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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 



