Until within the past few years, knowledge of the upper winds was imperfect and fragmentary. Sounding balloons and kites furnished with recording apparatus are beginning to supply humanity with much-needed information concerning horizontal movements of the upper air; the airmen are furnishing knowledge of vertical movements.
Research in recent years shows that the updraught in equatorial regions is not uniform in force nor continuous at all times. Neither is the overflow of rising air toward polar regions uniform or regular. Sounding balloons occasionally have been carried toward the equator instead of away from it. Stiff west winds also have been observed in equatorial regions at the height of a few thousand feet, surmounted by easterly winds at a still greater elevation.
Sounding balloons do not find the decrease of temperature with increasing altitude to be regular; on the contrary, they encounter layers of air throughout which the temperature is practically unchanged. They also encounter other layers in which an inversion occurs—that is, the temperature rises with increasing altitude. In other words, instead of a uniform temperature gradient from ground level to stratosphere, the air consists of a succession of layers, differing in temperature, humidity and horizontal velocity of movement. Usually the planes of contact between adjacent layers are indicated by clouds.
The air of adjacent layers, or strata, does not readily mix one with the other. Smoke, dust, and cloud matter, rising to the top, or sinking to the bottom of a layer does not always penetrate the adjacent layer. In the absence of strong winds such matter is apt to spread out laterally. Moreover, the aviator, in passing from one layer to another, is apt to receive a sharp bump at the plane of contact. In meteorology the plane of contact is commonly known as a ceiling or lid.
The convectional layer of air—that is, from ground level to stratosphere—is marked by constant motion as noted, the movements consisting of general circulation, local winds and the turbulence connected with vertical movements. There seems to be no such complexity of movement in the stratosphere; indeed the knowledge of the movements of the air in the stratosphere is next to nothing. Tidal movements probably warp
