470 



METEOROLOGY. 



posite character is described by M. A. Lancaster, of 

 Brussels, when mists were raised by the shining of 

 the sun into a narrow valley when the air was full 

 of moisture after a heavy dew. In a few moments 

 a white vapor rose from the ground, thickening 

 gradually to the height of about a metre, and then 

 stretched along, under the influence of the wind, 

 forming a long ribbon of mist. The vapors be- 

 haved exactly like the steam rising from a strongly 

 heated surface of water. New beds of the mist 

 were formed as the sun's rays advanced into the 

 valley, while the former ones disappeared. Similar 

 vapors arose from the trees and shrubs along the 

 road, to disappear very soon without greatly ex- 

 panding. 



None of the fogs in high latitudes, as observed by 

 Prof. W. H. Brewer, are so white and opaque as 

 those seen south of latitude 50 : and they are rarely 

 so opaque that large, dark objects can not be seen 

 at a distance of 200 feet. In the Greenland seas 

 the fogs were, as a rule, very much wetter. Often 

 when the fog was so transparent that objects could 

 be seen for half a mile or even a mile from the ship, 

 the water would drip like rain from the rigging. 

 On returning to the south, where the fogs were very 

 dense and objects could not be seen at a ship's lengt h, 

 there was a marked contrast in their wetness. The 

 air did not appear as if entirely saturated. The 

 dust particles in the air over the southern waterr 

 were ample to collect all the moisture, while in the 

 Greenland fogs condensation went on as if- these 

 was not nearly dust enough in the air to supply the 

 demand. 



It has been shown by R. II. Scott concerning the 

 difference between fogs as relating to the weather 

 systems that accompany them that at least two dis- 

 tinct classes of phenomena are described under the 

 generic name of " fog.' 1 In the case of anticyclonic 

 fogs, no rainfall takes place : the temperature is low 

 in the morning, and a considerable rise takes place 

 during the day ; while in the case of cyclonic fogs, 

 rainfall does take place, and the temperature is 

 high in the morning, frequently approaching or 

 even equaling the maximum for the day. 



The following definitions of fog, mist, and haze 

 are proposed by a member of the Royal Meteoro- 

 logical Society : Fog, an obscuration due to con- 

 densation of aqueous vapor when the particles are 

 too small to be seen by the naked eye; mist, when 

 the particles are large enough to be seen with the 

 naked eye ; smoke fog, obscuration without water 

 particles; haze, an obscuration of distant objects 

 so slight that the cause is not visible to the ob- 

 server. 



F. A. Rollo Russell observes that haze is most 

 prevalent when the wind is from the northeast, 

 and is due probably to excess of dust brought 

 about by conflicting currents. The causes of fogs 

 are to a great extent the same as the causes of haze, 

 although radiation in certain states of the air and 

 the ground plays a more conspicuous part. The 

 main cause of fogs is mixture of airs of different 

 temperatures, and the attainment of a size of water 

 particle so much larger than in the case of haze is 

 due to suddenness of mixture, greater humidity, or 

 greater differences of temperature. The conditions 

 favorable to visibility are dryness of the air near 

 the ground level, uniformity of temperature and 

 moisture, radiation below the mean, steady and 

 homogeneous winds through a great depth of the 

 atmosphere, approximation of the temperatures of 

 sea and land, and a number of dust particles less 

 than the mean. 



Observations made by Mr. E. D. Fridlander with 

 a form' of Aitken's pocket dust counter during a 

 voyage round the world showed that considerable 

 variations in the quantity of dust in the atmosphere 



often occur in a very short space of time. Dust was 

 found not only in inhabited countries and over the 

 water surfaces immediately adjoining them and up 

 to an altitude of 6,000 to l',000 feet among the Alps, 

 but it was also found in the open ocean, and that so 

 far away from any land as to preclude the possi- 

 bility of artificial pollution ; and its existence has 

 been directly demonstrated at a height of more than 

 13,000 feet. 



Winds. The committee of the British Associa- 

 tion appointed to consider the effect of wind and 

 atmospheric pressure on the tides reported the con- 

 clusions, from information obtained from various 

 parts of England, that the tides are influenced both 

 by atmospheric pressure and by the wind to an ex- 

 tent that considerably affects their height; that the 

 height of about one fourth of the tides is affected by 

 wind : that the atmospheric pressure affecting the 

 tides operates over so wide an area that the local 

 indications given by the barometer at any particu- 

 lar spot do not afford any trustworthy guide as to 

 the effect on the tide of that particular port ; that 

 although, so far as the average results go, there can 

 be traced a direct connection between the force and 

 direction of the wind and the variation in the height 

 of the tides, yet there is so much discrepancy in the 

 average results when applied to individual tides that 

 no satisfactory formula can be established for indi- 

 cating the amount of variation in the height of the 

 tide due to any given force of wind ; that the results 

 given in the tables attached to the report relating 

 to atmospheric pressure indicate that the effect of 

 this is greater than has generally been allowed, a 

 variation of half an inch from the average pressure 

 causing a variation of 15 inches in the height of 

 the tides. 



In a discussion of the scientific aspect of balloon 

 voyages, Prof, von Bezold, starting with the funda- 

 mental physical principles which underlie the events 

 taking place in cyclones and anticyclones as also in 

 the general atmospheric circulation, proceeded to 

 show the necessity for more exact measurements of 

 temperature and humidity in the upper strata of 

 the air, and of ascertaining the height at which air 

 passes over from a cyclone into an anticyclone. 



Thunderstorms. Prof, von Bezold-, speaking 

 on the unstable equilibrium that precedes a thun- 

 derstorm, observes that the fact that in the interior 

 most thunderstorms occur in the afternoon and dur- 

 ing the summer, whereas near the coast they are 

 most frequent at night and in the winter, shows 

 that there must be different causes for the insta- 

 bility. As a matter of fact the author shows that 

 not only overheating of the lower layers of air, but 

 also excessive cooling of the upper layers, must lead 

 to unstable equilibrium and a correspondinglypower- 

 ful upward current of air. A similarly unstable 

 state is brought about by the sudden solidification 

 of strongly cooled water drops, or the condensation 

 of air highly supersaturated with moisture. The 

 conditions for realizing these states are different in 

 the interior and at the coast or over the sea, and 

 the mode of formation of a thunderstorm is corre- 

 spondingly different. 



Mr. H. Harries has shown, in the Royal Meteoro- 

 logical Society, that hail and thunderstorms are 

 not, as has been supposed, extremely rare in the 

 arctic regions. He has examined 100 logs of ves- 

 sels that have visited those quarters and 75 of them 

 gave records of hail having been encountered at 

 some time or other. Thunderstorms are not so 

 freely mentioned as hail, but they have been ob- 

 served in seven months out of the twelve most 

 frequently in August. 



Miscellaneous. The International Meteorolog- 

 ical Congress, which met in Paris in September, 

 was attended by directors of meteorological stations 



