Fog, Mist and Haze - ATPL 050 — Meteorology

The previous lesson promised that a ground inversion on a clear night breeds fog; this lesson pays that debt and organises the whole family. Fog is cloud with its base on the ground (the same condensation physics, run at the surface), and it grounds more flights than any storm. The lesson defines fog, mist and haze by the numbers, separates the frozen variants, and then works through the five ways the atmosphere makes fog: by radiation, by advection, as sea smoke, along fronts, and over hills — each with its formation, its character and the conditions that end it.

Fog, mist and haze defined

Fog — water droplets at the surface reducing visibility to less than 1,000 m, by the WMO (World Meteorological Organization) standard.
Mist — the same droplets, thinner: visibility 1,000 to 5,000 m, with relative humidity near saturation, roughly 95 per cent or more.
Haze — dry particles, not droplets: dust, smoke and salt suspended in the air, in relative humidities well below saturation, giving the sky an opalescent, milky look; reported once it cuts visibility to 5,000 m or less.

The formation stories differ accordingly. Fog and mist are condensation at the surface: cool the lowest air to its dew point, or feed it extra vapour, and droplets form on the nuclei that are always present. The contributing factors follow directly:

moist air near saturation;
a cooling surface or a moisture source;
plenty of nuclei;
a light wind, to stir the cooled layer without diluting it;
stable air or an inversion, to hold the product down.

Haze needs none of that: it asks only for a supply of dry particles (industry, dust, smoke, sea salt) and stable air with an inversion to trap them in the lowest layers, where light winds let them accumulate.

Freezing fog and ice fog

Freezing fog — fog of supercooled droplets at temperatures below 0 °C, from the same 0 to −20 °C band where supercooled water is common. The droplets freeze on contact with any surface, coating aircraft, runways and antennas in rime: an icing hazard for an aircraft that is still on the ground.
Ice fog — fog of ice crystals suspended in very cold air, typically below about −30 °C in arctic and continental winters; colder than −40 °C, where no droplet survives, surface fog can only be ice fog. It glitters in low sunlight and, unlike freezing fog, deposits little on contact.

Radiation fog

The inland classic. On a clear night over land, the ground radiates its heat to space, a ground inversion builds, and the moist air trapped beneath it cools toward its dew point. The wind decides the outcome: about 2 to 8 kt is the fog window, enough stirring to cool a layer some tens of metres deep, not enough to mix in dry air from above. Calmer than that, the cooling stays in the bottom centimetres and only dew or frost forms; stronger, and the mixing deepens until the condensation happens aloft instead, as low stratus.

Its character follows from its birth: shallow (from a few tens of feet to a few hundred feet), patchy, and pooled in valleys and hollows where the coldest air drains, densest around dawn when the night's cooling peaks, and often thickening briefly just after sunrise as the first stirring lifts cold moist air into the fog. Dissipation reverses the recipe: the morning sun heats the ground, the ground warms the fog from below and it burns off, thinning from the edges and below, sometimes lifting through a stage of low stratus; or a freshening wind mixes drier air in and sweeps it out. Over land in winter, under a persistent anticyclone, the sun may be too weak and the fog can last for days.

Advection fog

Radiation fog waits for night; advection fog just needs a journey. Warm, moist air flows across a surface colder than its own dew point and is chilled from below until it saturates: no nocturnal cooling required, so it forms by day or by night, and it tolerates wind up to about 15 kt, which merely deepens the chilled layer. Stronger wind lifts the product into low stratus instead. Three geographies run this recipe:

Over land — mild, moist maritime air arrives over a cold or snow-covered continent in winter; the ground chills it and fog spreads far inland.
Over the sea — warm air crosses a cold ocean current or the still-cold spring sea; the classic sea fog of late spring and early summer, when the water lags far behind the warming air.
In coastal regions — the boundary traffic of the two: sea fog drifting inland on the onshore wind, or moist land air flowing out over cold coastal water and fogging the shore.

It is the more dangerous cousin: deep (often several hundred feet), extensive, persistent, and mobile, arriving with the wind at any hour and outlasting the daytime heating that would kill a radiation fog. It dissipates only when its supply is cut: the air mass changes (the wind veers or drier air arrives), the underlying surface finally warms, or the wind strengthens beyond about 15 kt and lifts the fog into a sheet of low stratus.

Sea smoke

Advection fog run in reverse. In sea smoke, the air is the cold party: very cold, stable air flows off ice or a frozen continent over much warmer water. The warm sea evaporates vigorously into the frigid air just above it, that air saturates almost instantly, and the surface appears to steam: ragged columns of fog smoking off the water. It needs a large air-to-sea temperature difference and cold, stable air to hold the vapour in the lowest layer, which is why it belongs to high-latitude winters: arctic coasts, fjords, and polynyas, the openings in the sea ice. It ends when the ingredients part: the cold air moderates as it warms over the water, the flow leaves the warm surface, or wind mixes the shallow steaming layer with drier air above.

Frontal fog

Fog can also be rained into existence. Ahead of a warm front, warm rain falls through the wedge of cold air below; the drops evaporate on the way down and load the cold air with vapour until it saturates: fog built by moisture addition rather than by cooling. The front's own lowering cloud base helps, stratus dropping to meet rising fog. Its character is set by its parent: it forms in bands along and ahead of the front, travels with it, and comes packaged with the front's rain and low cloud, a combination that can close an airfield completely. By the same token it dissipates with the frontal passage: the wind shifts, the rain stops, and the warmer, drier air behind the front ends it. Fronts themselves are the business of a later module; here it is enough that their rain can put cloud on the ground.

Orographic fog

The simplest recipe of the five: moist, stable air is forced up a slope, cools adiabatically, reaches its dew point and condenses — cloud sitting on the hill, which from inside is fog. To the pilot of a hilltop airfield it behaves as persistent hill fog: it blankets the windward slopes and summits for as long as the moist flow holds, day and night alike, while the valley below may stay clear. It clears only when the machine stops: the wind changes direction off the slope, drier air arrives, or the air descends the lee side and warms (the Foehn descent of the thermodynamics module, drying the mountain's far side).

What the exam asks

Fog questions are definition questions and recipe questions: the visibility numbers, the wind windows, and for each named type its formation, character and ending. The overview, following the learning objectives:

Define fog, mist and haze by the WMO visibility standards, explain their formation briefly, and name the contributing factors for each

Describe freezing fog and ice fog

Explain the formation of radiation fog, describe its characteristics and vertical extent, and summarise its dissipation

Explain advection fog, its land, sea and coastal variants, its characteristics and its dissipation

Explain the formation, development conditions and dissipation of sea smoke

Explain the formation, characteristics and dissipation of frontal fog

Summarise the features, characteristics and dissipation of orographic fog