THE HABITS OF FOGS

By E. GOLD, F.R.S.* g GREAT mist and a darkness fell upon them so that they

could not see," wrote John Bunyan, and the boy said, "Are we not yet at the end of this doleful place ? " Though not a dry subject fog is an amorphous one, deficient in the fundamental elements of humour. If meteorological phenomena were arranged in order of precedence as enemies of civilised man, fog would vie with the tornado for the position of enemy No. r. A tornado is active: if it comes near you, it hits you. Fog is inactive but insidious: it gets something else to hit you or you to hit something else.

Is there any knowledge about fog of general interest ? I think there is, both practical and scientific. I will try to indicate it by considering the questions, What are the characteristics and significance of fog ? How is it caused ? Can it be eliminated ?

Fog is due to microscopic particles of condensed moisture or of smoke in the air. It makes the air unpleasant to feel and to breathe ; in this respect town fogs are worse than country fogs or sea fogs. The interference of fog with transport is outstanding. Fogs are classified by the meteorological authorities, with the concurrence of the aviation and shipping authorities, according to the distance at which objects can be seen in daylight—a distance called the visibility. In a dense fog visibility is less than 55 yards ; this may appear too liberal an allowance but the classification is based primarily on the needs of aviation and shipping and for these a fog is prohibitively dense if the visibility is less than 55 yards. A thick fog is one with a visibility between 55 and 220 yards, while a moderate fog is one with a visibility between 550 and ritoo yards. The distances of visibility of lights at night are adjusted to ensure uniformity of classification ; an ordinary street lamp would be visible at too yards in a fog in which the daylight visibility was only so yards.

So much for classification. Now something about constituents and in particular moisture or water vapour. This is always present in the atmosphere but the amount which can remain as vapour and not turn into liquid water depends upon the temperature; the amount is three times as great at 6o deg. F. as at 30 deg. F. If air just full of water vapour were cooled from 6o deg. F. to 30 deg. F. two-thirds of the vapour would condense as drops of liquid water. This would produce an extremely dense fog—in fact the amount of liquid water would be about ten times the amount in a fog with a visibility of 5o yards. Visibility is often said to be nil, but in 40 years in London the worst visibility I have experienced was 6 yards on Saturday afternoon, November 27th, 1948. The worst visibility, measured at a station of the Meteorological Office during the recent fog was to yards. Objects, unilluminated, could not have been seen at all in such conditions but lights would have been visible through the fog at about double these distances.

The drops of water in a fog are extremely small, one thousandth (.00t) of an inch or less in diameter. A drop .00t of an inch in diameter falls through the air about 200 feet per hour. A fog of drops of this size would soon clear, if it were not renewed, as the height of fogs is generally less than t,000 feet. But fogs are renewed by cooling at their upper surface and-by evaporation from the ground: frequently, too, there are many smaller drops which fall more slowly ; a drop half the size (diameter) falls at one quarter the speed.

How is fog caused ? There are material differences between land and sea in this respect. Over land they are most frequently caused by radiation at night when the sky is clear ; the earth's surface loses heat to space and cools the air in contact with it. This cooling extends upwards in the atmosphere partly by mixing, "eddy diffusion," and partly by the upper surface of the resulting fog acting as a new radiating surface. The upward extension by mixing gets slower and slower as time goes on ; it takes 16 hours to extend it to double the height reached after 4 hours. Consequently a fog which is dispersed by sunshine during the day does not get very deep even if it is formed again at night. The top of the fog at the time of writing has fallen from 800 feet to 5oo feet near London and above the fog the temperature is 52 deg. F., about 20 deg. F.

* Late Deputy-Director Meteorological Office.

warmer than at ground level. Although the cooling of the air near the ground below the condensation temperature is a necessary condition

for the formation of fog, it is not a sufficient condition. Two other factors operate and may either retard or prevent altogether the formation of fog. One is the rate at which the amount of water vapour decreases from one layer of the atmosphere to the next layer above it: the greater this rate, i.e., the faster the decrease, the less is the likelihood of fog. The other factor,is the change of wind from the layer near the earth's surface to the layer a few hundred feet above it: the greater this change, usually a veer and an increased speed, the less is the likelihood of fog. These two factors may in fact retard the formation of fog at night so long that the new day begins before the fog is formed. Once fog has been formed in the night, it continues to intensify after dawn. The worst time of day for fog away from towns is about sunrise. In or near towns the worst time is just after the smoke from domestic fires and industrial furnaces has begun to reinforce the night fog. Over the sea, fog is usually caused by a warm current of air advancing to a region where the sea is appreciably colder. The famous (or infamous) fogs of Newfoundland are caused in this way.

Overland, too, fogs are sometimes caused by warm moist air from the sea advancing across cold ground. This is especially noticeable at the time of a thaw. Another cause of fog at sea is the advance of cold air from the land over a relatively warm sea this is not infrequent in Norwegian fjords and is described as "sea-smoke."

When fog is formed in air at temperatures below freezing point, the fog drops are not, as might be expected, ice but liquid water.

One of the most fascinating chapters in recent research is the investigation of condensation of water vapour in air at low tempera- tures. In air cooled rapidly by expansion ice begins to be formed

only when the temperature falls below minus 20 deg. F., i.e., more

than 50 deg. F. below freezing point. In really clean air the limit is 74 deg. F. below freezing point. Consequently fog (or cloud) at low temperatures may consist of super-cooled drops, i.e., drops which remain liquid at temperatures below that at which water generally turns to ice. If these super cooled drops come in contact with a solid object they turn, partially at first and then completely, to ice. The process can be seen on a motor-car wind-screen' in such a fog, and icing of aircraft occurs in a similar way. The super-cooled drops also freeze if they come into contact with ice crystals, and the success of the efforts to produce rain by dropping dry-ice into clouds has been due to the clouds extending above freezing level in the atmosphere and so containing super-cooled drops. The dry ice starts a freezing of the drops with a consequent warming of the air, which then rises and makes the cloud thick enough to produce rain.

Can fog be eliminated ? It can be cleared artifically over a limited area by warming the air as has been done over run-ways of aerodromes by Fido. An increase of temperature of about 5 deg. F, is sufficient to evaporate the water drops in a moderately thick fog. Over a small area the warming must be continuous ; otherwise fog from outside drifts over. It does not take much heat to warm a few cubic feet of air: but to warm the air 5 deg. F. over an area

20 miles square up to a height of 300 feet would require ro,000 tons of coal or its equivalent. The warming would not appreciably

affect the meteorological conditions producing the fog, so that it would need to be repeated, or continuously supplemented, to main- tain the initial clearance.

The possibility of starting a "chain-reaction" in fog to cause the waterdrops to coalesce and fall out cannot be excluded, though recent published investigations of the conditions favourable for chain-reaction do not lend it much support. It is more likely to be successful with super-cooled fogs. Spraying such fogs with dry-ice in sufficient amount would at least turn them to ice and diminish their interference with transport.

What about the present fog ? The ray of hope induced by the knowledge that fogs lasting several days are rare is tempered by the reflection that the weather delights in breaking records. The present spell, though not continuous, has been prolonged already beyond the spells of December t8th to December 22nd, 1941, and of Christmas, 1944, the only two spells in recent years lasting more than 3 days in central London. This year's spell began after an incursion of relatively cold " polar " air on November 20th had dis- placed the warm and humid Atlantic air which had been moving across Britain for some time previously. This " polar " air settled down and, by November 22nd, an anti-cyclone, in the centre of which the air was stagnant and the sky clear, gave conditions favourable for radiation fog to form over Britain. The anti-cyclone moved S.E. to central Europe and it looked as if there would be sufficient southerly wind across England to displace the fog. But gradually a wider area over central and western Europe became stagnant and air reaching England from the south arrived pregnant with fog. At the time of writing the Meteorological Office forecast contemplates no early changes; but the top of fog has fallen—a good sign.