Heat and its Importance in Meteorology. 375 



about an uniform temperature at all altitudes in the descend- 

 ing air. The fact that in high areas air is warmer as we de- 

 scend down to a certain altitude must therefore be held to prove 

 that the air is descending faster than this compensating rate, 

 just as it does in the case of the fohn. 



The resulting rate of warming expressed in degrees Centi- 

 grade per 100 meters of descent is given by the formula: one 

 less the quotient of 288 divided by the rate of descent ex- 

 pressed in meters per day. Thus if the descent is 144 meters 

 per day, the air will cool at the rate of 1°C. per 100 meters of 

 descent ; if the rate be 432 meters per day the air will be 

 warmed up at the rate of 0*33° C. per 100 meters. In the 

 upper layers of the maximum area of 1889, as investigated by 

 Hann, the air grew warmer at the rate of 0*38° C. per 100 

 meters, corresponding to a rate of descent of 457 meters per 

 day ; on the other hand in the lower layer, namely between 

 500 and 1000 meters the air grew colder as the altitude dimin- 

 ished at the rate of 1*8° C. per 100 meters, corresponding to a 

 rate of descent for the air itself of 103 meters per day. 



(13 ) These figures must for the present be considered merely 

 as illustrations of the quality of the influence that atmospheric 

 radiation must exert on the temperature and motions of the 

 air. If the coefficient of radiation of the upper air is less 

 than Maurer's figure or if the coefficient for the lowest strata 

 is as large as Hutchins' figures owing to the abundance of dust 

 and haze, or if, as is generally the case the enclosure is more 

 than 1° cooler, then the numerical rates just given will need to 

 be modified. In either case the general principle remains that 

 on the average throughout the whole atmosphere the cooling 

 by atmospheric radiation largely compensates for the warming 

 by compression and that in this way we must reconcile the 

 hydrodynamic theories with the facts of observation so clearly 

 set forth by Hann. 



The compensating process here discussed is illustrated by the 

 curves in the accompanying diagram which express the fact 

 that if air starts at a height of 10,000 meters at P at a tempera- 

 ture of —50° C. and descend to the earth's surface adiabatically 

 its temperature will be expressed by the straight line X and 

 will be +50° C. on reaching the earth's surface. But if it cool 

 by radiation its temperature at any altitude will be represented 

 by such curves as A, B, or C, whose curvature will depend 

 upon the rates of radiation and descent. The apices of these 

 curves, or the regions where the increase of temperature with 

 descent ceases and becomes a decrease, will be nearer the sur- 

 face of the ground in proportion as the coefficient of radiation 

 is smaller or the rate of descent is smaller and especially if the 

 rate of descent diminishes with the time. 



