The Salt Lake herald. [volume] (Salt Lake City [Utah]) 1870-1909, 1893-07-23

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sufficient to cOndense cart OF the vapor
in the air and make it cloudy; but it not
iufrequently happens that clouds are
| formed in this manner. especially in val
| legs, where the air lies rather quiet, and
where it is well cooled by loss Of heat to
the enclosing valley slopes. In noun
famous regions it is common
to find the valleys and low
lands covered over with dense layer of
| Jog, that is of cloud lying on the ground.
In damp summer weather the same result
is common in the open country. recall
many summer morning that has
opened apparently cloudy, only to clear
off when the warm sunshine above dis
solved away the fog.
| But this does not explain the Nam
| tucket clouds.
| When the Nantucket clouds are
watched, they are seen to DRIFT eastward,
being borne along by the prevailing west
erly wind of the summer season. On the
western border of the cloud patch the
clouds are small, but if they are carefully
examined they are seen to grow upward
to greater and greater size as they move
along, and then gradually to dwindle
away as they drift beyond the influence of
the sandy island. One cloud forms after
another, and what is at first taken to be
stationary mass is really slow process
sion of growing and fading clouds.
l measured them on one occasion,
and knowing the distance of Nan
| tucket from the mainland, found that
the even base of the clouds stood about
$500 feet above the sea, while their
| highest SUBMITS rose to height of i, 600
or 5000 feet, thus giving q vertical thick
ness of /, 800 or 1000 feet for the heav-
lest part of the cloud.
| Now what process OF cooling and con-
densation can be associated with clear
sunny weather in summer time and a
warm sandy island where the temperature
is several degrees higher than it is over
the surrounding waters7
| AT first no process of cooling is appar-
emit. There is, however another process
that may be inferred easily enough; this
is the formation of slow ascending or
| correctional current IS the gentle west
wind blows over the warm surface of the
| iSland and becomes warmer than the sur
rounding air. Perhaps the Nantucket
clouds may be dependent in some way on
each ascending current. The increase in
their size as they pass over the island aud
their disappearance beyond confirms this
idea. The we'll known occurrence of in
flowing sea breezes in the day time
around the margin Of warm islands gives
support to this suggestion, for there must
be an ascent about the central area OF
is probably somewhat oblique, leaning
forward in the direction OF the general
| movement or prevailing drift OF the at
mosphere at the time.
But why should such an uprising of the
air cause the formation of clouds2
1t is sometimes said that ascending cur
rents are cooled by the cold of the upper
atmosphere into which they rise; but
while the upper atmosphere is colder than
the lower. and the ascending current
| therefore more or less cooled at its con-
tact with the colder upper air, it is not
possible to explain the growth of massive
clouds in this manner. Clouds formed by
the cooling OF The ascending current at its
contact with the upper air would be
balloon like shells OF clouds with clear
air inside; bearing no likeness to the
compact masses that one sees in the sky.
The chief process of cooling involved in
this cloud making is curious one, but
Of great Importance in meteorology from
| its frequent occurrence. When the lower
air rises it expands, because the pressure
upon it is less and less the higher it goes.
In expanding it has to push away the
surrounding air, to gain the desired in
crease of room for itself; and this is a
considerable piece of work. The higher
it ascends, whether climbing vertically
or obliquely, the greater its volume be-
comes, by expansion, and the greater the
work it must have done in pushing away
the surrounding air. Now the curious
thing in this process is that the energy
needed for the work of expansion is sup
piled from the heat of the air, and as that
energy is exerted the heat is used and the
air cools.
1t has been we'll shown by physical ex-
pertinent that heat is not thing in itself,
as was supposed in the early part of this
century, but q condition of things; and
the best theory regards heat as the energy
of the molecular movement. Now the
lower air is heated as it advances from the
cool sea upon the sandy surface of Nan
tucket, warm in the sunshine; that's, the
molecules of the air are set 10 flying q bout
at O greater velocity.
Readers of this brief statement who have
not already encountered such an explana-
zion must beware Of gaining too gross
| an idea of the process here referred to-
The molecules of gases must be incon-
ceivable minute; they must swarm in un-
counted millions in the space of foot; it
is their incessant bombardment as they
fly about that causes the expansive pres
sure exerteG by gases.
While the air is near sea level the up-
ward bombardment of its molecules Just
suffice to support the downward bonn
| bardment OF the everything air; that is.
to counteract the downward pressure OF
the atmosphere. As soon as the ascent
begins the pressure on the ascending
mass decreases. and the balance of the
forces that existed at sea level is lost.
/ Expansion OF the ascending mass there
fore takes place, and the surrounding
air is pushed away by the energetic bonn
bardment of the ascending air. But in
very consequence of doing this work the
energy OF the molecular bonn bardment is
diminished. When bat strikes a ball
the movement of the bat almost ceases
while the velocity of the ball is greatly
accelerated. So when the expanding all
pushes away the surrounding air, the
velocity OF molecular movement in the
expanding mass must be lessened; that is
it must be cooled.
Changes OF temperature of this KIND
occur whenever mass of air changes its
altitude. and it is manifest that, if the
process is carried far enough, the tempe
ature of the air will fall so low that some
of the yapors of the rising current must
be condensed and become visible in
clouds. Here, then, we find an explana-
ton that fits the case of the summer day
clouds over Nantucket in most satisfac.
tory manner.
The even base of the Nantucket clouds
indicates the height at which con.
densation begins; from the measurement
quoted above, it appears that an ascent Of
about 1900 feet was needed to cool the
air down to the point of condensation
But the ascending current did not stop al
that height; 1t continued to climb higher
forming more cloud as 1t rose, until il
drifted away from the island. Then the
ascent weakened; the clouds ceased tc
grow higher, and az the ascending mo-
tied down again and dissolved or eyapo-
rated "a thew SLOWLY sank into thf