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 flies into 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.