[Frost and fire : natural engines, tool-marks and chips : with sketches taken at home and abroad by a traveller] . IUW a Railwj Cumulus clouds whicli form above London arc higherthan the Alps, and, like Alps, they must be seen from adistance before the eye can take them in. But when theyare seen, under favourable circumstances, tlicir movementstell of upAvard and downward currents in air—of expansion ATMOSPHERIC FORMS. 25 find contraction in particular regions—of ciirrents which areanalogous to those which move up and doAvn, and sideways, inboiling water. The lower edge of a distant cloud is o
[Frost and fire : natural engines, tool-marks and chips : with sketches taken at home and abroad by a traveller] . IUW a Railwj Cumulus clouds whicli form above London arc higherthan the Alps, and, like Alps, they must be seen from adistance before the eye can take them in. But when theyare seen, under favourable circumstances, tlicir movementstell of upAvard and downward currents in air—of expansion ATMOSPHERIC FORMS. 25 find contraction in particular regions—of ciirrents which areanalogous to those which move up and doAvn, and sideways, inboiling water. The lower edge of a distant cloud is often nearly astraight line ; it is, in fact, the outline of the under side ofpart of a dome of vapour, forming at a certain distance abovethe earths convex surface. The upper side is a heap ofgreat rolling mounds which are constantly moving, swelling,and shrmking ; rising and falling. As warm currents of air rise through the vapour, rollingclouds expand upwards, and change from rounded domes toconical piles, and they flow over, and spread out upon thehigher layer of atmosphere through which they have been. Fig. 7. Rising and Falling. thrust, taking the shapes of moimtains. So long as the sunwarms the cloud, or the earth lieneath it, the upward expand-ing motion continues. But when the sun disappears below thehorizon, the action grows less, and the movement is great boilmg mass ceases to boil; and settles down intolayers of even thickness. The Cumulus becomes a Stratus, 20 ATMOSPHERIC FOKMS. or jDerhaps a cold wet current of air joins company witli tliecloud ; drops grow larger and heavier, and the whole fabrictumhles do-wn as a heavy shower. Then the Cuniidus is a Nimbus, and the source of a flowing stream. It is the same whether the growth and decay of suchclouds be watched from below or from above. At sea there are no mountams to mterfere with windswhich blow along the surface of the water ; so clouds, if theychange their form, alter beca^ise of forces within
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Keywords: ., bookpublisheredinburghsn, booksubjectgeo, booksubjectmeteorology
