METEOROLOGY. 



masses, while it is horizontal below. The appear- 

 ance, increase, and evanishing of cumulus, in fine 

 weather, are often periodical, and correspondent to 

 the degree of heat. Generally, it forms a few hours 

 after sunrise, attains its highest degree in the 

 hottest hours of the afternoon, and decreases and 

 vanishes at sunset. If the upper region, with its 

 drying power, predominates, the upper parts of the 

 cumulus become cirrus ; but if the lower region 

 predominates, the basis of the cumulus sinks, and 

 the cloud becomes stratus, which appears as a 



Stratus. 



long horizontal band of moderate density, with its 

 lower surface resting upon the earth or the water. 

 Combinations of the above forms have been dis- 

 criminated under the names of cirro-cumulus, 

 cirro-stratus, and cumulo-stratus. The black rain- 

 cloud, which seems a general mixture and con- 

 fusion of all the clouds in the heavens, has been 

 called the cumulo-cirro-stratus or nimbus. 



The cirrus clouds are the most elevated ; and 

 must be often made up of snow-particles, for even 

 under the equator, water would be frozen at such 

 a height. In general, we may say of the light 

 fleecy masses that we see on a summer-day, that 

 they are made up of snow-powder or frozen par- 

 ticles. The cirro-cumulus is formed from the 

 cirrus by the fibres breaking and collapsing into 

 small roundish masses. It is this which is known 

 as a mackerel sky, and is attendant on dry and 

 warm weather. The cirro-stratus clouds consist 

 of horizontal masses, dense in the middle and 

 thinned towards the edges, so that a group of 

 them resemble a shoal of fishes. Their prevalence 

 indicates a coming storm. 



The cumulus clouds are evidently the precipita- 

 tion of the ascending vapour in the cold upper 

 regions ; for they generally increase with the heat 

 of the day, which disperses all superficial mists. 

 The stratus, on the other hand, has more the 

 character of a night-cloud : it is a result of the 

 cooling of the air in the evening, and comes out 

 in the lower regions of the atmosphere. All mists 

 and fogs are of this species of cloud ; which, in its 

 lightest state, does not wet leaves or any objects 

 with which it comes in contact. Sometimes it 

 remains quiet, and accumulates in layers, till the 

 atmosphere is incapable of sustaining its weight, 

 when it assumes the condition of the heavy and 

 dark nimbus, and falls in a shower of rain. 



The extreme height to which clouds may reach 

 has never been accurately determined ; but from 

 observations made in balloon ascents, it is probable 

 that the cirrus cloud is often ten miles above the 

 arth. 



The average amount of the sky covered by 

 clouds, is an important element in the climate of 

 a country. It is usually measured by the scale of 

 o to 10 ; 5 indicating that the sky is half-covered, 

 10 that it is wholly obscured. In Shetland, three- 

 fourths of the sky is the average space covered 

 with cloud ; the mean for the whole of Scotland 



is less than two-thirds, being 6-7 in the west, and 

 6 in the east and interior. 



Dew. When moisture is precipitated at night 

 in the form of wetness, and drops on the surface 

 of the ground and on the leaves of plants, it 

 receives the name of dew. This precipitation 

 arises when the surface of any body is cooled 

 below the dew-point temperature of the air at 

 the time. Thus, if the dew-point were 45, and 

 if by any means a glass tumbler were cooled 

 down to 40, the film of air lying next to it would 

 also be cooled down to 40, and would therefore 

 have to give out all the vapour which it could not 

 hold at that temperature; but the precipitated 

 surplus in this case would not appear as mist in 

 the air, but would adhere to the surface of the 

 glass. When cold glasses are brought into a 

 warm room, they sometimes become dewed all 

 over in this way. The bringing out of visible dew 

 is, as we have seen, the means of determining the 

 dew-point temperature in Daniell's hygrometer. 



Night-dews are most copious when the sky is 

 clear. The reason of this is, that the earth cools 

 faster under a clear sky than when hung over with 

 dense clouds, which prevent the radiation of the 

 heat ; just as bed-curtains make a bed warmer. 

 Wind also prevents the deposition of dew ; because 

 the air in contact with the earth is constantly 

 changed, so that the temperature does not fall 

 sufficiently low. The surfaces which naturally 

 radiate off their heat with most rapidity are the 

 first to sink below the dew-point temperature and 

 to become dewed. Thus, rough surfaces are wetted 

 sooner than smooth, plants sooner than glass, and 

 glass sooner than metals. Metals being good 

 conductors of heat, as fast as their surface cools, 

 heat flows to it from the interior, and consequently 

 the temperature of the surface cannot sink till the 

 whole mass throughout has parted with its heat 

 Woolly and fibrous substances cool very fast at the 

 surface, and are therefore rapidly bedewed. No 

 dew can fall on a surface till its temperature has 

 fallen below the dew-point ; hence, in the case of 

 a very dry atmosphere, there may be no dew formed 

 at the coldest time of the night. In arid deserts, 

 and in the countries where dry winds prevail, dew 

 is not often seen. 



When the surface dewed is below the freezing 

 temperature, the vapour is not only precipitated, 

 but is also frozen ; hence the origin of hoar-frost, 

 or frozen night-dew. The occurrence of hoar-frost 

 is a proof that the temperature of the ground has 

 fallen below 32, as well as below the dew-point 

 temperature. As in the case of dew, everything 

 that prevents the radiation of heat arrests the 

 formation of hoar-frost. During the chilly nights 

 of spring, plants that are sheltered by trees are 

 less liable to be frozen than those which are fully 

 exposed ; and a slight covering of straw, or even 

 of paper or netting, will often afford an effectual 

 protection. Vineyards, it is said, have frequently 

 been saved from the effects of frost by enveloping 

 them during the night in a cloud of smoke. 



Rain is the aggregation of the cloudy particles 

 into masses or drops. The causes of the formation 

 of clouds have been already explained. Every 

 cloud does not end in rain ; it is only when the 

 whole mass of air between the cloud and the earth 

 is saturated, that the watery particles fall down. 



SHOW. When the temperature of the stratum of 

 air from which the rain falls is under 32, the 



