METEOROLOGY. 



493 



(8.) The frequency of sudden irruptions of very dry winds (gen- 

 erally but not necessarily cold) which by their desiccating power de- 

 stroy tender vegetation, a factor that can of course be well presented 

 by the changes from small to great daily evaporation or from great to 

 small elevations of wet-bulb above dew-point temperature. 



In numerous suggestive notes Hann leaves the dry field of descrip- 

 tive climatology proper and wanders a little over into the fascinating 

 border-land of dynamic meteorology. Usually he thus intensifies our 

 interest in the subject by the clear and satisfactory explanations. But 

 confining ourselves to the problems of the future, we quote the following. 



The annual mean distribution of atmospheric pressure reduced to 

 standard gravity and at sea-level and for all meridians is given for each 

 5° of latitude in the second column of the following table : 



Mean annual — 



Latitude. 



Temper | 



FeTei l Pre88Qre - 

 and 

 Hann. 



Degrees. 

 North 90 

 85 

 80 

 75 

 70 

 65 

 60 

 55 

 50 

 45 

 40 

 35 

 30 

 25 

 20 

 15 

 10 

 5 

 



°C. 



—17.0 



Ferrel. 4 



26.8 



760.5 

 700.0 

 758.8 

 758.2 

 758.7 

 759.7 

 750.7 

 761.5 

 762.0 

 762.4 

 761.7 

 7C0.4 

 759.2 

 758.3 

 757.9 



Ratio of land 



to water. 



Foster and 



Dove. 



Per cent. 



48.3 



56.8 



56.3 



43.4 



30.8 



23.4 



758. 21. 6 20. 



1 Hann, Klimatologie, pp. 92, 93, and 742. 



'Dove, Zeit. Eidkunde, 1862, xii, and Hann, p. 90. 



3 Hann, pp. 92, 93. 



* Ferrel, Met. Researches, part 1. 



u The pressures here given agree with the distribution of the atmos- 

 phere required by the mechanical conditions for the general circulation 

 of the atmosphere as published by Ferrel in 1858 and 1860. We see 

 that the pressure diminishes from either circumtropic latitude towards 

 the corresponding pole, especially in the southern hemisphere, where 

 the whirl around the pole can develop with the least disturbance. The 

 zone of highest barometric pressure lies further from the equator in 

 the northern than in the southern hemisphere (it lies in the former 

 somewhat north of the 35th degree north latitude, but in the latter be- 

 tween the 25th and 30th degrees south latitude), and the accumulation 

 of air is less there since the process of circulation is more restricted 

 and therefore less energetic. 



" This circulation of the atmosphere (delineated in the previous par- 

 agraph according to Ferrel's views as given by him in his Motions of 



