[Frost and fire : natural engines, tool-marks and chips : with sketches taken at home and abroad by a traveller] . a Railway Tkmn. Cunudus clouds which form above London are liigherthan 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, their movementstell of upward and downward currents in air—of expansion i ATMOSPHERIC FORMS. 25 and contraction in particular regions—of currents which areanalogous to those which move up and down, 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] . a Railway Tkmn. Cunudus clouds which form above London are liigherthan 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, their movementstell of upward and downward currents in air—of expansion i ATMOSPHERIC FORMS. 25 and contraction in particular regions—of currents which areanalogous to those which move up and down, 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 moimds which are constantly moving, swelling,and shrinking ; rising and fallmg. As warm currents of air rise through the vapour, rollingclouds expand upwards, and change from roimded domes toconical piles, and they flow over, and spread out upon thehigher layer of atmosphere through which they have been. AND Falling. thrust, takmg the shapes of mountains. So long as the sunwarms the cloud, or the earth beneath it, the upward expand-ing motion continues. But when the sim disappears below thehorizon, the action grows less, and the movement is great boiling mass ceases to boil; and settles down intolayers of even thickness. The Cumulus becomes a Stratus, 26 ATMOSPHERIC FOiniS. or perhaps a cold wet current of air joins company with thecloud; drops grow larger and heavier, and the whole fabrictumbles down as a heavy shower. Then the Cumidus is a Mmbus, 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 mountains to mterfere with windswhich blow along the surface of the water ; so clouds, if theychange their form, alter because of forces within them. Thus, off the O
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Keywords: ., bookpublisheredinburghsn, booksubjectgeo, booksubjectmeteorology
