Evening star. [volume] (Washington, D.C.) 1854-1972, 1894-07-24
They Are Employed by Uncle Sam
..m to Make Forecasts.
THE DAILY PREPARATION OF MAPS
Data Of Atmospheric Conditions
rom All Parts of the Country.
WHY WE ARE HOT OR COLD


the sky watchers who maintain a lookout
from the isolated building on oath and NJ
streets. The roof of that structure is an
excellent point from which to view both
city and sky, but the watching that par
ticularly interests is done not there
but within the walls of the building.
and the field of vision is not a mere
local landscape, but the best part OF
North America. Electricity is the We
hicae of vision. and set of maps the
retina upon which it quickly records its
messages.
Back of this retina the brain cells which
attach significance to the visual impress
sions are Maj. Dunwoody, Prof. Hazen,
Prof. Able and Mr.. Garriott. In other
words. these are the gentlemen who are at
present available for forecast duty in the
weather bureau. and who. therefore, make
promises or threats in meteorological way
concerning the morrow. Each is assigned
to the work for one month at q time Mai.
Dunwoody being responsible for the July
predictions He was preceded by Prof.
Hazen, who refused to console the public
during the installment of the summers
hot weather that came in June.
Four maps are drawn simultaneously as
a basis for the decisions of the forecast
official. One displays about the same data
as that presented upon the published maps
weather. temperature. air pressure, wind,
&c.-another barometric changes, another
temperature changes and a fourth humid
ity changes and clouds. The upper clouds
are indicated by symbols in red, the lower
In blue, and areas quite overcast are in
closed by green lines. Not all this data is
exhibited upon the published charts, and
thus the forecast official enjoys advan-
tages beyond those enjoyed by progno5ti-
Gators outside of the washington office.
The Daily Predictions.
Ea n official follows his own lines in
making predictions One Viii perhaps give
more weight to changes of barometer. and
another TO conditions OF temperature or
humidity. All elements OF the situation
must be considered IN heir mutual bearing.
and yet no absolutely fixed rules have
been deduced. Experience and long ac-
quaintance with the habits OF the atmos-
phere are important factors in the work.
The daily predictions are based upon reports
from 120 telegr Iphlc stations. Observations
are taken for the morning map throughout
the country at a m, Washington time,
the atmospheric conditions from ocean TO
ocean as they exist at the instant being
thus, as it were. photographed upon A desk
in the forecast room. Within half hour
the cipher dispatches begin % reach the
head office. and as one clerk interprets and
maps In figures and symbols under the rings
designating the respective stations.
In the barometer readings the effect of
elevation is eliminated by the reduction of
the readings To sea level equivalents Grad
tally entire regions are represented ana
for these the lines are drawn-this is done
by the forecast official himself. Points hav-
ing the same air pressure are connected to
Indicate the iscbars, and those OF the same
Temperature to Indicate the isotherms BY
?.3o in favorable weather all reports are in,
and not long thereafter the remainder Of
the rough map is finished. Further study
of the conditions and preparation of preiic-
tons for the fDrty-four districts brings the
time to 1923. While a messenger is dis
patched to the representatives OF the press
the map is hurried TO the 18th ographic de-
partnient. and in half an hour the stone is
ready. By lf:2j several hundred copies are
printed by the big steam presses, and in
shape Tor immediate distribution. of these
about 2DO are posted about the departments
hotels and prominent places of business in
the metropolis. number of private stu
dents are supplied with the maps daily A
poard passes upon Applications for maps.
which are addressed to the chief of the bu-
reau Copies may also be purchased for =
cents each.
Fuller Details for This City.
No map Is made In Baltimore the quota
for that city being sent from the Washing
ton Office. About hundred neighboring
towns are also supplied. The weather chart
furnished to the People of the metropolis is
more complete and elaborate than those
produced elsewhere. although the stations
In the large cities display much more data
than is published upon the average sheet.
Only here are all telegraphic reports re-
ccived and utilized, and the conditions are
more graphically represented than in other
cities. Arrows indicate the wind direction
lines of equal pressure are depleted in
black, while those of equal Temperature
are distinguished by red: regions favored
by rain are darby shaded. and recently a
crooked sign has Ieen added to mark the
location of thunder storms .besides the
Usual symbols for rain and clouds
While years OF acquaintance with the
| service has led many lay students of the
/ weather to utilize the graphic publications
30 Prefuseiy distributed about the CITY, the
general public still remain content to ac-
cept or carelessiy reject the Interpretation
OF the forecast official printed in the left
hand corner of the sheet But wherever a
convert to the private study of the map
has been gained the ranks of those who
sometimes sheer at the efforts of the
weather officials has been to that extent
reduced. Acquaintance with a few prin-
cip!es of prediction enables the citizen to
apprehend the grounds for n given Prog
nost:cation, to understand the reasons for
failure in verifcaton, and particularly to
pore we the degree OF certainty with which
any statement concerning the future may
be venturei. Predictions vary widely in
tcsard to probability of veriication. and
IN general tne certainty or uncertainty
| may be recognizcl in advance Mr. Will
.iams. the chief clerk of the forecast office
| computes the percentage OF verifications
and stated to the writer that for weather
(rainfall or its absenca) the per cent l.,
! about 81 while for temperature n few per
| cent lower, that being field in which safe
predictions are made with more difficulty.
High and Lows.
| While some combinations upon the wealth
| er chess board may be called complex
! the alphabet of the game is simple. The
/ atmosphere Is gaseous sea subjected con-
| stantly to wave-like changes of pressure,
/ and these (whirlng) waves always move
| from the western to the eastern side of the
| country, though by irregular and often
very Oblique tracks, and at rates of speed
| varying from two or three hundred miles,
| or less per diem. to fifteen hundred miles
| or more. Technically, a crest is termed a
| "high." and q trough Is styled low"
/ Both are depleted upon the map by concen-
| trio isobars. Winds blow toward the center
| of the low and from the top Of high"
| with a whirlng motion about the former
! contrary to that of the hands of a watch
and with the watch hands about the latter
Other laws follow in consequence. Of
| course temperatures Will rise in the regions
| to the south Of an air whirlpool, because
there the winds are southerIy. To the north
! of the trough a cOrreSDondin- fell will no