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 a storm 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 cork on 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.