Some of the methods by which the hazards of flying in foggy weather are being reduced
STRIPS OF NEON LIGHTING are let into the ground surface at Croydon Airport to indicate the best landing and take-off directions during the calm conditions prevailing in foggy weather. The lighting is in the shape of a wide H, the middle bar of which is in a broken line and the side lines continuous. One of the side lines is seen in the foreground. The pilot lands along the direction of the broken line.
IF any experienced pilot were asked to say what caused him the most trouble in the course of his duties, the answer would almost certainly be “fog”. Every time he lands in foggy weather he knows that he is taking the lives of his passengers in his hands and that he must not relax until his wheels have touched the ground. Every winter we read of mishaps to aircraft forced down because of fog, and even of pilots having to use their parachutes for the same reason.
The fog menace becomes more and more serious as aviation grows. Authorities believe that the development of commercial aviation will be much hampered unless a complete solution to the problem is found. So seriously is the menace regarded that the Aeronautical Research Committee of the Air Ministry have made arrangements at a number of airports for an extensive series of large-scale tests on fog dispersal. Big efforts are also being made to make flying independent of fog, and it appears likely that this will be possible before long.
There are said to be fifteen different kinds of fog, distinguished by the various ways in which they are formed. Only a few of these varieties are of importance to aviation in Great Britain, but they all present difficulties. Most of the varieties are essentially the same in general nature, as they consist of minute water droplets more or less contaminated with particles of dust. It is the way in which the condensation of these droplets is caused that distinguishes the various types of fog.
Mist, cloud and fog all belong to the same family. Clouds are merely a form of mist or fog produced from moist currents of air, cooled because they have risen above the ground. Land fogs are seldom free from dust. Though this makes them more objectionable than sea fogs, they are not necessarily more dangerous.
The water droplets are so small that they can scarcely be seen even with the most powerful microscope. Their minute size keeps them in constant motion and prevents them from falling to the ground as raindrops do. A thimbleful of foggy air may contain a hundred thousand of these particles and in twenty-four hours a man would breathe a billion of them.
The kinds of fogs which affect a country depend largely upon its size, its situation, the extent of sea on its borders and the nature of the ground. Some localities of the same country suffer much less than others. A small country such as Great Britain has a different kind of fog from a big country such as America. A flat open country generally suffers more from fog than a hilly country, for a range of hills may act as a screen. If the Earth’s surface did not consist of a mixture of land and water, most fogs would never form. Some authorities believe that fog is essentially due to the fact that so many countries have coastlines.
Great Britain suffers mostly from two kinds of fog, the first termed radiation fog and the second a brand of what are termed “advection” fogs. The first kind can occur almost anywhere in varying degrees, whereas the southern districts of the country suffer most from the second type.
Radiation or ground fog is a particular nuisance to inland aviation, not because it is exceptionally dense or extensive, but because it occurs suddenly when least expected, and because it is so frequent and widespread. We can all recall those rather chilly, quiet evenings with a slight mist which develops overnight into a dense fog, and which is driven away as rapidly as it came by the sun the next day. The worst London fogs belong to this class, but except over big cities fogs of this class do not last more than one night.
Ground fogs are never more than about 300 feet high. They do not, therefore, interfere with an aeroplane’s ordinary flight, but they are a great nuisance when landing. They form on clear, quiet nights, over flat country when the lower layers of air have every opportunity to cool by radiation to the sky. If the air is thoroughly saturated with water vapour, after a wet day, or over large stretches of warm water, the tiny fog droplets condense and soon the whole mass is filled with them.
A slight air movement is necessary for the production of this type of fog. If there is dead calm, only a heavy dew is formed; if there is appreciable wind, the fog is dispersed as soon as it has formed.
A radiation fog might be described as a home product, whereas some British fogs are unwelcome visitors from overseas and for that reason are known as maritime fogs. A south-west wind in
winter is favourable to this type of fog and the south of England therefore suffers much from it. When warm moisture-laden air strikes the relatively cold land, the moisture is condensed and a fog is formed. An enormous area may be paralysed by this widespread fog. If the coast is protected by a natural barrier, as by the mountain ranges on the west coast of America, maritime fogs cannot occur.
A Common Fallacy
A fog is generally thought to be a winter product, because land fogs normally appear in the winter. Aviators and sailors will affirm that this idea is wrong; for sea fogs can occur readily in the summer, and sometimes have serious consequences. For example, warm air currents may pass over large areas of warm land and become chilled on reaching the sea. In other instances two air currents may meet, one having traversed an expanse of warm ocean, the other having passed over colder seas. This type is well known to aviators, and the districts in which they occur are avoided as much as possible. Thus is seen the importance of the part played by the sea and the coastline in promoting fog.
During the past few years scientists have given much attention to the question of dispersing fog at aerodromes. Before long some real progress may be made in solving the problem. Several ways of getting rid of fog have been tried with varying success. If expense were no deterrent, this could probably be done. It is easy to dissipate a fog on a small scale in the laboratory, but, when attempts are made to dissipate fog on a large scale, many difficulties arise.
One of the most obvious ways of dispersing fog is to evaporate the water droplets by heating them. The Aeronautical Research Committee, after having considered the various methods available, have decided that heating is the most feasible, and have decided to make some tests at a number of airports. Imperial Airways estimate that the heating plants should be large enough to keep clear for five minutes a space 750 feet long by 100 feet wide by 300 feet high.
Sir Napier Shaw, the meteorologist, has made some calculations which indicate that it would be necessary to burn coal at the rate of about 13 tons an hour to clear an aerodrome of normal size. This assumes that all the heat in the coal is efficiently used.
EXPERIMENTAL APPARATUS tor the dissipation of fog by means of chemicals. Calcium chloride has a marked arfinity lor moisture, and fog has been successfully removed over considerable areas by spraying a strong solution of this chemical into the air. A portable type of apparatus working on this principle has been developed that can be placed out of the way alongside the runway.
If electric heating were used the equivalent rate of consumption would be about 65 tons an hour. Electric heating would be more easily controlled, as it could be switched on for a short while when required. A consumption of 65 tons an hour seems excessive, but the heating would he required for only a short period at a time. The cost of coal might therefore be about £10-£15 for each landing. The initial cost of the heating installation would probably be high.
A method of fog dispersal which has been successfully demonstrated at an American airport uses a chemical called calcium chloride. This is a strong absorbent for water. It can be sprinkled into the fog as a solid or can be used as a strong solution.
In the American experiments a pipe 100 feet long was supported 30 feet above the ground and was fitted at intervals with special nozzles through which the calcium chloride solution was sprayed. Even in unfavourable conditions the largest fog particles likely to be encountered disappeared in thirty seconds.
A portable type of this apparatus has been developed at the Massachusetts Institute of Technology. It can be placed along the side of the runway where it is out of harm’s way and no drops of calcium chloride need enter the cleared space.
The Aeronautical Research Committee consider the calcium chloride method to be too costly to be practicable. A rough calculation, however, shows that probably not more than about one-tenth of a ton of chloride would be required to clear a site for ten minutes; so the cost should not exceed £1 for each landing.
Some interesting experiments have been made on the dispersal of fog by an electric discharge. Fog droplets remain in suspension because they are small and because they have an electric charge. If this charge can be removed, the drops unite together and fall as rain. A high voltage is required to produce the desired effect, but the real difficulty is to make the effect felt through a sufficiently big volume of foggy air.
This difficulty was cleverly overcome in one experiment by charging sand to 14,000 volts, and scattering this into the fog from an aeroplane. In this way a big volume of fog could be reached and it was possible, with only 100 lb. of sand, to clear an area two miles by 1,000 yards. Aeroplanes may one day carry a load of charged sand ready to clear their own path.
Although these methods are the most hopeful ones for the disposal of fog at aerodromes, others have been tried with good results. One method was to freeze the fog with a substance known as “dry ice”. Dry ice is solidified carbon dioxide gas, and is many times colder than ordinary ice. The dry ice was dropped from an aeroplane into the fog bank, which soon disappeared.
Suction Method
Still another method which has been considered is to suck the fog away. This could be done by having tunnels below the landing ground into which the foggy air would be drawn through gratings with big fans. This is not an impossibility, but introduces the difficulty that pilots would find themselves involved in strong air currents near the ground when landing.
It is well known that red light penetrates fog much better than ordinary light. Advantage has been taken of this fact at Croydon Airport, where some large neon striplights have been installed to show pilots the best landing direction in the calm conditions prevailing in foggy weather.
The lights are arranged in the form of a big letter H, the middle bar in a broken line and the side lines continuous. The neon tubes are placed in trough-shaped boxes below ground level and are covered with armour-plated glass to withstand the shock of the aeroplane wheels when landing.
Not long ago the interesting discovery was made that an intermittent beam of light has greater powers of penetrating fog than a continuous beam. A special type of fog beacon has been devised on this principle, which may have possibilities for the lighting of airports. The light beam is covered and uncovered five times in a second, and in this way visibility is almost doubled.
A FOG BALLOON which has been used at Croydon Airport to investigate weather conditions above fog. (It is often possible for aeroplanes to take off in thick fog if the weather above the fog is fine.) The balloon carried meteorological instruments and could ascend on the end of the cable to a height of two or three thousand feet. The instruments recorded conditions above the fog.
Beyond the visible red of the spectrum are the infra-red radiations which have even more remarkable penetrating powers than the visible red rays. A young British scientist has invented an ingenious piece of fog-piercing equipment which makes use of these rays and enables the pilot to see as clearly as in daylight. An infra-red floodlight attached to the aeroplane searches the ground below, and the scene is projected on to a screen fixed to the aeroplane in view of the pilot. To make this possible, the infra-red rays are transformed into visible rays through the agency of a photoelectric cell.
Another fog camera working on similar lines goes even farther, and photographs the scene so that a record can be preserved. In this clever invention a specially treated film is automatically exposed at intervals of thirty-two seconds, and develops and fixes the negative thirty seconds after the shutter has been opened. The pilot can view the picture of the scene ahead by pressing a button.
It is to wireless that civil aviation is turning hopefully for a solution of the fog problem. Wireless fog signals play an important part in aerial navigation, and have been extended considerably since the first permanent signal of this type was installed in 1921. Shortly before his death, the Marchese Marconi was experimenting with micro wireless waves from a radio beacon for giving information to aircraft and ships in fogs.
The greatest promise, however, appears to be afforded by methods of blind landing which have been developed. These methods relieve the pilot of much of the feeling of responsibility when he finds the landing ground obscured by fog. Experimental landings have been made with the cockpit completely enclosed so that the pilot was unable to see the ground and knew he had landed only when the wheels had touched the ground.
Fog Forecasting
Fog forecasting is one of the most difficult branches of meteorological science. As yet no practicable system of forecasting fog has been developed. The problem is difficult enough with sea fogs, with which most weather services are concerned; it is intensified a hundredfold with the variable and erratic land fogs with which air lines are so closely concerned. So many factors are involved in fog formation that it is impossible to predict with certainty when and where a fog will envelop a given district, although it can be said whether or not conditions are favourable for fog formation.
The present scope of the weather services in dealing with fog is restricted to keeping an accurate picture of the position throughout the country, of the range of visibility from place to place, and of the general movement or drift of the fog. From this some idea can be formed of what is likely to happen in the immediate future and pilots can then be warned.
A method discovered in the war of 1914-18 enables a fog to be detected much earlier than is possible by the naked eye. A searchlight shining through a slightly misty sky throws a whitish disk which is often a warning of a thick fog to come.
The liability of a particular district to these evil visitations is an important factor to be taken into account when deciding on the site of an aerodrome. Coastal sites are generally the best in the British Isles, because the important maritime fogs already mentioned do not get really thick until the air from the sea has passed some distance into the interior.
For inland sites the most important points to be considered are the elevation and the proximity to a large industrial city, especially if the wind blows the smoke in a particular direction. A high site above the cold air of the valley is usually best.
Records show that a situation east of a manufacturing area may be expected to experience abnormal fogginess. This is evident at Liverpool, Sealand (Chester), Renfrew and Birmingham. Birmingham is said to have the highest recorded frequency of fog over a period of years in winter in Great Britain.
TAKING OFF IN FOGGY WEATHER is often difficult because the far side of the aerodrome may be obscured and the pilot cannot judge the direction of his take-off by objects on the aerodrome boundary. To overcome this difficulty a broken white line was let into the ground at Heston Aerodrome along the best take-off direction. Cross lines on the broken line were arranged to indicate distances across the aerodrome.
