Page:Popular Science Monthly Volume 92.djvu/740

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��Popular Science Monihhj

���Air Waves Far Above the Earth

With the right conditions of temperature and humidity these waves are made visible by the formation of little clouds whose crests are marked as furrows. These are called cirrus, and are the light, fleecy summer clouds

��of diflferent temperatures and humidities, which glide over each other without much intermingling. At the boundary surface between them, friction sets up waves like those produced in water by wind flowing over it. When the two streams are moving in the same direction the waves are long and regular; when they are more or less crossed, the waves are short and choppy. The moisture at the crests or furrows of these waves may be cooled to such an extent as to condense into visible clouds, arranged in long continuous rolls or successive rows of detached patches; but more often the waves are entirely invisible.

Ascending and descending currents in the air are also sometimes made visible by the larger detached clouds of the type known as cumulus. Each of these clouds marks the summit of an ascending column of moist air, while in the spaces between them the air is generally sinking. The up- flowing air under a cumulus cloud may at- tain a vertical speed of 25 or 30 feet a second, while the descending current be- tween clouds is sometimes so strong that an airplane cannot force its way up through it. The most violent vertical movements are encountered in thunder- storms. The term "hole in the air" de- scribes a sudden downward tendency of the airplane, whether due to running into a descending current or to encountering a

��sudden change in wind velocity and conse- quent lifting force.

There has been much talk about "charting" the winds for the aviator — a project implying the assumption that the currents in the atmosphere are as regular and as constant in their location as those of the ocean, which is far from being the case. Even the most constant winds in the world — the trade winds — are subject to great fluctuations in force. What the aviator really needs to know is the typical behavior of the winds with respect to the distribution of barometric pressure at a given time (as shown on a daily weather map), and how they are likely to be af- fected by the topography of the country over which he is flying.

Fog — the Airman's Dread

One of the most serious weather prob- lems of the aviator is presented by fog. When flying above a fog (and the same is true of low-lying clouds) the airman has no landmarks to guide him. His compass is almost useless, because, while it tells him which way the head of his machine is pointed, he has no means of knowing how much he is being drifted out of his course by the wind. In a long flight his "leeway" may carry him scores or hundreds of miles wide of his objective point. Fog also pre- sents a grave danger when he is landing, as he knows neither his distance from the

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