Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/164

Rh 154 METEOROLOGY [SNOW. falls wholly during the summer months of the southern hemisphere. In South Africa pressure in January is lowest in the interior, towards which prevailing winds from the ocean blow, and as these advance into regions becoming rapidly hotter the rainfall all round the coast and for some distance inland falls to the annual minimum. But in more strictly inland districts which are at a considerable elevation the rainfall reaches the maximum at the same season. Thus the amounts in inches for January and July are for Pretoria, 6 07 and 071; Maritzburg, 4 23 and 21; Graham s Town, 2 89 and 1 51; Lower Nei s Poort, 1 33 and 49; and Aliwal North, 1 55 and O OO. In the winter months pressure in the interior is high, and the rainfall consequently small. Though on the coast winds from the arid interior frequently prevail, yet the storms that sweep eastward past South Africa precipitate over large portions of the southern slopes of this part of the globe what must in the main be regarded as a generous rainfall. It follows that the climates of these important colonies range themselves into two perfectly dis tinct classes, the climates of the inland regions and the Natal coast, where the rains occur during the hottest months, and the climates of the other regions, where the annual rains occur during the coldest months. Little is accurately known regarding the rain fall of the interior of Africa. It is certain, however, that it is small, or nil, over the extensive region of the Sahara, and that it is large from about 15 N. lat. to some distance south of the equator. Probably the rainiest part of Africa is the region extend ing from the Victoria Nyanza northwards to and including the gathering grounds of the two great tributaries of the Nile. Snow. Snow takes the place of rain when the tempera ture is sufficiently low to freeze the condensed moisture in the atmosphere. Snow is composed of crystals, either six- pointed stars or hexagonal plates, which exhibit the greatest variety of beautiful forms, one thousand dif ferent kinds having been observed. These numerous forms Scoresby reduced to five principal varieties : (1) thin plates, comprising several hundred forms of the most ex quisite beauty ; (2) a nucleus or plane figure, studded with needle-shaped crystals; (3) six-sided, more rarely three-sided, crystals ; (4) pyramids of six sides ; (5) prismatic crystals, having at the ends and middle thin plates perpendicular to their length. In the same snowfall the forms of the crystals are generally similar. The flakes vary from 07 inch to an inch in diameter, the smallest occurring with low temperatures and the largest when the temperature approaches 32. If the temperature is a little higher, the snow-flakes are partially thawed in falling through it, and fall as sleet. The white colour of snow is caused by the combination of the different prismatic colours of the minute snow-crystals. The density of snow is far from uniform ; it is generally from ten to twelve times lighter than an equal bulk of water, but varies from eight to sixteen times lighter than water. The limit of the fall of snow near sea-level coincides roughly with the winter isothermal of 52, since in places where the mean winter temperature is no higher than 52 that of the air falls occasionally to 32 or lower during the winter months. As regards Europe, the southern limit is about Gibraltar; in North America it is Savannah, New Orleans, the mouth of the Rio Grande, the head of the Gulf of California, and San Francisco. In Europe, north of lat. 60, snow falls generally on an average of from 80 to 110 days in the year. At Upsala the number of days is 61, at Warsaw 45, Aberdeen 42, Oxford 18, Ostend 15, Brus sels 27, Tarum (in the south-west of Jutland) 12, Copen hagen 23, Vienna 33, Odessa 19, Sebastopol 12, Milan 11, Trieste 6, Saragossa 5, Madrid 3, and Lisbon 1. In Greenland the number of days exceeds 80, and this figure is nearly reached in Newfoundland and the north east seaboard of Nova Scotia. At Quebec the mean days of snow are 66, Halifax 64, Winnipeg 54, Detroit 34, Cape Henry 13, St Louis 11, mouth of the Columbia River 7, and Charleston 2. In Russia the time of the year when snow falls most frequently is December and January, except in the south of the empire, where February is the month of the most frequent occurrence of snow. But to the north of a line drawn from the entrance of the Gulf of Finland through Warsaw, Cracow, Salzburg, and Santiago March is the mouth of maximum occurrence in the great majority of instances ; while to the south of this line it is January and in several cases December. The largest falls of snow occur in the Antarctic regions, as is well attested by the magnificent icebergs of solidified snow which break off all round from the lofty walls of ice that engirdle the Southern Ocean. Excepting perhaps in the Dominion of Canada, no data have been anywhere collected from which even a rough estimate could be formed as to the mean annual amount of snow that falls in different parts of the globe. Snow-Line.- The snow-line marks the height below which all the snow that falls annually melts during summer. No general rule can be stated for this height in different climates owing to the many causes determining it. These are the exposure of mountain slope to the sun (and hence, other things being the same, it is higher on the south than on the north sides of mountains), exposure to the rain- bringing winds, the steepness of the mountains, and the degree of dryness of the air. Hence the position of the snow-line can be known by observation only. It falls only little on either side of the equator to lat. 20 ; from lat. 20 to 70 it falls equably, but from lat. 70 to 78 much more rapidly. To this general rule there are many excep tions. It is 4000 feet higher on the north than the south side of the Himalayas, owing to the larger snowfall on the south, and the greater dryness of the climate of the north side, and therefore the greater evaporation from the snow there. It is higher in the interior of continents than near the coasts, because the precipitation is less and summer heat greater. In the Caucasus it is 11,063 feet high, but only 8950 in the Pyrenees. In South America it rises from the equator to lat. 18, and more on the west than on the east slopes of the Cordilleras, owing to the large precipitation on the east and small precipitation and arid climate of the west side of that chain of mountains. It is as high in lat. 33 S. as in 19 N., but south of that latitude it rapidly sinks owing to the heavy rains brought by the moist N.W. winds of these regions. In the south of Chili it is 3000 feet lower than in the same latitudes in Europe, and 6000 feet lower than in the extremely arid climates of the Rocky Mountains. Storms. If weather charts representing a large part of the northern hemisphere be examined, two distinct systems of pressure are seen which change their forms and positions on the earth s surface from day to day. The one set are systems of low pressure marked off by concentric isobars enclosing pressures successively lower till the centre is approached ; and the other systems of high pressure marked off by concentric isobars enclosing pressures becom ing successively higher towards the centre. The former of these are called cyclones, and the latter anticyclones. These areas of low pressure are the distinguishing characteristics of the hurricanes and typhoons of tropical regions, and of the ordinary storms of higher latitudes, and they may all be conveniently grouped under the general name of cyclones. Fig. 19 shows a storm which was passing across north western Europe on the morning of November 2, 1863, and it may be taken as fairly representing the general features of cyclones. In the figure the arrows fly with the wind, and the force of the wind is indicated by the number of feathers on the arrows. It will be seen that the winds indicate, not a circular movement round the centre of lowest pressure, but a vorticose motion inwards upon that centre, the motion being opposite to that of watch-hands. In other words, the wind follows Buys Ballot s law, already explained. The winds are strongest where the isobars are closest together ; or they are generally proportioned to the &quot; baro-