The herald., 1897-09-17
OBSERVATION-DISCLOSURES
: Dew and FROST IN CALIFORNIA :
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tain section that there was no difficulty
about getting fine drinking water there
because the drip from tin roof a night
would keep large cistern full. Many
others have said that the talk about our
dry air is all nonsense, for the dews are
as heavy as anywhere, and the frost IL
winter cannot be surpassed for the
amount of moisture.
1t is commonly supposed that dew falls
from the sky. and that frost is but frozen
dew The latter part of the sMpposition
is correct enough, but if dew fell from
| the sky, should there not be more on a
cloudy night than on clear one? And
when the fog bank moves in from the
ocean after nightfail and stands till
morning only five hundred feet or 80
above the earth it surely should drop
some thing. Sometimes there is faint
drizzle from its lower edge, and If 1t
rests upon the ground it vets many
! things. But there are few nights so
dry as many of those in which this great
amount of water remains in suspension
above the ground.
But this is not so strange as the fact
that If that bank moves off during the
night and leaves space open to the sky
there may be heavy dew where it would
have remained dry all night had the fog
remained there.
| But if the moisture does not fall from
the sky, why is there no dew on the grass
that grows just inside the protection
from the edge of an open shed while
there is plenty half an inch outside the
line of the roof? And why is there some
on the edge Of the shed, almost perpen
= a,1x- ho1 ft,ac OF of hot
| is so dry?
| Hard questions may sometimes be
best answered by harder ones. In the
cold weather of December three winters
ago the top of the hedge of lime trees
on Alvarado street in Los Angeles near
Hoover was yellowed with freezing
throughout almost the whole length ex-
cept where trees nodded over it. Under
each of these was green spot corre-
spending to the shade Of the tree with
the sun vertically above it. 1t was plain
| in scores of places that the trees had
kept off the frost and had done it even
where the foliage was thin. 1t looked as
| if The frost had fallen from the sky just
as dew appears to do. But in this case
there was no white frost or any damp
ness whatever upon the lime hedge. The
lime will st and white frost. But this
was a dry freeze and the trees kept it
off the same as they would dew or white
frost. And they kept it off merely by
shading and not on the principle of pro-
lection like that of a house. The air
beneath the trees must have been as
cold as it was an inch or two outside of
the line of their shade, yet they acted
precisely as the she'd acts in stopping
dew just inside it's edge. How does
shade stop cold?
| And if dew fails why do the big stones
remain so dry amidse the dripping green
grass. And on a frosty morning 1t is
common to see the railroad ties quite
dry until you come to a culvert or bTigde
and there you find them white with
frost. Here is something still more re-
| markable-two punches Of dead grass
quite dry but on the spider web that
stretches from one to the other sparkle
hundreds of beads of water. And here
is piece of rusty sheet iron lying on a
pile of straw and dripping with dew.
while a piece lying on the ground be-
/ side it is but slightly damp and piece Of
solid iron partly buried in the ground
is perfectly dry. How happens all this
if dew falls from the sky?
Dew does not fall, but is deposited by
condensation on bodies colder than the
air. It is the same invisible moisture in
the air that is deposited on the outside
of pitcher of cold water, and the prin-
ciple is exactly the same. The same proc
ess can be seen when the weather first
becomes cool in the fall. On the north
side of the street the cement sidewalks
begin to become damp at sundown, while
those on the south side that have been
warmed all day by the sun remain dry
all night.
So far it is very plain and then the
question, puzzing to many, arises, how
does one body become so much colder
than another in the same air that 1t will
condense moisture from the air while the
other remains dry? 1t is by radiation of
heat into the clear sky above. All bodies
cooling in the air lose most Of their heat
in millions of fine lines that carry it away
into space. But mere trifies check the
velocity of these arrows of heat. A
redhot iron will not cool as fast in the
air near the ground as If hung in space
a thousand miles above. Nor will it cool
as fast in a room as in the open air,
though the difference may be too fine to
nOtlce without instruments. moisture IN
the air is one of these checks, especially
when visible in the form of clouds or fog.
Almost all air contains moisture
enough to make dew and frost. But the
point at which it will be condensed varies
greatly with the amount of moisture and
temperature of the air. Consequently
a dry air might yield more dew upon a
cold body than moister air would give
to one less cold. So California with a
much dryer air than that of most all
parts of the east by having a greater
radiating power, which would reduce the
temperature of the radiating surfaces
below what they would be in a damper
air, could deposit more dew than coun-
try having more moisture in the air. It
is mainly question of the radiator.
Hence if a good one be exposed to the
clear sky on nights when the upper air is
very dry a rapid fall of it's temperature
is certain and a rapid deposit of dew,
even though the air seems dry as at mid
day.
But this Goes not explain why some
things on the ground are dry in the
morning and others only slightly damp,
while others above the ground are per-
fectly wet. A wagon with leather cush-
ions is one of the best examples of the
| whole thing, and many a time in camp