abstracts: meteorology 29 



a killing frost has occurred in spring. (4) The earliest date on which a 

 killing frost has occurred in autumn. (5) The average length of the 

 crop growing season, or the number of days between the average date 

 of the last killing frost in spring and the first killing frost in autumn. 



The data consist of the 10 to 30 year records of about one thousand 

 country stations, and, therefore, being free from city disturbances, are 

 of especial climatological value. W. J. Humphreys. 



METEOROLOGY.— The Aleutian and Icelandic lows. W. J. Hum- 

 phreys. Bulletin of the Mount Weather Observatory, 4. 1911. 



The high regions of Greenland and Iceland are perpetually ice covered 

 and perpetually cold. Hence at their level the air is, in general, warmer 

 over the intervening ocean than over the land. There must therefore 

 exist a nearly continuous atmospheric overflow from above the ocean 

 onto these two land areas and a drainage in turn of the air from them to 

 the water. But this is the nature of a cyclonic circulation, and there- 

 fore this region is almost continuously surrounded by counterclockwise 

 winds of varying intensity. 



A similar temperature distribution, with its resulting cyclonic winds, 

 obtains during the winter in the neighborhood of the Aleutian islands. 

 But the Siberian and Alaskan peninsulas, since their elevation is small, 

 are relatively warm in summer and hence the Aleutian low is only a 

 seasonal phemonenon. 



With the exception of the polar regions, in contrast with the equatorial, 

 there are no other places with such pronounced and, at the same time, 

 so continuous temperature gradients to either side of them, and there- 

 fore no other permanent or semipermanent low. W. J. H. 



METEOROLOGY. — Vertical temperature gradients and convection limits. 

 W. J. Humphreys. Bulletin of the Mount Weather Observatory, 

 4. 1911. 



By day the surface layer of the air is warmed mainly by contact with 

 the heated earth, and the layers next above by convection. Hence the 

 afternoon temperature gradient must closely follow first, the dry air 

 adiabat, and then, if convection has extended high enough, the satura- 

 tion adiabat. 



By night the lowest air is cooled by contact with the relatively cold 

 earth, and the layers next above partly through mixture with the cooler 

 under air and partly by radiation, and thus an inversion gradient — one 

 that shows warming with elevation — is often produced near the surface. 



