Fog is only cloud resting on the Earth’s surface, yet it can kill. Aircraft and ships get lost in it, and so do people.
It is responsible for some appalling accidents on motorways. Cars travelling at high speed on a fine sunny morning can turn a corner and suddenly be in thick fog. If they manage to slow down or stop without hitting anything, they can then become sitting ducks for the traffic bearing down on them. This is particularly a problem in the Waikato, but it is far worse on busy roads in England and Europe, where pile-ups of as many as 50 vehicles have occurred, with multiple deaths.
Fog has often been used as a cloak when men set out to kill each other. In December 1944, Hitler launched his last offensive in the west after his forecasters had predicted that extensive fog should last for days. Striking through the Ardenne, his armies almost broke through to Antwerp on the coast of Belgium. But when the fog cleared after ten days, the Allied airforces took advantage of their air superiority to cut the German armour to ribbons.
Heavy fog in Auckland on the morning of May 24, 1942 served a double purpose: it hid ships in the harbour from a Japanese reconnaissance plane operating from a giant submarine waiting offshore, and it hid the plane from ground observers. Susumu Ito, the pilot of the float-plane, records that he became lost in the fog. However, Auckland airport’s lights were turned on, possibly in response to sounds of the lost plane circling, and Ito was able to regain his bearings and retrace his path to the submarine.
How does fog form? Air with high humidity has to be cooled so that a lot of the water in it condenses from gas to liquid. One of the most common ways this happens is on a clear night with light winds. The ground radiates heat out, to space and becomes cold. The ground then cools the air from below. As the air cools, its humidity increases until condensation begins. If the ground is dry the air will lose some water to the Earth’s surface as dew. Once the ground is wet, fog will develop after a few hours. If it is cold enough for frost to form, then fog formation is likely to take another six hours.
If there is no wind at all the fog will often be only a metre or so thick, and will form only over favoured surfaces such as grass. If the wind is around one metre per second then the cooling from the ground will be gently mixed through a deeper layer, and the fog may be around 30 metres thick.
Forecasting the difference between one metre per second and no wind at all is well nigh impossible, and even measuring it is difficult. Anemometers are designed to be robust enough to withstand wind speeds of more than 200 kilometres per hour, and so are not sensitive to tiny fluctuations around zero.
If the wind is too strong, then the cooling from the ground will be mixed through a deep layer, and there will either be cloud formed above the ground or the skies will remain clear.
Fog can also form when warm humid air blows over a cold surface. This happens in Christchurch when air over relatively warm sea moves over cold land during the night. It also happens when a cold ocean current meets a warm current. For example, fog occurs about 120 days a year east of Canada where the Labrador Current meets the Gulf Stream (which is about 17°C warmer).
Although many fogs evaporate because of heating by the sun’s radiation during the morning, there are other ways they can clear. At Christchurch airport, for example, nearly half of the fogs clear before dawn. This is because of wind changes, or insufficient moisture to replenish the fog as it slowly falls to the ground, or heating from cloud that moves over the top of the fog.
Freezing fog occurs when the fog droplets are at a temperature below zero. The liquid droplets freeze when they come into contact with solid objects. This can cause the landscape to be covered in ice, as occurred in central Otago in July 1991 (see issue 11).
Recently the tattooed body of a man who died 5000 years ago was found high in the Italian Alps. The probable cause of death was freezing fog. Unable to see his way off the mountain, he took shelter in a crevice, then died of exposure in the storm that followed. During the next winter he was covered by snow, and then by an advancing glacier as global cooling occurred.
The effect of fog in depriving us of our sight is so dramatic that it has entered our language as an image for confusion and uncertainty. The word nebulous derives from the Latin for fog, and we use phrases such as “a fog of uncertainty.” One of the oldest works of Western literature is Homer’s Iliad, composed almost 3000 years ago. In it, the author describes the death of a warrior from a sword thrust as “a mist of darkness closed over both eyes.”
When the Revolutionary Government of France renamed the months of the year on a scientific basis in the late 18th century, they called one autumn month Brumaire after the French word for mist.
The difference between mist and fog is one of definition. In mist the visibility is greater than 1000 metres; in fog it is less.
Fog forms more easily in dirty air, and can combine with air pollutants—sometimes lethally. In December 1952, London had a fog that lasted almost five days and was estimated to have contributed to 4000 deaths, mostly from bronchitis and pneumonia. Anyone living in London at that time would have found breathing painful. A portion of the 1000 tons of dirt particles suspended in London’s air was sulphur dioxide, which combines with water droplets and oxygen to form sulphuric acid.
But fog can also be a life-giver. In deserts situated next to cold ocean currents, rain almost never falls, but fog is common. Plants living in such conditions are able to take in water through their leaves rather than their roots, and animals lick the fog droplets that collect on their bodies. In the Namib Desert of southern Africa, there is a beetle that stands on its front legs with its back to the wind so that the fog trickles down from its back and is channelled into its mouth.
And in California orchardists actually try to produce fog artificially, in order to prevent the severe frosts that would kill their fruit.