Page:The American Cyclopædia (1879) Volume XI.djvu/455

 METEOROLOGY 443 Diurnal Variation in Cloudir HOUR. AT CREFELD. AT PRAGUE. January. July. January. July. Midnight. 75perct. 49perct. 1 A.M. 2 73 45 8 4 65 46 5 6 74 si 7 82 63 8 76 49 9 77 62 10 74 54 11 76 66 Noon. 73 58 1 P.M. 76 67 2 is 56 3 74 66 4 73 57 5 73 63 6 73 54 7 ' 69 59 8 75 54 9 67 58 10 73 48 11 70 53 Midnight. 75 5i An annual periodicity in the cloudiness is also observed (see the following table), having its maximum in the winter time throughout the temperate zones, while within the polar cir- cle the maximum cloudiness may be placed in the late summer and fall. Annual Variations 'in Cloudiness. MONTH. AT OREFELD. AT PRAGUE. 74 per ct 78 per ct. 70 Ma rch .... 66 62 April 62 54 May 66 52 June 65 53 July 62 51 58 49 57 51 October 65 November 77 76 December. . . 74 74 As regards the geographical distribution of cloudiness, it may be remarked briefly that the annual averages show a minimum cloudiness in the regions of the trade winds and in the interior of continents, and a maximum in moun- tainous districts, the polar regions, and on the western coasts of the continents. The relation between the direction of the wind and the av- erage degree of cloudiness is known to all, and is rendered apparent by the following table : Percentage of Cloudiness for each Wind Direction. WINDS. CARLSRTTHE. TIENNA. Winter. Summer. Winter. Summer. N.. 63 56 56 69 80 82 80 77 63 46 32 29 43 59 62 58 55 46 75 74 63 71 70 69 65 72 75 45 26 25 25 87 43 56 55 45 N.E E 8. E. S 8. W W... N. W. .. N.. The connection between the lunar phases and the cloudiness has been already spoken of. The influence of cloudiness in protecting the earth's surface against the direct rays of the sun, and against nightly radiation from the earth, has been investigated for a few stations only. According to Weilenmann (1874), the atmosphere, when the sky is clear, protects the earth against radiation by an amount equal to one third of the protection afforded by a cloudy sky, whence the latter may be concluded to allow on the average scarcely 15 per cent, of the solar heat to reach the earth's surface. The connection between cloudiness and baro- metric pressure has not yet been studied with the care it deserves; but it is certain that a low barometer corresponds to an increased cloudiness, and an important portion of the daily variation in the barometric pressure de- pends upon the formation of dew or fog at night and of cumulus clouds by day. The rain- fall may be considered as the completion of the process of the formation of clouds, although probably the majority of clouds are dissipated without producing rain. The larger part of the rainfall is probably deposited by uprising currents of air, and therefore it has its daily and annual periods. The formation of snow is apparently more directly dependent upon noc- turnal radiation of heat. The measurement of rainfall by means of the simple rain gauge needs to be made under circumstances of great uni- formity, if for different stations we would at- tain results comparable with each other. The most minute investigations of the laws govern- ing rainfall have been made by Symons, and published since 1866, in his annual volumes of " British Rainfall." According to these results, which are abundantly confirmed by other mea- sures taken throughout the world, the quanti- ty of rain received by a gauge diminishes in an irregular manner with the height of the gauge above the earth's surface. The diminution is apparently to be attributed principally to the greater velocity of the wind at the higher sta- tion, and amounts to 10 per cent, of the whole rainfall for an elevation of 20 ft., and in one exceptional case, at the height of 50 ft., to 40 per cent, of the rainfall at the surface. The study of the diurnal period of the precipitation (including rain and snow) has as yet led to only an imperfect result ; but it is believed that as a general rule, in the temperate zones, a maxi- mum occurs in the afternoon in the summer, but before sunrise in the winter months. The annual variation in rainfall depends almost ex- clusively upon the relation between the winds and the geographical position and topographi- cal details of the country. On the W. coast oi North America and of Europe the greater part of the annual rainfall occurs in winter, but in the interior of Europe and on the E. coast of the United States in summer. The geographi- cal distribution of the snow and rainfall over the world can be properly presented only by means of a very large and detailed chart ; its