The morning news. [volume] (Savannah, Ga.) 1887-1900, 1898-11-11
INTEREST TO WEATHER sHAnPs.
FOreteIIing FrOsts by Means of Wet
and Dry Bulb Thermom eters.

From the New York Tribune.
There are two kinds of fall in tempera-
ture-one due to an advancing cold wave,
which may cause the thermometer to read
10, Is or even 20 degrees lower one MORN
ing than on the day before at the same
hour. This can be foreseen by a man
who has Information about the condition
of the weather 1000 or 1900 miles west of
him. but otherwise can be expected only
as the result of a shrewd guess.
The other drop In temperature Is one
half of the ordinary diurnal oscillation.
The mercury in the thermometer goes up
ward during the hours from or a. m.
to 3 or p. M, and after the latter hour
it falls again. This daily change varies
in extent in the same place from time
to time. On some days the range will be
10 or 20 degrees, and others only or 10.
Now, the same general condition of af-
fairs that makes wide range in the di-
urnal wave possible is Likely to be the
fore-runner of a cold wave of the first
mentioned kind. If, for instance, a cold
wave is coming and the situation in any
locality is favorable to a deeper dip than
the average in the ordinary daily oscil-
iation, the two will combine and make
fall of from 20 or % POSSIBLY even 800 de-
grees from early in the afternoon one day
to the hours just before dawn the next.
Those are the nights to look out for. But
if cold wave from the West is coming
and the situation in a certain region is not
favorable to good diurnal fall of tenn
perature, the result will be different When
the cold wave arrives there will be cloudi-
ness or rain, and a raw, chilly day, but
no frost.
That which makes this difference is the
dampness of the air. Moisture in the at
mosphere serves as a sort Of blanket even
though it may be invisible. When the
temperature falls to a given limit, the
moisture condenses in the form Of clouds
or rain, and this operation partially checks
the drop. But if the air is very dry the
temperature not only can, but probably
will, make a much holder tumble.
Dryness of the air can be ascertained
in two ways. Some hint of it is given
when the clouds disappear and the wind
swings to the northwest A clearing sky,
with the wind stin in the south, or no
wind at all, is seldom attended with a
dry air
But the best way to tell about the hu-
Nudity of the air is by means of the wet
and dry bulb thermometer. When the
wet bulb reads within three or four de-
grees of the dry bulb, the air is damp.
But NNSn lHG interval Is dS great dS
or degrees-that is, in COOL autumn
weather-the air is dry and there is dam
ger of a drop. The limit to which the tenn
perature can fall before condensation be-
gins is called the dew point and 1t may
be found by this way: Suppose the dry
bulb instrument reads 80 degrees and the
wet bulb Al; the difference is 8 degrees.
Multiply this by 215 and you get 719 de-
grees. Call it 8 for convenience. Sub
tract this from the dry bulb reading 600,
and you get Az That's the dew point.
In such a case there is no danger from
frost. But suppose that the dry bulb
stands at 50 degrees and the wet bulb at
A, which Is degrees difference. Multi
ply this by 215 and you have 70, and SUBJ
tract the 20 from the dry bulb reading
go and you set so. Thus one discovers
that. the temperature may drop to 30 de-
grees without any interference from con-
densation. Such a difference does not in
sure a frost, but il shows that a certain
protection that exists at other times has
been withdrawn and that the situation is
risky.
Complete or comparative CAIN is also es-
sential to a severe frost. If there is a
high wind, with no indication of subsi-
dence, the temperature will not fall, as it
viii in a calm. A plant radiates heat in
a calm and often acquires a temperature
u or Is degrees lower than that shown by
a thermometer hung six or eight feet above
the earth But If the air is kept circu-
hting freely no such difference will oc-
cur. In a calm, with a drop in the night
to 50 or IS degrees indicated by nstru-
ments, the temperature in the plant may
go down to % degrees, which means de-
struction to some kinds of vegetation.
On clam night the coolest air viii at
first be on the hills, but later the colder
air will slide gently down the slopes and
accumulate in the hoIlows. Hence a frost
is most likely to occur in the latter locali-
ties The temperature is some times five
or ten degrees lower in valley than on
the adjacent hills, for a period of a few
hours.
Here. then is the combination of circum-
stances most favorable to frost: A clear or
clearing sky. a subsiding northwesteTly
wind that dies away almost 10 a calm, a
temperature of 80 degrees or less al night
fail and a very dry air, as shown by wet
and-dry bulb hygrometer With such a
state of affairs and a difference of from
seven to nine degrees In the readings of
the two thermometers of that instrument
the occurrence of a hard frost Is highly
probable. But if one consults only dry
bulb Thermometer and has no clew to the
amount of moisture in the atmosphere he
is good deal in the dark. And yet, If a
man will watch an ordinary thermometer
closely and note how fast IL falls from 8
10 D. M, and from to 9 D. M, he can
ten from the rate of the drop what the
Temperature will be between midnight and
M