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Jacques W. (Jacques Wardlaw) Redway.

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inches in circumference fell. In July, 1880, a hail-storm destroyed
the crops in the vicinity of Waupaca, Wisconsin. The shower
covered an area of forty square miles. Stones from six to ten
inches in circumference fell. In July, 1881, the fall of hail at
Cumberland, Maine, was so great that drifts two feet deep were
observed twelve hours afterward. In June, 1882, at Dubuque,
Iowa, stones Aveighing twenty-eight ounces were found. In
August, 1883, at Gray, Iowa, the drifting hail covered the fence
tops. In June, 1886, so much hail fell in Grrand Forks County,
Dakota, that it did not all melt for thirty hours. In a single
storm that passed over a small area in Dakota, a quarter of a
million acres of wheat were destroyed.



CHAPTEE XIV.



THE MOISTURE OF



THE ATMOSPHERE.
STORMS



CYCLONIC



Both on the land and at sea there are regions of con-
siderable area that normally are not swept by regular and
constant winds. On the sea these are the calm belts ; on
the land they are regions from which the winds are shut
off by mountain-ranges or disturbed by broad stretches
of land. On the sea the shifting of the calm belts with the
season brings various parts successively under the influ-
ence of the regular winds.
On land the regular Avinds
usually exist as upper cur-
rents, while at the surface
the wdnds are local and vari-
able ; the upper currents,
moreover, are so high that
they are too cold to contain
much moisture.

Such regions do not re-
ceive seasonal rains. The
land areas, in some instances, receive none at all, except from
an occasional cloud-burst ; but in many cases a consider-
able rainfall results from the movements of local winds.
That part of the United States east of the Rocky Moun-
tains is an excellent illustration. It receives no moisture
directly from the constant winds ; yet about every part of
it east of the 2,000-foot contour is so generously supplied

248




STRATUS CLOUDS DISTURBED BY
AN UP-DRAUGHT



CYCLONIC STORMS



249



with raiu that it is one of the most productive regions
of the workl.

Whenever a local wind occm's, one of two conditions is
pretty apt to exist. Either there is an up-draught toward
Avhich the wind is blowing, or else there is a great accumn-
latiou of air from which the air is spreading outward.
These local disturbances
constitute the condi-
tions jDopularly known
as storms. Moreover, in
either case the move-
ment of the air sooner
or later develops into a
whirl. The wind that
blows toAvard an up-
draught or a depression
forms a cyclone; that
which blows outward
from a high bank of air,
an antic ijclone. These
disturbances originate
both on the land and at
sea. They are usually
indicated by a changing
barometer ; hence a cy-
clone is often described as an area of low barometer — or
simply a "Low" — and the anticyclone, one of high ba-
rometer. As a rule both the cyclone and the anticyclone
are local disturbances, and therefore they are carried along
by the great currents of the air, just as an eddy formed in
a river is carried along in its flood.

Cyclonic movements thoicfon; travel westward! y in low
latitudes and eastwardly in latitudes beyond the trojiics,
because these are the prevailing directions of the winds.



50


N. HEMlSPHCRE


50


-iC^^


40


rWyiJ


40


30

20
10


Crr^^'^


30
20
)0


^^



10


EQUATORIAL CALMS



10


W^$^"


20


j^P^'^


20


30


^^5r\. - ^


30


40


^V^~^


40


50


^-"v^^


50


S. HEMISPHERE



NORMAL CYCLONE TRACKS



250



PTIYSTCAL GEOGRAPHY



Because of this fact, when a cyclone has once formed, the
track alono- which it is likel}- to move can be predicted
with considerable accuracy.^ The direction of the whirl
has been learned by experience : in the Northern Hemi-
sphere it is opposite that of the clock's hands ; in the
Southern Hemisphere, the reverse.^' A knowledge of
these facts enables the mariner to avoid a cyclone, and
also to steer out of it when overtaken by one.

Tropical Cyclones.— Tropical cyclones usually origi-
nate within a few degrees of tlie equator. They are the
hurricanes of the West Indies and the h/pJioo?is of the
China Sea. The storm area extends over a surface vary-
ing from a few hundred to
more than a thousand miles
in diameter. The preced-
ing illustration, p. 249, shows
roughly the track which, or-
dinarily, one of them fol-
lows. What is its direction
in tropical latitudes ? in lati-
tudes beyond the tropics?
Note the direction of the
whirl in each hemisphere.
It rarely extends beyond the 60th parallel.

The real beginning of the tropical cyclone is the dead
calm that for a few days precedes the disturbance, for
it is only when the air is in a state of rest that the
necessary conditions can obtain.^ The first essential con-
dition is the overheating of the air next the sea — pre-
cisely the same condition that formed the beginning of
the desert whirl (p. 224). But while the stratum of air
tliat causes the desert whirl is only a few hundred feet
in heiglit and involves a very small area, the atmo-
sphere disturbed by the tropical cyclone is, perhaps,




"STREAMERS" OF CIRRUS CLOUDS
-THE FORECAST OF A CYCLONE



CYCLOiS^IC STOEMS 251

several thousand feet high and many thousand miles in
extent.

The longer the sun beats down on the glassy surface of
the water the greater will be the energy of the storm when
it begins. Moreover, there is one element present in tlie
tropical cyclone tliat is not found in the case of the desert
whirl — namely, the vapor of luater — and this is the most
important distinction between the two. Finally the equili-
brium becomes so imstable that a slight up-draught of air
occurs where the resistance is least. The moment this
occurs, the rising air already near the dew-point is chilled
by its own expansion, and a part of its moisture is precip-
itated. The fall of rain sets free an enormous amount of
latent heat, and a furious up-draught at once takes place.

It is the latent heat of the moisture set free that gives
to the cyclone its great energy. This indeed is its fuel,
and so long as the supply lasts, just so long will the cy-
clone continue. The ascending air at first is very moist
and tolerably Avarm. But after its moisture has been con-
densed the latent heat set free renders it dry and very
much warmer, thereby increasing the up-draught.

The nearer the centre of the cyclone, the stronger is the
wind. The "eye" of the storm, or the centre of the
whirl, is the up-draught of the cyclone, and here brief in-
tervals of sunshine alternate with torrents of rain. In the
centre of the storm the barometer stands lowest— perhaps
two inches lower than it is beyond the edge of the storm.

The path of the cyclone seems at first to be one of un-
usual shape, but when examined in relation to the in-evail-
ing winds the mystery disappears. It is not unlikely that
the temperature of the upper air has much to do with the
northerly tendency of the cyclone;. Because cold air is rel-
atively heavier than light air, the colder the upper air
that surrounds the up-draught, the more vigorous will the



353 PHYSICAL GEOGRAPHY

latter be. In the Nortliern Hemisphere the colder air lies
to the northward of the storm, and this will be the direc-
tion of least resistance.

Knowing the direction of the whirl and the path of the
storm, it is not difficult to lay the course of tho vessel out
of the way of the cyclone. For this purpose " storm-
cards," or diagrams similar to that on p. 2G3, are conven-
ient. The distance of the storm-centre can be estimated
only to a rough degree, but the bearings can be obtained
with a high degree of probability. Facing the wind the
storm-centre is on the right hand.''

Winter Cyclones. — Some of the fiercest storms of the
higher latitudes, however, do not originate anywhere within
tropical regions. These are the extra-tropical or winter
cvclones, and the fierce winter storms of the North Atlan-
tic Ocean are examples. It is evident that these storms
cannot originate in a dead calm, because there is no long-
continued calm weather where they form ; and it is equally
apparent that they are not formed by the overheating of
the air next the surface of the water.

It is thought that these storms result from the intrusion
of cold, noi'th winds into the region of warm and moist
air, to the southward. In any case the condensation of
moisture creates an up-draught that quickly develops into
a whirl. But if, at the time of intrusion, the cold air takes
the upper position, the equilibrium becomes much more
unstable, and the storm very likely develops into one of
great fury.^

Land Storms. — The occasional local squalls excepted,
all the storms of the land are cyclonic in nature, and
except in violence they do not differ materially from the
cyclones of the sea. In nearly every case they follow the
same courses that are taken by the latter — westerly in
tropical and easterly in temperate latitudes.



CYCLOmC STORMS



253



Since the establishment of the various weather bureaus,
the storm-tracks have been closel}- studied, and it is found
that most storms follow certain lines or belts.

In the United States two storm-tracks are apparent.^
The lesser number follow the trend of the Atlantic coast.
The storms usually overlap the shore and the coast plain,
but they seldom extend west of the Appalachian highlands.
These storms
belong to the
class of West
Indian cyclones.
They originate
in the Caribbean
Sea, and turning
nort hward,
finally reach the
latitude of the
Middle Atlantic
coast.

Most of the
storms that pre-
vail in the Unit-
ed States form
near the great
highlands of the
west — very fre-
quently near the

eastern base of the Rocky Mountains, crossing the con-
tinent in a northeasterly direction. These storm -tracks
have a distinct tendency to shift north or south with the
apparent motion of the sun, the belt being a little farther
north in summer than in winter. The valley of the St.
Lawrence liiver and the basin of the Great Lakes is a
common track.




A STORM, OR AREA OF I.OW BAROMETER

The shaded part is the area of rain ; the dotted region the area
of cloudiness. The arrows fly with the wind.



254



PHYSICAL GEOGRAPHY



Altlioiigh tliey are sometimes accompanied by local
squalls, laud storms rarely exhibit the fmy of ocean
cyclones. The area of the storm is usually larger, but
the wind seldom attains a Telocity greater than forty
miles an hour. The storm-centre is distinct, but the
barometer may not fall more than half an inch.

Clouds, and rain or snow, accompany the majority of
storms, but the area of rain does not always cover the
whole extent of the storm ; as a rule, most of the cloud
area, and the rain as well, occur in front of the storm-
centre. With the passage of the latter there are occasional
hard showers in which the rain falls almost vertically, or

perhaps drives slightly tow-
ard the east. These are the
" clearing showers."

Because the wind blows
toward the storm-centre, it
is evident that storms of the
second class will be pre-
ceded by easterly and will
clear with westerly winds.
Those from the West Indies
will begin with northeasterly and clear with southwesterly
winds — the "nor'easters " and " sou' westers."

In some instances general storms are accompanied by
disturbances of a very violent character. Of these the
most important are thunder-showers, cold waves, and
tornadoes and waterspouts. Thunder-storms and torna-
does are local in character, and often occur independently
of general storms. Waterspouts and tornadoes are local,
the former being confined to the water. Cold waves are
general.

Cold Waves.— Just as the trough of a wave of the sea is
followed by the crest of another wave, so in the aerial




CLEARING WEATHER CLOUDS



CYCLONIC STORMS 255

ocean an area of Ioav barometer is followed bj one of liiffb
barometer, and if tbe latter be an anticyclone of cold air tbe
result is a cold ivave.

Not infrequently it happens that the barometer is con-
siderably higher on one side of a storm-track than on the
other. In such a case, it is evident that most of the air
flowing in to fill the depression will come from that side
on which the barometer is the higher. If the air is drawn
in from the south side, it is pretty apt to be a mass of
warm, moist aii', and the farther north the storm track, the
higher in latitude Avill the body of warm air intrude.' On
the contrary, if the bank of cold air lies to the northward,
the depression will fill chiefly with cold air from this
direction.

In summer neither the cool air nor the warm air, follow-
ing the passage of a storm, varies much more than eight or
ten degrees from the usual temperature. In winter, how-
ever, if the storm-track lies well to the south a large
volume of very cold air will be drawn far to the south aud
the temperature may fall forty or fift}' degrees in a day's
time, or even in a few hours.^ Ordinarily, the cold wave
flows in not more forcibly than a brisk wind, but occa-
sionally it advances with the force of a hurricane, lower-
ing the temperature to thirty degrees or more below zero
(F.). In such cases the cold wave is called a blizzard ^ aud
it is marked by a furious downfall of snow.

Tornadoes. — Tornadoes are whirling storms of the
land. Though they cover a smaller area than any other
storm, they are probably the most violent atmospheric
disturbances known. ^° The path of the tornado seldom ex-
ceeds thirty or forty miles in length, while the destructive
part of the whirl is not more than a few rods in width.
Like other cyclonic disturbances, the tornado is formed in
an area of low barometer. Seen at a distance of one or



256



PHYSICAL GEOGRAPHY



two miles, the tornado appears as a dense, black, funnel-
shaped cloud hanging from rapidly whirling clouds above.
The funnel is tlie centre of the storm, and so rapid is
the wliirl that it forms almost a vacuum. The rotatory ve-
locity of the wind is thought to be not far from two miles
a minute.

Between the terrific wind and the vacuous centre noth-
ing can Avithstand the force of the tornado. The stout-
est tree-trunks are twisted as though they were ropes,

and in many instances
pulled clear out of the
ground. Buildings in
the way of the funnel-
cloud burst into pieces
outwardly the moment
the latter envelops them ;
heavy locomotives are
lifted from the railway
track ; and iron bridges
are blown from their
foundations, twisted into
shapeless tangles, and
carried long distances.
Another noticeable feat-
ure is the lane or " wind-
road " made when a tor-
nado passes through a
forest.

A close study of sev-
eral hundred tornadoes in a measure has shown the
manner in which they originate. At the beginning of
a storm it sometimes happens that a great volume of cold,
dry air lies on one side of the disturbance, while a mass
of warm, moist air lies on the other side. Such a con-




A TORNADO TRACK.

The position and direction of the rails show the
direction of the whirl.



CYCLONIC STOEMS



257



dition, iudeed, is not iufrequently the immediate cause of
the storm.

Duriiig the progress of the latter large volumes of cold
air are whirled iuto regions of warm and moist air. Now,
if the heavier cold air lies next the earth, no disturbance
follows. But if it comes to rest on the top of a thick layer
of warm air the case is diflerent. The conditions are those




A TORNADO AND ITS FUNNEL CLOUD.

of unstable equilibrium, and the latter will sooner or later
be upset. There results an up-draught of warm air, and
soon the whirl is in full vigor.

In about ninety-five per cent, of all the tornadoes
studied the whirl accords with that of other storms in the
Northern Hemisphere. Almost always they move from the
southwest to the northeast." In nearly every instance
thus far recorded the tornado track lies south of a general
storm.



258



PHYSICAL GEOGRAPHY



All parts of the United States are subject to tornadoes,
but they are most prevalent in the central part of the Missis-
sippi Valley. West of the 102d meridian they are extremely
rare, because there is so little moisture in the atmosphere.
There is also a belt south of the Ohio Kiver, in which

they are infrequent. They
rarely occur in mountainous
regions.

Tornadoes are more frequent
in summer than winter. The
greatest number occur in May
and more oc-
cur in May,
June, and
July than dur-
ing all the
remaining
months. They
;ire more fre-
quent in the
afternoon
than in the
morning, and
rarely occur
at night.
Waterspouts. — A waterspout is a whirlwind of the sea
or other large body of water. The whirl is so rapid that
the water is carried upward to fill the vacuous centre. The
lower part of the waterspout is probably a nearly solid
column of water ; the upper part is a rapidly whirling
mass of spray. AVaterspouts are most common in the
region of cyclone tracks — especially along the track of the
Gulf Stream. It is usually asserted that the water that
composes them is fresh. This is not always the case,




EFFECTS OF A TORNADO.



CYCLONIC STORMS 259

however; in many instances it is salt — sea-water, pure and
simple. In the lower part the column is not more than
ten or fifteen feet in diameter ; in the upper part it is
whirled into a balloon-shaped cloud of spray and mist
several hundred feet in width.

The white squall is similar in origin to the whirl that re-
sults in a waterspout ; in fact, it ma}' properly be called a
fair-weather whirlwind of the sea. It is sufficiently violent
to whirl a considerable volume of sea-water into spray,
but hardly strong enough to form a waterspout.

Weather Forecasting. — Knowing the laws of storms
and normal atmospheric movements, it is not a difficult
matter to predict Aveather conditions with considerable
accuracy. In the temperate zones weather conditions
originate to the w^estward or southwestward of the ob-
server ; in tropical regions, they progress from the east-
ward.

Except in the extreme southern part, where disturbances
are occasionally tropical in their movements, the Aveather of
the United States is essentially of the westerly type. That
is, all disturbances progress from the west or southwest
to the east or northeast.

The United States AVeather Bureau'- was organized
for the purpose of protecting agriculture, navigation,
and commerce by furnishing information of coming storms,
dangerous coast-winds, threatening Hoods, cold waves, and
killing frosts. Scattered over the whole territory in
selected locations are upwards of six hundred observers
who, twice a day, at the same actual time, observe tem-
perature, barometric pressure, relative humidity, direc-
tion of wind, amount of rain or snow, etc. Tliese re-
sults are telegraphed to Washington and entered upon a

weather map.

Lines are drawn th If )U.l;1i localities of ecpial barometric




STORM CENTER : FIRST DAY




STORM CENTER: SECOND DAY



CYCLOXIC STORMS 261

pressure, and also through localities Laving the same tem-
perature. The former are isobars, the latter isotherms.
In this manner areas of high, normal, and low barometer
are readih' mapped and located. When the direction of
the wind is plotted it will be found that it is everywhere
blowing toward the area of low barometer.

Twelve hours afterward, when a new set of observations
is plotted, it will be found that the area of low^ barometer
has advanced eastward with about the velocity of an or-
dinary express train. With this information both the
direction and the velocity of the storm can be quite ae-
cui'ately forecast for the succeeding twenty-fom- lunirs.
Practically all general storms begin with easterly and clear
with westerly winds.

About ninety per cent, of the predictions may be verified
and the number actually- veritied is very close to the possi-
ble limit. Failure of verification is due to several causes —
the sudden swerving of a storm from its track, the dissipa-
tion of a storm once formed, and the unforeseen develop-
ment of a local storm. The shifting of a storm one hun-
dred miles on either side of its predicted track may nullify
the forecasts over a very large area.



QUESTIONS AND EXERCISES.— Why does the wind blow toward
a low and away from a high barometer ?

Why do cyclonic movements of the wind move toward the west in
tropical, and toward the east in temperate latitudes ?

Why does the water flowing out of a sink through a discharge-pipe
at the bottom form a whirlpool ?

In the map at the top of p. 260, near what city is the centre of the
storm ? What is the direction of the wind at New Orleans and Baton
Rouge?— at St. Louis and Cairo ?— at Chicago and Davenport ?— at
Duluth? — at Cheyenne?— in the greater part of North and South
Dakota ?

Name one or two places at or near which the barometer is 29.5 inches;
29.7 inches; 29.9 inches; 30 inches.



263 PHYSICAL GEOGRAPHY

About how far has the storm advanced at the time of observation on
the second day ?

Note the direction of the wind at Pittsburgh, Cleveland, Wilming-
ton, N. C, Cincinnati, Indianapolis, Chicago, Springfield, 111., Mil-
waukee, New Orleans, Mobile and Little Rock.

The wind whirls warm, moist air from the south to colder, northerly
latitudes ; what will be the effect on the moisture ?— on the temperature
of the region over which the storm passes ?

In what position, with reference to the storm centre, is most of the
rain, as indicated by the shading ?

Whence comes the air in the western part of the whirl — from
northerly or from southerly regions ?

Will it probably be colder, or warmer ? Why ?

Make a forecast for Cincinnati for each of the two days.

Make forecasts for New York, Denver, and Chicago for the third day.



COLLATERAL READING AND REFERENCE.

Greely. — American Weather — pp. 178-272.

United States Weather Bureau. — Daily Weather Maps.



NOTES

' In the tropics the cloud-ring rarely exceeds five hundred miles
in diameter, and the circle of dangerous winds is scarcely more
than half as great. In higher latitudes, however, the diameter
of the storm increases. The wind is more violent in tropical,
and less severe in higlier latitudes.

^ The direction of the whirl is thought to result from the con-
flict of winds as they approach the up-draught. Of all the cur-
rents setting toward the storm-centre, the northeast Trade Wind
is the strongest. As it approaches the storm-centre it is opposed
by weaker winds from the north, northwest, and west. As a re-
sult, the Trade Wind is bent toward the east and forced to rotate
in the manner described.

' The barometer gives first warning of the approach of the cy-
clone. During the few days preceding, tlie barometer is perliaps
above its normal heiglit and tlie weather pleasant and clear.
Sooner or later the barometer begins to show signs of unsteadi-



CYCLONIC STORMS 263

ness, and at the same time a long, low, ocean swell becomes per-
ceptible. Possibly a streamer or two of cat-tail clouds pointing
toward the zenith is seen in the south or southwest, and a whit-
ish arc near or on the horizon indicates the bearing of the cen-
tre. In a few hours or less the barometer begins to fall — slowly
at first, and then more rapidly. A halo gathers around the sun
or the moon ; the ocean .swell increases, the sky grows purple,
and fitful f)ufTs of wind come from the noi-th. There can no
longer be any doubt of the approaching storm, and the prudent
master has already made everything snug and ready for the com-
ing blow. Soon a heavy, mountainous bank of cloud looms up
fi'om the horizon. This is the cloud-ring that marks the edge of
the storm, and the circle of dangerous winds is not far away.
Finally the wind, already very squally, bursts into a gale, and
veers to the noi-thward, and soon the storm is on, in full force.
If, by any means, the course has not been altered, or if, through
accident, the ship is carried with the wind, the latter will in-
crease to hurricane strengtli, and not even the smallest storm-
sail will stand against its force. Soon, in almost a twinkling,
the wind lulls and the ship is in the eye of the storm. Then the
sky alternates between inky blackness, with terrific down-pours
of rain, and moments of misty, yellow light. Perhaps half an
hour pjisses, and the opposite side of the cyclone strikes the
vessel. At. that moment the wind again bursts upon the
ship from the opposite direction. Nothing but the stanchest
vessel can ride through such a storm. A square-rigged ship is


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Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 17 of 25)