“The air is so sensitive,” writes the late well-known British flyer, Gustav Hamel, “that it is affected even by the color of large patches of vegetation. Whether this be entirely due to the different heat-radiating power of different colors it is impossible to say, but invariably an aeroplane on passing from grass land to a field covered with yellow flowers experiences a certain amount of air disturbances only less noticeable than the inevitable bump experienced in passing from green fields to ploughed land, or from ploughed land to meadow.” When the wind is blowing, the air for at least a few hundred feet above the ground is nearly always in a state of turmoil. This is partly due to the friction of the moving fluid against the irregular surface of the earth, and partly to the ascending and descending currents caused by differences in temperature. The latter effect is illustrated in the rapid rise of air over a bare sunlit plain and its fall over an adjacent forest or body of water. Ascending currents are often made visible by the formation of detached cumulus clouds, each of which marks the summit of a rising column of moist air, while in the spaces between the clouds the air is generally sinking. Measurements with balloons have shown that vertical currents often attain speeds of 600 feet a minute or more, while the process of hail formation appears to indicate that in thunderstorm clouds there are violent uprushes amounting to 2,000 or 2,500 feet a minute, and possibly much more. The descending air current between clouds is sometimes so strong that an aeroplane cannot force its way up through it. [Illustration: THE FLOW OF AIR OVER TWO RIDGES (_After Dr. Franz Linke._) Notice the eddy in the valley to the leeward of the first ridge] [Illustration: A SHELTERED LANDING PLACE MAY BE DANGEROUS (_After Dr. Franz Linke._) A landing place surrounded by trees is dangerous in windy weather on account of the air waves formed between the moving air above and the calm air below.] [Illustration: AIR WAVES NEAR THE EARTH’S SURFACE The waves are made visible by smoke] The turbulence of the lower air--a phenomenon that adds so much to the difficulties of starting and landing--extends to various heights, depending especially upon the strength of the wind. A rough rule, evolved by the Zeppelin pilots before the war, was to expect turbulent conditions up to an altitude equal to from ten to twenty times the force of the wind in meters per second. Thus, for a wind of 10 meters per second, the turbulent layer would be from 100 to 200 meters thick. A good picture of the atmospheric ups and downs encountered by the airman when flying low is furnished by the behavior of the smoke from a factory chimney with a moderate wind blowing, forming smoke waves. [Illustration: THE WIND’S AUTOGRAPH ON A GUSTY DAY, RECORDED WITH A PRESSURE-TUBE ANEMOMETER