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S r a T L L L X i X c s . “ “ “ ft™ ovaporatioBovor tha wholc smface of the
1375. A cloud is a mass of vapour, more or less opaque, formed and sustained at
considerable height HI the atmosphere, probably by tho joint agoneios of heat and
electucity. The first successful attempt to arrange the divcrsifled forms of clouds under
a few general modifications was made by Luke Howai-d, Esq. We shaU give here a
brief account of his ingenious classification. " o suan give licrc a
1376. Tlw dm p k modifkations ara thus named and defined I. Cirrus a narallel
flexuous, or diverging hbre, extensible in any or in all dii'cctioiis ( / / 24fo a.) ;
3 S to Z L L vviftdelXxtc f t r X X ’ upwards from a horizontal base (A) •
, continuous, horizontal sheet increasing from below (o')
1377. The intermediate modifications wliioh. require to be noticprl ivp a n r
X ’ h X 'X f t ' ’ - n f r i s h L a sse s, in close Z i L n t a r a X g X L w ?
”i.odijkatiom are, 6. Cumulo-stratns, or twain cloud ■ the cirro
stratus blended with the cumulus, and either appeming intcnnixcd with the heaps o fth e
latter, or supcradding a wide-spread stracture to its base ( / ) ; 7. Cnmulo-ciiTO-stratus,
or JNimbus ; the rain-eloud, a cloud or system of clouds from whicli rain is falline' It
IS a horizontal sheet above which the cirrus spreads, while tlie cumulus enters it laterally
L o u n d “ i) ™ ™ apparently on the surface of the
1379. Tlw cirrus appears to have the least density, the greatest elevation, the greatest
variety of extent and direction, and to appear earliest in serene weather, being indicated
by a few threads, pencilled on the sky. Before storms, these tlu-cads appeal- lower and
denser, and usually m the quarter opposite to that from wliicli the storm arises. Steady
high winds arc also preceded and attended by ciiTous streaks, raiming quite across the
sky, according to the dfrection iu which they blow.
1380. The cumulus has the densest stnicture, is fbi-med in the lower atmosphere, and
moves along with the cun-cnt next the earth. A small in-egular spot first appears and
IS, as It were, tlic nucleus on which the cumuli increase. Tlio lower surface continues
iiTcgiilai-ly plane, while the upper rises into conical or hemispherical heaps, wliich may
af terwards continue long nearly of the same bulk, or rapidly rise into mountains. They
will begin, in fan- weather, to foi-m some hours after sunrise, an-ive at thefr maximum in
tlie hottest part of the afternoon, thcn_ begin to diminish, and totally disperse about sun-
sct. I reviously to rain, the cumulus increases ra])idly, appears lower in the atmosphere,
aud with its surface full of loose fleeces or protuberances. The foi-mation of large
cumuli, to leeward, m a strong wind, indicates the approach of a calm, with rain. When
they do not disappear or subside about sunset, but continue to rise, thunder is to be
expected in tlie night.
. stratus has a mean degree of density, and is the lowest of clouds, its
inferior surface commonly resting on the earth or water. This is properly the cloud of
night, appearing about sunset. I t comprehends all those creeping mists which, in calm
weatlier, ascend in spreading sheets (like an inundation of water) from the bottoms of
valleys and the surfaces of lakes and rivers. On the return of the sun, the level surface
of this cloud begins to put on the appearance of cumulus, the whole at the same
time separating fi-om the ground. The continuity is next destroyed, and the cloud
ascends and evaporates, or passes off with the appeai-ance of the nascent cumulus. This
has long been experienced as a prognostic of fair weather.
1382. Transition o f forms. The ciri-us, having continued for some time increasing
or stationai-y,^usually passes either to the cin-o-cuinuJus or the cirro-sti-atus, at the same
time descending to a lower station in the atmosplicre. This modification forms a very
witli the cin-o-cumulus in the same cloud, when the different evolutions foi-m a curious
spectacle. A judgment may be foi-mcd of the weather likely to ensue, by observing
which modification prevails at last. The solar and lunar haloes, as well as the parhelion
and paraselene (mock sun and mock moon), prognostics of foul weather,
ai-e occasioned by tliis cloud. Tlic cumulo-stratus precedes, and the nimbus accompanies,
rain.
1383. Regarding the uses o f clouds, Dr. Prout says, “ they arc one great means by
which water is transported from seas and oceans, to be deposited far inland where water
otherwise would never reach. Clouds also greatly mitigate the extremes of temperature.
By day they shield vegetation fi-om the scorching influence of the solai- heat ; by night,
the cai-tli, wrapt in its mantle of clouds, is enabled to retain that heat which would otherwise
radiate into space ; and is thus protected from tho opposite influence of the
noctni-nal cold. These benefits arising fi-om clouds ai-e most felt in countries without
the tropics, which are^ most liable to the extremes of temperature. Lastly, whether wo
contemplate clouds with respect to thcir foi-m, thcir colour, thcir numerous modifications,
or, more than all, tlicir incessant state of change, they prove a source of never-failing
interest, and may be classed among the most beautiful objects iu nature.” (See Hutchison’s
Meteorological Phenomena, p. 178.)
1384. Dew is the moisture insensibly deposited from the atmosphere on the suiface of
the earth. This moisture is precipitated by the cold of the body on which it appears,
p d will be more or less abundant, not in proportion to the coldness of that body, but
in proportion to the existing state of the air in regard to moisture. It is commonly
supposed that the foi-mation of dew produces cold ; but, like every other precipitation of
water from the atmosphere, it must eventually produce heat. Aristotle justly remarked,
that dew a p p c .p only on calm and cleai- nights. Dr. Wells shows, that very little is
ever deposited in opposite circumstances ; and that little only when the clouds are very
high. I t is never seen on nights both cloudy and w in d y ; and if, in the course of the
mght, the weather, from being serene, should become dark and stormy, dew, which
has been deposited, will disappear. In calm weather, if the sky be partially covered
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