Evening star. [volume] (Washington, D.C.) 1854-1972, 1893-11-06
FORECASTS
How Predictions Are Made in the
Weather Bureau.
HlGHSAND Lows IN TIE ATMOSPHERE
Storm Centers And How the Direc-
ton is Determined.
A TY PIC A L S T o R M .

Do you read the weather predictions in
The Star every evening? Is II a matter of
any interest to you to know how those pre-
dictions are made and upon what basis they
stand? If it is, just pay attention for a few
moments and the matter will be explained
to you. explained not from an abstruse,
scientific standpoint, but in a way that
layman can understand.
The map of the weather prevailing in the
United States and Canada at the time speci-
fled is constructed from observations taken
simultaneously over the country, the re-
suits being telegraphed to Washington and
other cities in an abbreviating cipher code
The times Of the observations are am.
and 8 pm, of the seventy-fifth meridian
west of Greenwich, which passes just east
of Philadelphia, and is minutes II seconds
east of Washington. The observations con-
sist Of readings of the barometer which
measures the pressure of the atmosphere
the thermometer which gives its tempera-
ture, the wet and dry bulb thermometers
from which is deduced the dew point in
records of the direction and velocity of the
wind, the clearness or cloudiness of the sky,
and the amount of rain or snow fallen. The
weather map exhibits graphically a large
part Of these data from about 155 stations.
The High and Low.
To show the pressure of the air all over
the country, the readings of the barometer
are reduced to sea level, and then solid
lines, called isobars, are drawn through
places having the same pressure. The re-
suit is a series of contours one-tenth Of an
inch pressure apart, which gives the highs
and lows in the atmosphere
The thigh" is a clear. cool region: the
low" is apt to be cloudy or rainy. and
warmer.
Where the isobars are close together the
pressure changes rapidly, and where they
are far apart the change is gradual. SINH
larly places having the same temperatures
are connected by dotted lines, called iso-
therms. There are sometimes added heavy
dotted lines inclosing space where there
has been a great change in temperature
within the last twenty-four hours. Shaded
areas show where rain or snow has been
falling since the last observation, that is,
within twelve hours. The direction of the
wind is indicated by arrows flying with it.
or opposite to the ordinary vane; these also
have circles which are SO shaded as to give
the state of the sky. clear. partly cloudy.
CLOUDY, rain, snow; and they have square
ends wherever cautionary storm signals are
displayed. more frequently on the Atlantic
coast and near the lakes.
Forecasting the Weather.
The best way to learn to use the map for
forecasting weather is to take three or four
maps of consecutive dates and observe the
changes in the lines. 1t will be seen that
highs and lows move eastward, at the aver
age rate of 600 miles daily, and that the
winds blow around a high and low in
definite way. as it were in the form Of the
two adjacent figures. Thus they go out of
and around the high in the direction of the
hands Of watch, called the anticyclone,
and then around and into the low in the
opposite direction. called the cyclone.
The winds on the Sc. and L. of a low are
warm and moist from the S.; those between
the high and low are cool from the NV,
stronger. and bring clearing weather: those
on the SW. and w. of the high blow from
the Sc, or are variable.
The forecast consists in picturing to the
mind the storm centers as moved to their
probable position for the next day, and
then describing such weather conditions
as would there exist. The tracks of GEN.
tiers Of low areas are given in heavy arrow
lines. The movement of storms is often
erratic. depending upon many hidden causes
and thence arises some uncertainty in max
ing the forecasts. The average success in
predicting is about IS per cent. Special
reports Of heavy rainfall of the depth of
water In the rivers and of the movement of
the river stage, whether rising or falling
synopsis of the variations of the conditions
since the last report at all the stations, and
statement of the weather to be expected,
Is added on the margin of the map.
The word storm, in the language of the
weather bureau. refers to a widespread
condition of low atmospheric pressure.
which, when well defined, is accompanied
by general rainfall, cloudiness, often very
high winds, and marked changes of tenn
rerature. This condition is likewise called
a cyclone, from the fact that the circulation
of winds within its influence is rotary in
character, blowing toward and around the
center in a very definite manner. The word
cyclone is incorrectly used when applied
to those violent tornadoes, which, though
extremely destructive, are very narrow and
last but a few hours, while cyclone is
often 15O0 miles in width and preserves its
identity for many days.
A Sample Storm.
Now, having learned these rudimentary
principles, you are ready to follow the pro-
gress of typical storm which appeared in
the western portion of the United States
and Travelled across the country. affecting
almost every section. This storm occurred
in April, 1882, and Is used by the weather
bureau as an example in Illustrating the
matter at issue. The weather bureaus de-
scription is as follows:
Saturday, April 2, 8 pm.
Attention is first directed to the distribu-
ton Of the atmospheric pressure. A low
is just passing northeastward above Lake
in the far northwest, with a long trough
like extension over the Rocky mountain
states and western plains. The pressure
is as vet only slightly depressed, but large
areas of marked increases of Temperature
have occurred, and the law of cyclonic cir
culaticn of winds is drawing into this re-
gion warm and moist air from the gulf
The moist winds from the Pacific have al
ready shed much of their moisture over an
extended region along the coast. The area
of highest pressure is central OFF the South
Atlantic coast.
sunday, April 3, 8 am.
The isobars on this map show that the
region of lowest pressure is now central in
eastern Colorado and that the whole condi-
ton of low pressure is widening in an east
and west direction and the isobars becoming
more circular in character. This means
that the distribution of the. pressure is be-
coming more symmetrical, the pressure be-
ing lowest in a small region in the center
and gradually higher and higher at greater
and greater distances away from this CEN
ter. The arrows again show a continued
and general flow of atmosphere northwest
ward from the gulf. The temperature has
risen from 10 degrees to 20 degrees over an
extended region southeastward of the storm
center. The condition of low barometric
pressure and the flow of warm and moist
air into ii are favorable for general precipi-
tation, and scattered showers have already
occurred throughout different portions of