THE EXPLORATION OF THE FREE AIR. 323 



heights of 100 to 11,000 feet above Blue Hill, in all kinds of weather. 

 Most of thein are discussed by Mr. Clayton in Appendix B to Part I 

 of Vol. XLII of the Annals of the Astronomical Observatory of Har- 

 vard, and some general conclusions can only be stated here. As 

 regards wind, the observations show that, as a rule, the wind steadily 

 increases with elevation, confirming the measurements made upon 

 clouds. The tendency of the kites as they rise is to come into a cur- 

 rent from the west, and it is possible, with no great diiference of 

 height, to find currents almost diametrically opposed to each other. 

 The decrease of temperature with increasing elevation varies under 

 different conditions. On most days when there are no clouds the tem- 

 perature falls at the adiabatic rate for unsaturated air, i. e., 1° F. for 

 each 180 feet of ascent, to the height of a mile or more. On fair days 

 with clouds the fall is at the above rate to the base of the clouds. In 

 the cloud the rate of fall is slow, and it is still slower above the cloud. 

 Kite records have been obtained during nearly twenty-four consecutive 

 hours at a height of about half a mile above the ground. From these 

 records it appears that the diurnal change in temperature in the free 

 air nearly disappears at about 2,500 feet, which is much lower than has 

 been hitherto supposed. During calm nights there is a marked inver- 

 sion of temperature, so that the air near the ground is usually much 

 colder than at the height of a few hundred feet. Indeed, on some 

 occasions the air may be colder at the ground than at the height of 

 several thousand feet. Kite flights, which were made daily for a week, 

 confirm the theory that temperature changes in the upper air are 

 cyclonic in character, being due to the passage of " warm and cold 

 waves," that are more strongly felt in the upjjer air than near the 

 ground. The changes with altitude which j) recede a warm wave are 

 these : During the day a decrease of temperature at the adiabatic rate 

 from the ground up to more than 1,000 feet, then a sudden rise of 

 temperature, amounting to perhaps 15", followed by a slow fall. 



Clouds form when the dew point of the warm current, which over- 

 flows the cold current, is sufficiently high. Such conditions announce 

 the arrival of a "warm wave" eight to twenty-four hours in advance 

 of its appearance at the earth's surface. The conditions which indicate 

 the coming of a "cold wave" are a rapid fall of temperature, which 

 exceeds the adiabatic rate up to about 1,000 feet and above that is the 

 adiabatic rate to 3,000 feet or higher. During the prevalence of the 

 cold wave the temperature at the height of a mile, which is sometimes 

 its upper limit, may be 25° or 30° F. lower than at the ground. After 

 the cold wave has jjassed, and with the coming of a southeast storm, 

 the temperature rises rapidly up to a height of 1,000 or 2,000 feet and 

 then slowly falls. Cloud usually occurs where the temperature begins 

 to fall, and sometimes this cloud extends downward to the earth as fog. 

 The relative humidity generally increases to saturatioi^/ in the clouds 

 and above them rapidly decreases. In clear weather there may be no 



