If you're reasonably warm and comfortable, then a thick cloak of fog can be one of the most sublime pleasures weather has to offer. Every hard edge in the landscape softens; a moist cocoon of cloud envelops both your physical and mental world, and everything around seems to shrink to a more manageable size.
Fog is literally a cloud on the ground. Like clouds in the sky, it comes in myriad shapes and forms. Fog can span a single valley or a thousand miles of ocean. Lowlands, mountains and the sea each have their own preferred types. Natives of some tropical islands have never seen fog, while the people in sections of coastal California or the Scottish highlands see it day after day, a backdrop to their everyday lives.
Nature's fog machine
As with any other cloud, the trick to making fog is to either cool the air so much that some water vapour is effectively squeezed out, or to add so much water vapour that some of it is forced to condense into cloud. Either way, you're left with more moisture than can remain in vapour form at a given temperature. Some of it then gets deposited onto salt, dust, soot or whatever else is lying around. Each of the resulting water-coated particles measures in the region of 10-20 micrometers/400-800 millionths of an inch.
It doesn't take much moisture to make fog. If a batch of fog materialized in an ordinary living room, it might represent only about 3.1 grams of water, or about one-tenth of an ounce. That's barely enough to coat the bottom of a drinking glass. An identical amount of water can produce a thin fog or a peasouper, depending on whether its distributed across a small or large number of particles. Back in Victorian England, when unrestricted coal-burning was the rule, there was so much airborne soot for moisture to cling to that the eras “stinking fogs” were far thicker than they are now.
Fog close to the ocean tends to form on airborne salt, a process that can happen even when the relative humidity is as low as 70 percent. The thin obscuration that results – haze – can lower visibility to a few miles or kilometres. In cities, the tons of pollution spewed out by cars and factories leads to the urban equivalent of haze: smog, a word that conjoins smoke and fog. (Hawaii even has something called vog, a product of water adhering to the ash and other debris emitted by the island's volcanic peaks.)
The lines between haze, smog and fog are rather fuzzy. Water is more attracted to some particles (like salt) than to others. If lots of these waterattracting (hygroscopic) particles are at hand, then fog can form at humidities on the order of 95 percent rather than the fully saturated 100 percent. In major urban areas, pollutants alone can restrict views below the fog criterion without the help of moisture. Some dank days are informally tagged as foggy or hazy when they're actually just dirty.
Where and when
The season of fog depends on what kind of landscape you inhabit. On some mountaintops, it's year round. Air that blows toward mountains cools down as it's forced upward, so that cloud banks commonly envelop higher elevations, especially when they're close to a moisture source such as a large lake or the sea. In an average year, fog shrouds the summit of New England's Mount Washington sue out of every seven days, and Kenya's Mount Kirinyaga gets at least some fog almost every day.
The waning sunlight and shortened days of autumn and winter set up ideal conditions for radiation fog to form across large stretches of the mid-latitudes, such as central Europe and central China. Usually this happens under clear skies after dark, as Earths surface radiates heat to space and cools down. Moisture left behind by rains earlier in the day can help. Before long the ground-level air approaches 100 percent relative humidity. If there's a slight breeze, some moisture might be deposited on the ground as dew, but when conditions are calm the moisture is more likely to cling to airborne particles. At first this happens above those land features that conduct the least heat, such as snowbanks and river valleys. In time these foggy patches spread to cover the entire ground, although perhaps to a depth of only 3-6m/10-20ft at first. The top of the fog bank now serves as the radiative equivalent of the Earth's surface. As it transfers heat to space, it cools further, thus helping the fog to grow upward. By morning a deck of radiation fog might be over 30m/100ft deep.
Another kind of inland fog occurs more often in the spring. When a warm, moist air mass spreads over colder ground, it can be cooled to saturation and advection fog may develop (in weather jargon, advection refers to the wind moving some atmospheric element, such as moisture). Advection fog can occur any time of day; it tends to form en masse and sweep across the landscape, sometimes in a stiff breeze, rather than following the patchy formation sequence of radiation fog.
The ocean can produce fog in two ways: by warm air passing over cold water, or cold air passing over warm water. Cold, dry air masses that cruise southeast off Canada's ice sheets may pick up moisture that congeals into fog as they flow onto the northwest Atlantic off Newfoundland The same is true for bitter Siberian highs that move over the Sea of Japan. The wispy, dramatic fog that results is called “sea smoke” – it's not unlike the clouds of steam you might see radiating from hot pavements after a summer shower.
Off the west coasts of the Americas and southern Africa, the chilly ocean currents generate regular fogs, especially during the spring and summer. Local air circulations that change by the hour help determine whether the fog rolls in or stays out to sea. The San Francisco Bay is a perfect haven for fog. During the warm season, each day's sea breeze pushes Pacific fog into the bay through the Golden Gate, the break in the coastline that gives the famous bridge its name.
A clear demise
Breaking up fog can be a difficult process. Fog particles actually sink to the ground at about lcm/0.4in per second, so an ordinary fog would settle to the ground in about an hour or so if cooling at the top didn't keep it replenished. Morning sun can be an effective way to disperse radiation fog. Sunlight filtering through the fog warms the ground, and the near-ground air may warm enough to dissipate the lowest fog layer by allowing the cloud droplets to evaporate into water vapour. Tiny eddies of air then help the evaporation and clearing to work its way upward We often say such a fog is lifting, although to be more precise, it's eroding from below. Sunlight also warms the air within the fog bank itself, helping evaporate and dissipate it from within – a process often called burning off fog.
Another, quite different way to dissipate fog is to spread a bank of douds well above it Whether it's day or night, overhead clouds restrict the heat loss from the top of the fog, which puts a damper on the fog replenishment process. Brute force will also help. Strong winds ripping across the top of a fog bank can stir up turbulence and disperse the moisture.
Sometimes none of these natural processes does the trick. Winter fog can last for weeks in sheltered valleys like those of the American West or northern Italy. Some airports have gone so far as to inject fog with sprays of salt, ice or other particles that attract moisture away from the fog droplets. One technique applied in Utah has turned fog banks into light snow. For sheer drama, it's hard to outdo the strategy of Britain's Royal Air Force during World War II. Along the runways of key air bases, trenches full of fuel were set ablaze in foggy conditions. The fire-driven updrafts cleared the way for the Allies.