184 



THE POPULAR EDUCATOR. 



METEOROLOGY. HI. 



ENOW SLEET HAIL WHIRLWINDS WATERSPOUTS LIGHT- 

 NING AND .THUNDER. 



WHENEVER the temperature of the air is so much diminished 

 that the vapour can no longer be suspended in it, it falls in the 

 form of rain. Various causes may lead to this, but the most 

 common one is the ascent of warm moist air into higher and 

 cooler regions. Sometimes, too, a cold wind suddenly coming 

 on will produce a condensation of vapour. On a few occasions 

 rain has been known to fall with a cloudless sky ; this 

 is, however, rare. 



The climate of any place is a good deal affected by the 

 amount of rain that falls there. Instruments for mea- 

 suring this are called pluviometers, or rain-gauges, and 

 many different forms of them have been constructed. 

 The simplest and best consist essentially of a tin fun- 

 nel, the aperture of whose mouth is accurately known. 

 The water is collected by this, and allowed to flow into . 

 a graduated measure, by which its exact amount is 

 ascertained. The object of the funnel is to prevent 

 evaporation. 



Fig. 9 represents a good form of instrument. The 

 area of the funnel is exactly known, and as the gra- 

 duated measure is smaller than this, an inch of rain 

 will fill several inches in it, and thus the readings are 

 rendered more accurate. The rain is often collected in 

 a metal vessel, and then poured into the measure, as the 

 glass is liable to be broken by a frost. 



One remarkable fact in the use of these instruments, is 

 that the height above the ground at which they are placed 

 makes a very great difference in the reading. When on 

 an elevated position, as, for instance, the top of a house, 

 the reading will be found to be much less than when it 

 is on the ground. In one instance, the amount 

 shown by a gauge at an elevation of forty-four feet 

 was 19'85 inches, while a precisely similar gauge on 

 the surface of the ground recorded 25'71 inches. On 

 this account great care is necessary in comparing 

 measures of the rain-fall made by different observers. The best 

 plan is to let the top of tiia funnel be just twelve inches above 

 the ground ; if all are placed thus, results can easily be compared. 

 The reason of the discrepancy is by no means clear. Many ex- 

 planations have been proposed, but none of them appear altogether 

 satisfactory. It is, probably, to be attributed mainly to eddies or 

 currents in the air, whica play round the instrument when it is 

 ut an elevation, and thus blow away some of the finer drops. 



There is a very great difference in the amount of rain-fall at 

 different places on the earth's surface. In two or 

 three districts it is hardly ever known to fall ; these 

 lire known as the rainless districts of the world. In 

 the Old World, they are the Sahara, part of Arabia, 

 and the Desert of Gobi ; and in the New, the Coast 

 01 Peru and a portion of Mexico. On the other 

 hand, there are isolated stations where the rain-fall 

 is very great. In the Khasia Hills, north-west of 

 Calcutta, 600 inches fall in the year, and as much as 

 30 inches has been known to fall on each of five 

 successive days. This is the largest rain-fall known. 



Such heavy showers as these are never known in 

 more northern latitudes ; but in places in Cumber- 

 land and Scotland as much as six or seven inches 

 has been recorded in the day on more than one occa- 

 sion. 



The rain-fall, as a general rule, diminishes in 

 amount as we recede from the tropics to the equator; the 

 number of rainy days, however, increases. The reason of this 

 apparently strange fact is that in the tropics the showers are 

 usually much more heavy and continuous, but are almost con- 

 6ned to the wet season. During the dry season, months often 

 pass during which scarcely a single drop of rain falls. In the 

 northern hemisphere, from 12 to 43 latitude, the average num- 

 ber o{ rainy days is 78 ; from 43 to 46 it is 103 ; from 46 to 

 i>0 it is 134 ; and thence to 60, 161. In this statement a fall 

 ot i5o f an i* 10 * 1 is taken to constitute a rainy day, this standard 

 being now very generally adopted. 



At London the mean rain-fall is about 23 inches ; the average 

 for Great Britain is a little above this ; while in some of the 



10. 



hilly districts it ranges as high as from 80 to 150 inches. The 

 proximity of hills, however, always greatly increases the amount 

 of rain, by condensing the vapour of the air in the manner 

 already explained. Other local causes likewise exert a powerful 

 influence. 



When the temperature of the air is down to the freezing point, 

 the minute vesicles of vapour, as they condense, are frozen into 

 particles of ice, which unite together, and fall to the earth in 

 the form of snow. When collected on a cool black surface, 

 and viewed through a microscope or a powerful lens, the 

 snow-flakes are found to consist of a great number of 

 beautiful crystals. The shapes which they assume are 

 very varied indeed, but nearly all have somewhat the 

 form of six-rayed stars. Fig. 2 in "Mineralogy," page 

 49, represents a few of the more common forms, but 

 more than a thousand varieties have been observed,, 

 many of which have been sketched. The exquisite 

 symmetry and beauty of some of them cannot be de- 

 scribed ; they must be observed, and will repay the 

 trouble taken in the examination. 



The flakes fall but slowly, owing to the broad sur- 

 face they present as compared with their weight. 

 When the temperature is a little above 32 they often 

 unite, and form larger ones, which sometimes attain 

 the diameter of an inch. During any one fall of snow 

 there will be found to be a strong general similarity 

 in the forms of the crystals ; they vary, however, 

 greatly in different storms, though the hexagonal 

 structure is found to prevail almost universally. 



Snow but rarely falls in Europe at a lower latitude 

 than 30 N. ; over the sea this limit is considerably 

 higher, and on the west coast of America it attains the 

 latitude of 45 N. The line indicating this limit very 

 nearly coincides with the winter isothermal of 52. 



In the polar regions the surface of the ground is. 

 constantly covered with snow, and in all latitudes 

 the snow lies permanently on the tops of the highest 

 mountains. The line marking the limit above which 

 the snow lies on the ground all the year round is 

 called the snow-line. The elevation of this under the equator 

 is about 16,000 feet above the sea-level; from thia it declines, 

 till, about latitude 78, it is level with the ground. Its eleva- 

 tion does not, however, decrease very regularly, being consider- 

 ably affected by local causes, the principal of which are the 

 situation of the mountain with respect to the rain-bringing 

 winds, the slope of its sides, and the general humidity of 

 the locality. On that side of a range from which the moist 

 winds usually blow the amount of snow which falls will be much 

 greater, and therefore it will lie at a lower level 

 during the summer, as there is not time for it to 

 melt. 



The most remarkable irregularity of this kind 

 occurs in the Himalayas, where the snow-line is 

 nearly 4,000 feet higher on the north than it is on 

 the south side. This is principally to be accounted for 

 by the greater dryness of Thibet, which affects it in 

 several ways. It diminishes greatly the amount of 

 enow which falls on that side, as compared with the 

 southern slope; it causes that which does fall to 

 evaporate much more rapidly, since the air is not 

 nearly saturated with moisture ; and, further, the 

 dryness of the air allows more of the solar heat to 

 pass and warm the earth, since air when charged 

 with vapour acts as a screen, and keeps off the heat. 

 The white colour of snow arises from the combi- 

 nation of the various rays which are given off by the crystals 

 of which it is composed ; when viewed separately, they reflect 

 all the various primatic colours, but seen collectively, these 

 blend and form the pure white. 



Red snow and green snow are occasionally met with in Arctic 

 regions ; it is also met with during the winter in parts of the 

 Alps. These appearances are caused by the presence of very 

 minute plants which flourish in the snow, and impart to it their 

 own tinge. 



As the snow falls very loosely, it entangles a large amount of 

 air between its particles, and thus becomes a very bad con- 

 ductor of heat. It serves in this way a very important purpose 

 in the economy of Nature, by protecting the surface of the 



9. 



