordinary conversation. In weather terms they are three different things, and the difference is practical: it tells you whether your windscreen will bead with water, whether the air will clear by mid-morning, and whether it is worth setting off at all.

The number that settles most arguments

Visibility is the deciding factor, measured horizontally at ground level. In UK forecasting practice, fog is reported when visibility falls below 1,000 metres. Mist covers the band between 1,000 metres and 2 kilometres. Both are made of the same thing — tiny water droplets suspended in air — and the only real difference is how many of them are in the way.

Haze is a different animal. Nothing has condensed. Visibility is reduced by dry particles: dust, smoke, sea salt, pollen, traffic pollution. Many meteorological services use a threshold of around 5 kilometres, and the air stays well short of saturation, which is why a hazy day leaves you dry.

  • Fog — visibility under 1,000 m. Droplets thick enough to hide familiar landmarks.
  • Mist — visibility between 1,000 m and 2 km. Same droplets, fewer of them.
  • Haze — visibility reduced by particles, not droplets. Often 2–5 km or more.
  • Freezing fog or mist — droplets that stay liquid below 0°C and freeze onto surfaces as rime.
  • Hill fog — cloud that has reached the ground. Meteorologically it is stratus; to a walker it is fog.

One caveat: visibility is usually measured at an airfield or official site. Fog at the airport says nothing certain about your back garden, which may sit in a hollow that fills half an hour before anywhere else.

How each one forms

Fog and mist: cooling to the dew point

Warmer air holds more water vapour than cool air. As a parcel of air cools, there comes a temperature at which it can hold no more — the dew point — and the surplus condenses onto specks of dust, salt or pollen floating in it. That is fog and mist in a sentence.

Radiation fog, the classic autumn and winter type, needs a clear sky so the ground can radiate heat away, light winds strong enough to mix the lowest few metres but not the whole night, a long night, and moist air near the surface. High pressure brings those ingredients together, which is why the Thames Valley, the Midlands, Cheshire and the Yorkshire valleys are regulars. Cold air drains downhill, so valley bottoms and low points of a motorway can fog over while the ridge above stays clear.

Advection fog works the other way round: warm, moist air drifts over a cold surface and is chilled from below. That is the mechanism behind the haar of the east coast — a cold North Sea in late spring, warm land, a north-easterly breeze, and fog rolling in over Whitby or Aberdeen while inland stays sunny.

Haze: particles, not droplets

Haze needs no condensation, just material in the air and enough dryness to keep it there. Saharan dust carried north on southerly winds in spring, smoke from moorland fires, sea salt whipped up by an onshore gale, pollen in May, and the chemical soup over a large city on a hot, still afternoon all do the same job. Light is scattered and absorbed on its way to your eye, distant objects lose contrast, the sky turns milky and a low sun reddens.

Telling them apart on the spot

Once you know what to look for, the distinction takes seconds.

  1. Check the dew point. Most weather apps show temperature and dew point. Within a degree or two of each other, expect fog or mist. A wide gap points to haze.
  2. Feel your sleeve. Five minutes of walking in fog or mist leaves hair, glasses and coats distinctly damp. Haze leaves you dry.
  3. Pick a landmark about a kilometre away. Gone entirely, with white air around you: fog. Visible but soft: mist or haze.
  4. Look up. A milky, featureless sky with a pale sun is the signature of haze. Fog and mist sit lower and often leave blue sky overhead.
  5. Watch the clock. Radiation fog and mist usually thin after sunrise and lift into low stratus by mid-morning. Haze sits there until the wind changes or rain washes it out.

Colour helps too. Fog and mist are white to pale grey and glow around streetlights. Haze looks flatter, with a yellow-brown or grey tinge to the distance, and it rarely deposits anything on your window.

Where you will meet them in the UK

Fog shows up most often in still, high-pressure spells between October and February, when clear nights and light winds let the ground cool hard. Hill fog is a year-round feature of the Cairngorms, Snowdonia, the Pennines and Dartmoor, and can sit on the tops all day. Mist is a dawn and dusk affair: over Windermere before the boats go out, across the Norfolk Broads, and over wet fields on the first clear morning after rain. Haze belongs to spring and summer — Saharan dust on southerly winds, pollen in May, photochemical smog over London on still afternoons, and a brown-grey film along the coast when smoke drifts in from fires further afield.

What to do when it matters

Reduce speed and increase the gap before anything else. In the UK, fog lights should be