Jacques Wardlaw Redway

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”