Handbook of Meteorology/Atmospheric Visibility

by Jacques Wardlaw Redway

CHAPTER XII

THE PRINCIPLES OF ATMOSPHERIC VISIBILITY

The transparency of the air is a matter which affects every organization engaged in transportation; the impairment of visibility has led to wrecks without number upon land and sea. Within the past few years wrecks from this cause have become a serious menace to air transportation.

Standards for testing the transparency of the air and for measuring the impairment of visibility are used here and there, but there is no uniformity among them; for the greater part they are local as to usage. A few, such as the visibility of the sun's disk, the variability of certain stars, and the sharpness of a shadow cast by a rod on white paper, are pretty general so far as overhead observations are concerned.^[1] Seamen all over the world are pretty apt to judge visibility by the clearness of the horizon; and the principle of camouflage is not so much to conceal a vessel as to blend it with sea and sky so that its outlines are indistinguishable. The locomotive engineer gauges visibility according to the plainness with which he can see semaphores during the day and signal lights by night. The marine pilot must be able to distinguish the colors of code flags and smoke stack markings, as well as colored lights. The air pilot must be able to discern the condition of the atmosphere by the refraction of the light passing through it or reflected from it. For almost all purposes, the problems of visibility must be determined along horizontal lines. The impairment of visibility along vertical lines becomes a danger when an airman cannot see his landing place.

Good visibility is safety; poor visibility is danger.

Several factors are concerned in the change from good to poor visibility, and these are all contained within the air itself. The factors which impair visibility are:

Fog.

Thickly falling snow.

Dust storms in arid regions.

Very fine rain—many drops per cubic foot.

Heavy rain—large drops. Dust storms in swarded regions.

A smoke pall held down by a “lid.”

Moisture haze in a smoky air.

Moisture haze or dust in clear air.

Refraction caused by the mixing of warm air and cold air.

Experience and judgment have taught pilots how to avoid dangers that confront them. It remains for meteorologists to fix definite standards and to express varying visibility in terms that are comprehensible and intelligible to all. Moreover, in many instances, changes in visibility may be forecast with a high percentage of verification.

Fog is generally regarded as the chief factor in the impairment of visibility, but the various other factors cannot be arranged in an inflexible order. A desert simoon may impair visibility quite as much as the worst fog; but simoons are rarely in the van of transportation, while fog is practically coincident with every line over which commerce is carried. On the other hand, though the blurring of outlines by refraction, and the slight discoloration of the air by a dust haze may reduce transparency, neither one is a menace to safety. Not even a moisture haze is disconcerting unless it conceals the horizon.

In general, the impairment of visibility is due to various movements of the air within itself. Except as the wind picks up fine dust, nothing is added to or taken from the air to make the difference between transparency and opacity. The great planetary movements of the air need not be considered here. The cloud belts incident to them and the conditions which produce precipitation are fairly understood; they are regular and periodical. Such movements as are not planetary—the cyclones and the anticyclones are understood as to cause and effect, and the forecasting of them has become a science. But there are other movements, more or less superficial and local, that are not so well understood. The causes of them may be known, but the effects cannot always be forecast. These movements are commonly grouped under the generic name of turbulence, and much of the impairment of visibility is due to them. Air in convectional equilibrium as to temperature and humidity may be clear and transparent at one time; a mixing process may cause it to be opaque a few minutes later. The change may be due wholly to turbulence.

Turbulence.—Sudden and local movements of the air are due usually to changes of temperature. A change of tem- perature produces a change in pressure; a flow of air results, and the flow—that is, the wind—continues until equilibrium is restored. But the moving of the mass of air does not always produce a mixing; indeed, the plane of contact where it meets another body of air differing in temperature and humidity sometimes is sharply defined. The friction between the two masses frequently causes the condensed vapor to be rolled into long windrows or billow clouds. The airman has learned that visibility is impaired in this plane of contact and, that in passing from one mass to another, he is likely to experience a sharp bump.

As a matter of fact, most of the turbulence which results from the mixing of air begins at the ground. The “skin friction” of the wind dragging over water reduces its velocity along the plane of contact about one-third; over the ground the reduction is roughly twice as great.^[2] The drag rolls great sheets of air into volutes. These, as they are pushed upward, bend into fantastic shapes, but continue to rotate upon many axes of many angles. Sometimes a volute bends into a ring, and the ring itself rotates on a constantly changing diameter in irregular librations. The movement, however, is upward as well as onward.

Now, this process of mixing is wholly different from the ordinary convectional movements of the air. A knowledge thereof is important to the marine pilot because it is the chief cause of sea fog along the Atlantic steamship lanes. Thus, a warm, moist wind blows into a region of the drift of a cold current. The chilling of the water vapor quickly condenses it to fog, and the churning movements of turbulence carry the process of condensation higher and higher.

To the pilot, the result is not merely impairment of visibility; it may be almost obscurity. The airmen who crossed the Atlantic emerged from the sea fog with plane wings thickly covered with ice.

The upward movement of turbulence, “the railway of the air” continues until resistance balances initial force—that is, to an altitude which practically is a lid. At this plane the fog spreads out, forming stratus clouds.

Convectional Movements.—A similar movement takes place when air is warmed. An ascending movement occurs at the focal area of warmth; descending air flows in to take its place. To a certain degree, these movements are planetary; in the tropics, ascending currents are the rule, and these are balanced by descending currents in higher latitudes. Planetary convectional movements are pretty well known and the limits of their procession and recession with the apparent motion of the sun are also known. The time, limits and location of the impairment of visibility resulting from these movements are also pretty well established. Indeed, the Coast Pilot Charts of the Hydrographic Office afford the information by which the loci of impaired visibility may be determined.

There are other examples of convectional movements which may be regional but are not planetary. The cyclonic movements are ascending convectional currents; the anticyclones are descending currents. The cyclone is very apt to be an area of impaired visibility, especially on the southeastern half, in which rain, snow and fog may be expected. The anticyclone may sweep snow or dust in blinding quantities up to a distance of a few hundred feet above ground; but it generally brings the conditions of best visibility.

Various causes bring about local updraughts of small area. Thus, during hot weather, a large area of bare rock surrounded by greensward becomes a local radiator of heat, and a sharp updraught results. Vision through such an updraught may become blurred, but it is not greatly impaired. The airman entering it gets a bump that rattles his plane, however. Descending currents in the shape of downdraughts of small area if rather strong are apt to be dust-raisers; they are “woollies” over water. There may be a slight blurring of outline due to refraction, but there is otherwise but little impairment of visibility. The obscuration of outlines of objects at ground level may warn the airman in some cases that he is nearing a downdraught. The obscuration may be a dust storm; if under a cumulo-nimbus cloud it is pretty certain to be a dust-storm; in snow-clad regions it may be due to wind-blown snow.

In any case, although they do not materially impair visibility, such local convectional movements are more disconcerting than the cyclones and anticyclones. The forecasting of these and the charting of their tracks has become a science. Small local convectional movements cannot ordinarily be predicted along with general forecasts; but in various cases they may be predicted locally. Thus, during abnormally hot days in the valleys east of the Coast Ranges of California, the updraught is so great that strong sea winds prevail along the coast. The air is clear until after sunset; then, because of rapid cooling, fog billows roll in through the Golden Gate and cover much of the lowlands.

The Ceiling or “Lid.”^[3]—The paradoxical epigram, “air to be warmed must first be cooled” and vice versa is strictly true. If a body of air, having been thoroughly mixed, comes to rest, the temperature is not the same throughout its mass; it is cooler at the approximate rate of 1 degree Fahrenheit for every 183 feet of ascent (about 10 degrees centigrade per kilometer). It may be defined as being in convective equilibrium while in such a condition. Now, if a body of air at the top be cooled a few degrees, it contracts and becomes relatively heavier. Because it is heavier, it begins to drop. It is warmed by compression as it descends, but it is always surrounded by warmer air; so it drops until it reaches a plane which it cannot penetrate—sometimes a layer of colder air; sometimes the ground.

Similarly, if a mass of air at the ground be warmed ever so slightly above the surrounding air, immediately it begins to ascend, being floated upward because it is lighter. It is chilled by its own expansion as it ascends, but as its temperature remains higher than that of the surrounding air it continues to rise until it reaches a layer of air as warm as itself. At that level it ceases to ascend, and spreads out instead. This plane therefore becomes a “lid.” Perhaps it may reach the stratosphere, for the stratosphere Is a planetary lid that envelopes the convectional air; but a lid may form anywhere between the ground and the stratosphere. Wherever a layer of warm air rests on one of still, cold air a lid is formed. Smoke, dust, and other fine foreign matter spread out to form stratus cloud when it reaches such a lid. If the two air layers are not turbulent there is little or no mixing.

Low stratus clouds indicate the height of a lid near the ground—half a mile or more; but the stratiform appearance is seen to best advantage when the clouds are not higher than 30 or 40 degrees above the horizon. Near the zenith they lose their stratiform shape, being then seen in “elevation” and not in “plan”; but frequently they indicate themselves to the practised eye of an observer. The strato-cumulus clouds that follow an anticyclone also indicate a lid. The high fog that completely covers the sky at heights varying from 7000 to 10,000 feet—practically a stratus or an alto-stratus cloud—is a lid. Cross-winds at a very considerable height likewise may indicate a lid.

The presence of a lid has much to do with the comfort of the airman. In penetrating a lid the plane is apt to get a sharp bump. If lightly ballasted, a free balloon may rebound after descending upon a lid and shoot upward several hundred feet. Sir Napier Shaw has called the attention of aeronauts to this possibility.

A low lid affects visibility to a marked degree. Under the lid, fine floating dust, smoke and the various gases of combustion spread out in stratus cloud and greatly impair visibility. The famous London fog is due to the persistence of a low-lying lid.

The impairment of visibility depends partly on the height of the lid and partly on the character of the content of the air underneath it. In open, swarded regions where the air is free from pollution, not much impairment of visibility is likely to exist. In regions where soft coal is used as power fuel, chimney products may accumulate to such an extent that impairment becomes a very serious matter; and the lower the lid, the greater becomes the impairment. Obscurity is apt to grow until increasing pressure breaks the lid and brings about a clearing of the air.

Pressure is an important factor in visibility. When the air is misty and the seeing generally is poor a very slight increase in pressure clears it up at once. As a cyclonic depression advances the seeing becomes poorer, because of rain or snow, until the trough passes. Then the seeing at once begins to improve, with increase of pressure.

The foregoing, turbulence, convection, and inversion—that is, the formation of a lid—are the principal movements of the air which impair visibility. The factors themselves are moisture, dust, smoke and refraction of light. The dust and smoke differ merely in origin; the moisture may appear in the form of fog, mist, rain, or snow.

Fog, Cloud, Mist.—In marine transportation, fog is the worst factor in the impairment of visibility. Practically it is the only one. If the temperature is brought below the dew-point, fog results from condensation of the water vapor. The brisker the wind, the thicker the fog blanket. A convectional updraught does not destroy the turbulence; convection merely carries it higher.

The thickening of a sea fog is an illustration. If the sea water is colder than the air, which is the case when polar waters intrude within lower latitudes, warm air blowing over it will give up its vapor in the form of fog. So long as the eddying movements of the air are constantly bringing warm air next the cold water, fog condensation increases. The fog blanket thickens; its upper part marks the height at which the temperature of the air is above that of saturation.

The fogs of the Newfoundland Banks have been the terror of the sailing route between American and British ports. They will prove a much greater hazard to air transportation unless a circuitous southerly route is followed. An Arctic current and a southwest wind laden with water vapor constitute the working machinery of this fog factory. Radio-telegraphy now forewarns the sailing master when and where he will encounter the fog blanket that hovers over an ocean graveyard. Times and positions of these fogs are approximately known, but definite forecasts cannot now be made.

During summer, fog along the steamship lanes in the vicinity of the Banks averages between ten and twenty per cent of the time; in winter it may be expected about one-third of the time. If an iceberg is sighted dead ahead during foggy weather it is usually so near that avoidance is difficult. A steamship can stop or it may back. An air plane can do neither.

From time to time experiments in oiling the area of fog-covered waters have been tried; but the oil film has no effect on the fog. The fog comes from the air and not from the water and the oil film does not warm either one.

The fogs of the Pacific Coast occur usually in early evening and may continue after sunrise. In southern California the coast fog may be high above ground. It is then the “velo,” or veil. The fogs of the middle Atlantic Coast are usually associated with cyclonic movements. To a certain extent they are of the nature of city fogs, being encouraged by the smoke and dust incident to city industries. In the larger harbors fog may be forecast, but not with a high degree of verification. Even a light fog ties up shipping pretty effectually.

A light fog may be more opaque than a heavy rainfall. Direct rays of light do not penetrate a dense fog more than a few rods. The light is scattered by reflection. The amount of water contained in a cubic foot of saturated air at 67° is 6.2 grains. If the temperature be reduced to 42°, approximately one-half the vapor, 3.1 grains, will appear as fog, and this amount is sufficient to produce a very dense fog.

In practise, a single rule must guide a pilot; when the limit of visibility is less than the distance required to make a stop, there is danger. In traversing sea fogs, where other vessels are not likely to be encountered, sailing masters have expressed the opinion that quite as- much danger exists at half speed as at full speed. Perhaps this is true if one considers a collision. Nevertheless, at full speed an average of twice as many chances of collision will occur, for the vessel will meet an average of twice as many other vessels in a given time.

To the airman there is no difference between fog and cloud, so far as the impairment of visibility is concerned. For the greater part, a pilot may fly above stratus clouds if the air is not clear below them; and airmen usually can find plenty of room under the alto-stratus clouds of an overcast sky. But an airman who has once encountered a cumulo-nimbus cloud, or even large masses of low cumulus clouds, is not apt to repeat the experience. In many cases both fog and cloud may be avoided; but one cannot avoid a fog when it shrouds a landing place. And while fog turbulence is slight, cloud turbulence may be very great, and this is notably the case with cumulus, and cumulo-nimbus clouds.

It is not easy to draw the line between fog and mist. Inasmuch as the droplets of mist are much larger than those of fog, they do not scatter so much light; moreover, measured per cubic unit of air, there are not so many of them. At times one may see a gray moisture haze in the direction of the horizon. A little experience enables one to distinguish it from a dust haze. It is never thick enough to impair seeing materially.

Humid air is not quite so clear as dry air, but it rarely loses transparency to the extent that it impedes transportation. Nevertheless, the impairment of visibility by air that was merely very moist has been the critical point of several suits in which railways were involved.

Rain and Snow.—It is not often that rain, per se, falls so fast that the seeing is badly impaired; but now and then this happens. Very heavy downpours may limit the vision to less than a few rods. But downpours of this sort are not common, and if the seeing is passable for 1000 feet ahead, danger is largely avoidable.

Some of the light passing through raindrops is refracted; some is reflected and otherwise scattered. The outlines of an object which normally is distinct may be obliterated, but its mass is likely to be seen. In this respect rain differs from fog. Even if the combined surface of the drops next the observer is sufficient to form a screen, the screen is partly transparent, but a fog screen is practically opaque. If a rain-drop be broken into water particles of fog size, their aggregate surface is several million times that of the rain-drop. The screening power of the fog, therefore, is vastly greater than that of the rain-drop and so also is the amount of light scattered.

A fast-falling snow is about as bad for visibility as an ordinary fog. If blizzard conditions prevail, the snow may be broken into a fine ice dust quite as opaque as a thick fog. The airman may avoid a snow squall of small area by flying around it; the locomotive engineer and the marine pilot must push through it. The danger point is reached when a snowfall hides semaphores or obscures signal lights.

Dust Storms.—In arid regions winds of the simoon type are not uncommon. Frequently they carry heavy clouds of dust far beyond desert boundaries into fertile regions. The Santa Ana of southern California is an example. Dust storms originating in the plains states sometimes carry their content as far east as the Mississippi. So far as the impairment of seeing is concerned their effect does not reach more than a few hundred feet above ground. The haze of fine and highly electrified dust which commonly hangs in the air after a dust storm is not very opaque, but it extends much higher above ground. It may persist for several days. The airman may fly above a dust storm, but if it blankets a landing place it becomes a positive danger.

The ordinary dust haze, a bluish tinge observable against a dark background, does not impair seeing. Frequently it has the density which the landscape artist terms “atmospheric effect.” The sea haze, on the other hand, may be disconcerting because it may hide the distinctive marks of nearby vessels. The sea haze has been an interesting factor in naval strategy because of this fact. Frequently it is dense enough to interfere with signaling, even with helio-apparatus. Calmness of the air is a condition necessary to the formation qf the blue dust haze and the sea haze.

The Smoke Pall.—The smoke that hovers over manufacturing districts differs materially from that of forest fires, being composed largely of free carbon and hydrocarbons. In moist weather water drops varnished with tarry matter are mixed with smoke carbon. Mixed with stack products are sulphur gases—sulphur trioxide and sulphur dioxide. These are chemically active and in the presence of moisture become very effective nuclei of condensation.

In manufacturing districts where soft coal is extensively used as power fuel the smoke pall may be dense enough to hide the outlines of large objects at a distance of 4 or 5 miles. A low-lying lid holds the smoke pollution close to the ground and impairs seeing very materially. The airman may fly above the lid into a region of clear air; the marine pilot, except in harbors, is out of the way of smoke; the locomotive engineer, who cannot avoid the smoke pall, sometimes finds it disconcerting. It rarely interferes with signaling.

Under ordinary circumstances the diffusion of smoke is so rapid that it is rarely visible at a distance of more than 40 or 50 miles from the source of pollution. At this distance a dirty-appearing horizon is about the extent of the impairment of seeing. From Chicago to South Bethlehem the region is one of almost continuous manufacture; nevertheless, the combined smoke pollution of the wide region is rarely discernible at the Atlantic Coast.

Refraction of Light.—Rays of light passing through bodies of air differing in density are bent from their original direction. The outlines of objects therefore reach the observer more or less distorted. The blurring of outlines one notices along a railway track is an example. At a distance of half a mile an approaching locomotive appears as a dark mass without outline. The imperfect mixture of warm air and cold air causes the scattering of light. A boss of rock projecting from the coast, or surrounded by greensward, produces a similar effect noticeable to the air pilot.

Refraction of this sort is a menace to safety whenever it conceals the outlines of objects which should be recognizable beyond stopping distance. A locomotive engineer who loses time in order to make certain of semaphore signals, and is censured therefor, is in about as bad a position as one who is disciplined for running past them for the same reason. Usually the judgment that comes with experience enables the engineer to observe the necessary precautions.

The mirage of arid regions, especially of the desert, is disconcerting at times. It hides landmarks which are necessary to the safety of the traveler; along the railways of arid regions it may disconcert trainmen. When it is below the eye of the observer it has the appearance of a distant body of water which reflects the sky.^[4] It is observable only when the eye is not more than 4 or 5 degrees above the level of the apparent surface. The angle is so critical that a change of level of 2 feet on the part of the observer may destroy the illusion. Such a mirage may be apparent to a man, and not to a child standing beside him.

The desert mirage is disconcerting as well as deceiving. Surveyors occasionally are compelled to suspend work, and locomotive engineers are sometimes deceived concerning the locations of sidings and signals. A cattfeman who unwisely attempted to drive a herd of several thousand cattle across the Colorado Desert lost the entire herd. The cattle, becoming thirsty, grew very nervous. The mirage deceived them and they stampeded to their death. During a battle between British troops and Turks in the arid plains of the Tigris, a desert mirage concealed the Turks so effectually that fighting was temporarily suspended.

From the nature of the case, forecasts of the impairment of visibility due to refraction are out of the question. However, it may be safely assumed that when a light wind is blowing there will be no trouble from this source. In desert regions the whirls, sometimes known as “sand spouts,” indicate the absence of surface winds. They indicate the breaking up of a lid, but their effect on visibility is slight.

Forecasting Conditions of Visibility.—Some of the fundamental conditions of visibility have been discussed in detail. Fogs cannot be forecast with any degree of certainty, but local conditions may indicate their probability, especially in the case of the coast fogs already noted. Along the steamship lanes between American and British ports, time and place are indicated, not by forecast but by probability. City fogs, which are due largely to pollution, cannot be forecast. They are indicated when the humidity is high and the smoke pollution great. They disappear with a slight rise of temperature. It is well to bear in mind that, with temperature close to the dew-point, a fall of a very few degrees may fill the air with a dense fog; a rise in temperature ever so slight may change foggy air to clear air. Impairment of visibility due to smoke pollution cannot be forecast. When due to a lid, a rise of barometric pressure indicates a clearing of the air, which may take place in an hour. In other words, a lid indicates suspended convection.

The best seeing comes with an anticyclone, the forecasting of which is pretty certain to be verified. Other highs indicate good seeing if the air is dry, and dryness of the air, when not polluted, is fundamental to its clearness. Rain or snow, and mist are pretty certain to accompany a cyclone. A cirrus haze at the eastern horizon and a white sky overhead are followed by gathering clouds which increase in thickness, and by precipitation. These changes can be forecast, both as to time and place, with a fair degree of certainty. The advancing half of a cyclonic depression is an area of increasing impairment of seeing; the receding half is one of improving visibility. The same is true of the V-shaped depressions of western coasts in high latitudes. On the front of the V the seeing grows worse; at the rear, it constantly improves. In each case the decreasing pressure brings foul vision; the increasing pressure, good seeing.

Stagnation of the air almost always brings haziness, but rarely to an extent that interferes with good seeing. In some cases, such as the “stranded Bermuda high” it may be roughly forecast. The haziness resulting from stagnation may interfere with the long-distance helio-signals occasionally necessary, or with the long-distance sighting in geodetic surveys; otherwise, the impairment is not of consequence.

1. Sir Napier Shaw suggests the degree of visibility of the Zodiacal Light, during the season when it is visible, as a test of the clearness of the air. In many laboratories observers note the degree of clearness.
2. G. I. Taylor, Meteorological Office, London.
3. This term was adopted by Sir Napier Shaw in a monograph on atmospheric transparency. It would be difficult to coin a more appropriate name.
4. Trained observers in arid regions are of the opinion that the ordinary desert mirage is due to the reflection of light from the plane of contact of two layers of air resting one upon the other. The experience of the author, covering many years in the desert region of western North America, favors this explanation. In his “Light,” Professor Hastings explains it as a case of refraction, and this view is held also by Humphreys.