448 



METEOROLOGY. 



Prof. Hellmann in 1875, when studying the 

 Mount Washington observations, and the same 

 fact has since been remarked at Ben Nevis and 

 other observatories. The wind has also a vertical 

 as well as a horizontal motion, which has 

 amounted to 7 miles an hour in a storm. The 

 normal temperature at the summit of Blue Hill 

 is 2 lower than at the base, giving a difference 

 of 1 for each 220 feet of ascent ; but inversions 

 frequently occur when the temperature at the 

 base is lower than at the summit. 



A study of the influence of forests on the daily 

 variation of air temperature has been made by 

 Prof. Muttrich from data collected at stations in 

 Germany and Austria. The influence seems to 

 be greater between May and September or Oc- 

 tober than in the other months. In pine and 

 fir woods it rises gradually from January to a 

 maximum in August or September, then falls 

 more quickly to a minimum in December ; but 

 in beech woods a minimum occurs in April, then 

 there is a quick rise, till the maximum is reached 

 in July. The daily variation itself is greatest in 

 June, both in forest and open country. The in- 

 fluence of the forest is to lower the maxima and 

 raise the minima, and the former influence is 

 greater in most months than the latter ; in De- 

 cember and January, and occasionally in neigh- 

 boring months, it is less. The influence on the 

 maxima in summer is greatest in beech woods, 

 less in pine, and least in fir. The absolute value 

 of the influence of woods of a given kind of tree is 

 affected by the degree of density of the growth, 

 being higher the denser the forest. The char- 

 acter of the climate (oceanic or continental) also 

 affects the results. From daily observations in 

 forest and open country, every two hours in the 

 second half of J une, it appears that, soon after 

 5 A. M. and 8 P. M. the air temperature in the 

 wood was equal to that in the open ; that the 

 maximum was about 0'9 lower in the wood, and 

 the minimum 0'6 higher; that from May to 

 September the difference sometimes reached 

 2'7 ; that the maximum in the wood occurred 

 about half an hour later, and the minimum a 

 quarter of an hour later, than in the open ; and 

 that the daily mean temperature was about i 

 less in the wood. 



The influence exercised by the mixture of 

 strata of air of different temperatures and near- 

 ly saturated with vapor on the formation of fogs 

 and clouds, has been investigated by Herr von 

 Bezold. Thermodynamic considerations derived 

 by graphic methods seem to the author to dem- 

 onstrate that the mixture of warm air saturated 

 with vapor with nonsaturated cold air, more 

 readily causes condensations than a current of 

 saturated cold air with a stratum of nonsatu- 

 rated warm air. The quantities of water con- 

 densed in this way are very small, for the action 

 of adiabatic expansions and direct cooling re- 

 moves much. If the air contains suspended par- 

 ticles of water, evaporation and depression of 

 temperature may be produced under the influ- 

 ence of a current of warm air. If the air is me- 

 chanically but not hydrosco^ically saturated, de- 

 pression may be produced, even if the warm air 

 that comes in is saturated with vapor. But if the 

 cold air is also saturated, the air that comes in 

 should be dry. It is concluded that mixtures of 

 liquid water and nonsaturated air will cool, and 



that the cooling will be more sensible according 

 as the air is further removed from its point of 

 saturation and the quantity of water is larger. 



It appears from a map constructed by M. Lan- 

 caster to show divergences from normal temper- 

 ature in Europe during the five years 1886-'90, 

 that the center of the "island of cold " lies over 

 the north of France, the south of Belgium, and 

 the most western parts of Germany. From this 

 center the cold decreases with an approach to 

 regularity outward on all sides to a nearly circu- 

 lar line of nil divergence, which, embracing the 

 whole of Great Britain, crosses the south of 

 Sweden, then goes along the German-Russian 

 frontier, through Hungary, the south of Italy, 

 the north of Africa, and across Spain. Through- 

 out this inclosed region abnormally low temper- 

 atures have prevailed. Siberia, too, shows ther- 

 mal depression, which M. Lancaster thinks may 

 be connected with that in western Europe. 



The details of the process by which all the 

 heat our planet receives from the sun is eventu- 

 ally lost by radiation evident according to Prof. 

 Cleveland Abbe has not been completely worked 

 out. Such studies of it as have been made en- 

 force the necessity of carefully distinguishing the 

 influence of the absorption and radiation by the 

 soil, the vegetation, the snow, and the ocean, re- 

 spectively ; and in the atmosphere itself, between 

 the absorbing and radiating powers of the dry 

 air, of the clouds or haze, and of the dust, re- 

 spectively. Besides what is conducted and con- 

 veyed to the atmosphere from the immediate 

 surface of continents and oceans, the radiant 

 heat from those surfaces is in part directly ab- 

 sorbed in the atmosphere, and in part transmit- 

 ted to outer space. All of this heat must be 

 ultimately lost through a process of radiation 

 from the atmosphere, as distinguished from ra- 

 diation through the atmosphere. A part of it 

 may even escape from the inner layers through 

 the overlying layers, without being taken up by 

 them, into ethereal space. The researches of 

 Ferrel, Helmholtz, and others show that areas 

 of high pressure and horizontal or inclined or 

 vertical whirls in the atmosphere are kinetic 

 movements, not dependent on conditions of heat 

 or density. The factors that principally affect 

 temperature in extensive areas of high pressure 

 are: 1, the direct absorption by the air of the 

 solar radiation ; 2, the direct radiation by the air 

 of its own heat ; and, 3, the thermal changes due 

 to changes of barometric pressure on the radi- 

 ating mass, which are themselves due to its de- 

 scent in the atmosphere. The cooling by radia- 

 tion is to only a very slight extent offset by the 

 direct absorption of solar heat, and on the aver- 

 age it is largely compensated for by the warming 

 from compression. The general tenor of obser- 

 vation is that under a cloudy sky the diminution 

 of temperature with ascent is "slight and uni- 

 form ; within a cloud it is given by the laws of 

 evolution of heat; above all clouds and in all 

 clear air, as in areas of high pressure, the tem- 

 perature depends upon the radiation of heat and 

 the thermo-dynamic changes of a compressible 

 atmosphere. Having reviewed the deductions of 

 various authors relative to these points, Prof. 

 Abbe mentions as among other important prob- 

 lems in the mechanics of the ,atmosphere that 

 are elucidated by the study of radiation : 



