CHAPTER XI
AERIAL NAVIGATION
ATMOSPHERIC CONDITIONS—WINDS AND THEIR WAYS—CLOUD FORMATIONS, NAMES, AND ALTITUDES
Justas the navigator must know the sea, so the aviator must have a knowledge of the heavens and the basic principles of aerodynamics in order to become a successful pilot. Although the air is volatile like the water, the aviator flies through it as a fish moves through water. Therefore the aerial navigator must know enough about the medium through which he travels to know what to do in an emergency. Through a knowledge with the fundamental principles of meteorology the fliers may know what to expect in the form of disturbances to the atmosphere, and how to meet those conditions.
For aeroplane flight a calm clear day is the best. Then eddies and storms are not encountered, although the air is never absolutely free from the former in some degree. Even a strong gale is not a hindrance to flying, as the United States aero-mail and hundreds of machines on the battle-fronts have repeatedly demonstrated. Mists, fogs, and low-hanging clouds are the greatest impediments to flying where the machines are not fitted up with wireless telephones or directionalwireless. For first flights the early morning and late evening afford the calmest atmospheric conditions.
Air, like water, seeks the level where the lowest pressure exists. It is 1,600 times lighter than water, and it extends to some 50 miles above the earth. One half of its weight is below the three-mile limit. Atmospheric pressure is variable, and the temperature of the air usually decreases with the altitude, so that it is often very cold up in the air when it is comparatively cold on the ground. For that reason electrically heated clothing or cabins, heated from the engine, are used to keep the pilot and passengers warm.
The change in the temperature of the earth sets the air in motion, so that portions that are heated by the sun’s rays faster than other portions affect the atmosphere more quickly in that locality than in others, for the heated air rushes up by expansion and the cooler air will rush into the vacated place. With the repetition of this the movement of the air increases. Thus high-pressure areas and low-pressure areas are formed. A glance at a United States Weather-Bureau map will show the location and the atmospheric pressure at various places in the United States, and the intelligent reading of the same will be of infinite usefulness to the aviator. The atmospheric pressure is measured by a barometer. It is measured by a column of mercury necessary to balance it. This same atmospheric pressure is used to operate the altimeter, which tells the aviator how high he has climbed.
A falling barometer indicates the approach of astorm and a rising barometer fair weather. Wind strength is usually indicated by miles at which the storm is raging. In the early days of aviation the aviator used to wet his finger to see if the wind was stirring and what quarter it was from. If it was blowing many miles an hour, he would not venture forth.
In starting or landing a machine it is always desirable to head into the wind. It is true that in forced landings pilots have come down with the wind, but for every foot they must make an allowance.
Atmospheric pressure also has much to do with the flying efficiency of the wings. The heat generated on the surface of the planes used by the United States army in Mexico caused the dope to peal in some cases and rendered the planes unfit to fly.
The flier should, however, know something about the kinds of winds which prevail and the times of the day when the most violent are to be encountered. At the earth’s surface the day winds are stronger than the night winds, and the average velocity of the day wind is about eleven miles an hour. Because of the similarity of the movements of the winds to those of water, many of the terms applied to air movements are the same.
When an upward movement of wind rises from barren land or conical hills, it is called an aerial fountain. Sometimes this air rises at a velocity of twenty-five feet per second. Sometimes an aeroplane when caught in one of these fountains will rise like a corkon the top of a water-spout, or the wing will be tilted if it is hit by this column of hot air.
An aerial cataract is caused by descending cold air, and has the opposite effect on an aeroplane flying through the air to that of the fountain. These are encountered in flying over very broken ground.
Aerial cascades are encountered often in flying over narrow valleys or steep hills. The contours of the land cause the air to follow down into the valleys suddenly, thus often making it dangerous for fliers to attempt to land on rivers enclosed in steep banks, unless of course they fly up or down the river.
With aerial torrents the same principle applies, except that the area of disturbance is broader and more powerful. Great velocity is attained near open valleys, due to the cold air rushing to replace the hot air moving upward. A cross, choppy wind will cause choppy air surfaces and bad eddies, and can be discerned on a cloudy day by rips in the surface of clouds.
Over the crests of hills vertical eddies are encountered. They are usually called pockets by fliers. Often the machine drops straight down, and the pilot should immediately head his machine into the current. Sometimes winds will be found blowing in different directions and passing in layers above one another. These have a tendency to turn the ship about, and is one of the reasons why the aviators prefer to get altitude before doing any stunt flying. Except close to the ground these contrary winds are not dangerous. So just as a vessel is safest far from a coast in a storm,so an aeroplane is safest at a reasonable altitude where the wind is not so bumpy.
Clouds and mist are two of the worst enemies of the aerial navigator; first because it shuts off the observer’s vision of the terrain, preventing him from knowing exactly where he is, and because it makes it difficult for him to locate his landing-field. Directional wireless and the wireless telephone do help a great deal in giving information about the lay of the land beneath the clouds or mist, but of course it cannot visualize the ground on which the aeroplane is to land for the pilot to see exactly where he should set the wheels down. For that reason a knowledge of clouds is essential to piloting aircraft.
There are many different kinds of clouds, but they are all formed by condensation when an ascending volume of moist air mingles with another mass of a different temperature, or when a mass of arising vapor condenses. With a knowledge of the direction clouds are moving in it will reveal certain facts about the weather to the pilot. Clouds take almost every conceivable shape.
A general knowledge of the movement of the clouds is a valuable asset to the flier, for they indicate the air-currents and also the condition of the atmosphere in their neighborhood. Unbroken clouds indicate smooth-flowing air, while the more a cloud is broken the more bumpy the air-currents are in that neighborhood. From the formation of clouds then the atmospheric conditions may be realized by the pilot before he fliesinto them. In general the following types of clouds indicate certain specific facts to airmen.
A mackerel sky, called technically Cirro-Cumulus, which is formed of small globular masses, or white flakes showing only light shadows, or at most only very light ones, or arranged in groups or in lines, usually at a height of 10,000 to 25,000 feet, denote fine weather, and for commercial flying afford ample opportunity for smooth flying below that altitude.
Very light, whitish wisps of clouds, fibrous in appearance, with no shadows which appear at 30,000 feet altitude, or more, are the highest clouds in the firmament, are called Cirrus or Mare’s Tails, because they are scattered like hair over the sky. They indicate wind and a cyclonic depression.
The next clouds in altitude are the Cirro-Stratus, which float 29,500 feet, and look like a thin sheet of tangled web structure. They are whitish, and sometimes completely cover the heavens, giving it a milky appearance. This cloud is one of the most beautiful, and often creates moon and sun halos. It indicates bad weather.
The Alto-Stratus is a thick extensive sheet of bluish or gray cloud, sometimes composed of a thick fibrous structure which is very dense and impossible to penetrate with the eye. They are at an average height of from 10,000 to 23,000 feet, and cause a luminous crown or aureole around the sun or moon.
Woolpack Clouds, or Cumulus, as they are designated, are thick, and the upper surfaces are dome-shaped,with many sharp protuberances, and with horizontal bases. They are low-lying and indicate violent disturbances of the air, and are dangerous for any kind of aircraft when passing above them or through them.
Thunder-Clouds, or Cumulo-Nimbus, are formed in heavy masses rising in the forms of turrets, mountains, or animals. They are usually surrounded by a screen or sheet of fibrous appearance, having its base in a similar formation. The highest points of these clouds reach an altitude of 10,000 to 26,000 feet, and they are as low as 4,000 feet at the base. They indicate lightning and terrific gusts of wind, and are very dangerous to aerial navigators.
The whitish-gray globular masses partly shaded, piled up in groups and lines, and often so thickly packed that their edges appear confused, are called Alto-Cumulus. They are arranged in groups at an elevation of from 10,000 to 23,000 feet. They do not look unlike the mackerel sky. The cross-lines indicate strong currents of air.
Strato-Cumulus are dark globular masses of large clouds, often covering the whole heavens in the fall and the winter. They hang as low as 6,000 feet, and always predict changing weather.
The lowest-hanging cloud of all is the Stratus, which is uniform at a height anywhere from 100 to 3,500 feet. It may be either drifting or stationary. It is a uniform layer, and resembles a fog, but, unlike the latter, it does not rest on the ground.
The Nimbus is a thick layer of dark clouds with ragged edges but without shape. Rain or snow usually falls from this formation. There are many rifts in these clouds, and through them many higher clouds are seen. The Nimbus usually occupy altitudes from 300 to 6,500 feet.