METEOROLOGY. 



537 



Wind*. The results of observations on the 

 daily periodicity in the velocity of the wind 

 extending over two years have been communi- 

 cated by Dr. Vettin. From direct determina- 

 tion of the movement of smoke coming from 

 a chimney and from observations with a home- 

 made anemometer, the author found that, in 

 addition to the well-known maximum velocity 

 of the wind which occurs at midday, there is 

 a second maximum just after midnight. The 

 latter maximum is very small in summer, but 

 in winter it is much greater. It is, however, 

 not very marked as an average over the whole 

 year. 



Falling winds that is, winds that blow down 

 from the heights of mountain- crests into the 

 valleys and depressions may evidently have 

 different characters. Two classes of them are 

 usually distinguished, warm and cold falling 

 winds. To the former class belongs the Folm 

 of the Alps, on the northern slopes of the 

 Pyrenees, of the Ferral in Spain, etc. ; to the 

 latter the Mistral of the French Mediterranean 

 coast and the Bora of Istria and Dalmatia. H. 

 Meyer lias shown, from the labors of Hann and 

 Von "Wrangell. that these winds are not intrin- 

 sically different from one another. A vapor- 

 bearing mass of air is warmed by compression 

 about 0'97 C. for every hundred metres of 

 descent, while the temperature of the still at- 

 mosphere descends about 0'5 for every hun- 

 dred metres of ascent. 



The phenomenon of the Fohn is supposed 

 by Hann and Wild to be simply a kind of gust 

 or eddy which, blowing down from motmtain- 

 heigbts. is warmed by compression. This ex- 

 planation is confirmed in a popular essay pub- 

 lished by Erk in a Bavarian journal by descrip- 

 tions of certain marked examples of Fohns 

 that had been observed in 1885 and 1888. On 

 two of these occasions a marked east Fohn 

 blowing over the southern side of the Alps was 

 regarded as the effect of a high pressure in 

 Eastern and a low in Western Europe. 



Prof. F. Waldo has determined that too high 

 a factor of multiplication 3, when it should, 

 be 2'15 has been used in deducing wind-press- 

 nres from velocity-anemometers ; and, conse- 

 quently, that the pressures thus deduced have 

 been greatly exaggerated. 



Salke's observations at Tarnople on the daily 

 periods of wind-velocity indicate, in harmony 

 with the theory of Espy and Koppen, that the 

 period is more sharply defined according as the 

 conditions are favorable to a vertical circula- 

 tion of air, and, consequently, to its descent 

 from above to the surface of the earth. The 

 difference between maxima and minima veloci- 

 ties diminishes with increasing strength of 

 wind, and the daily period becomes indistinct 

 in very strong winds. The duration as well as 

 the velocity is influenced by the temperature; 

 and the maxima of frequency swerve around 

 the horizon, following the sun at a distance of 

 about 90, with a regularity corresponding 

 with the warmth of the periods. 



The records of the stations of the Engli.-h 

 Royal Meteorological Society tor the ti^ht. 

 years 1878-'85 show that the" southwest wind 

 is the most prevalent, and blows on the aver- 

 age seventy-four days in the year, while the 

 we>t wind occurs almost as frequently, blow- 

 ing for sixty-five days. The least dominant 

 winds are the southeast and north, which oc- 

 cur on twenty-seven days, and the northeast 

 on thirty-two days. Thunder-storms are most 

 frequent in the eastern and midland counties, 

 and least frequent in the north of Wales. 



The name derecho, or ''straight blow," has 

 been proposed by Dr. Gtistavus Ilinrichs, of 

 the Iowa Weather Service, to designate a kind 

 of storm on the prairies which has been <! 

 with tornadoes, but is distinguished from them 

 by the absence of spirality in the motion. It 

 is described by him as " a powerfully depress- 

 ing and violently progressing mass of cold air, 

 moving destructively onward in slightly di- 

 verging straight lines (in Iowa), generally to- 

 ward the southeast, with its storm-cloud front 

 curving as the storm-lines diverge. The ba- 

 rometer bounds upward and the thermometer 

 falls greatly under the blow of this cold air of 

 the upper strata suddenly striking the ground. 

 The derecho will blow a train of cars from its 

 track, unroof, overturn, and destroy houses; 

 but it does not twist the timbers into splinters 

 and drive these firmly into the hard soil of the 

 prairie," as does the tornado. The latter is 

 described as "a powerfully lifting column of 

 violently revolving air, describing a narrow- 

 path of destruction as it moves along the 

 earth's surface in a northeasterly direction ; it 

 is surmounted by a cloud from which the col- 

 umn seems to hang down. Its track is gener- 

 ally marked by stakes driven into the ground 

 beyond where it has destroyed buildings, these 

 stakes being the longer fragments into which 

 the tornado has torn such buildings." As the 

 storm-front of the derecho sweeps onward and 

 spreads laterally over the prairies, it is plainly 

 the more extensive of the two storms ; but the 

 tornado in its narrow track is by far the more 

 destructive. The annual period of the two 

 storms is very marked. Neither of them oc- 

 curs in the cold months. In Iowa, the rising 

 tornado season, beginning with the sudden 

 heated and moist spells in April, continues for 

 three months, till early in July, and is most 

 intense in June. Derechos may also occur at 

 this season, and in midsummer are the only 

 storm forms by which the unstable equilibrium 

 of the atmosphere is suddenly restored to sta- 

 ble equilibrium. Tornadoes may again occur 

 in September and October, but have not been 

 observed from November till April. While 

 the Uizzard is a winter storm, bringing the 

 surface air of colder regions, the d 

 summer storm, in its mode of progress and in 

 some other features resembling the blizzard, 

 but more restricted in extent, confined to defi- 

 nite limits, and supplied with cold air coming 

 down from higher strata of the atmosphere. 



