HOW TO FORECAST FROM THE DAILY WEATHER MAP
IT IS NOT DIFFICULT TO BECOME WEATHERWISE AND THEREBY TO GAIN
ADVANTAGES IN HEALTH, HAPPINESS, AND BUSINESS
The person who will take the time to learn to interpret the daily
weather map has a decided advantage over those who are less
progressive. The maps may be secured by applying to any Weather
Bureau station. Many members of commercial associations, having the
advantage of seeing the large glass weather map that is made each
morning by an observer of the Weather Bureau and displayed on the
floor of the association, have become expert weather forecasters. The
value of the principal crops of the country is largely influenced
by the weather, as are the prices of transportation and industrial
stock; and there is hardly a business that directly or indirectly is
not influenced by the prospects of coming weather.
Vessel masters, long accustomed to forecast the near approach of
storms from the action of their “glass” (barometer), now have learned
that the daily weather map shows them at a glance the height of not
one but of many barometers scattered over a wide area and read at the
same moment of time. They see that the direction and the force of the
wind are the results of differences in air pressure; that the air
flows from a region where the air pressure is great, that is to say,
where the barometers are high, towards a region where the pressure is
less, or where the barometers are low; and that the velocity of the
wind will be in proportion to the difference in the pressure of the
air. Coast-wise and lake shipping are therefore not only affected by
the forecasts made by the Weather Bureau but by the forecast made by
the masters themselves when they can get access to the daily weather
map. Their own lives and the lives and property of others are in
their keeping. But the great mass of intelligent people have no idea
of the methods employed in the making of the weather map and of the
many and widely diversified uses to which a study of its data would
lead.
One first must learn of the simple manner in which the map is
constructed; then, by a comparison of the map each day with the
preceding chart, he soon will be able to detect the beginning of
storms, trace them through their various migrations as they cross
the continent and finally pass out to sea, bidding them bon voyage
as they go in quest of a more eastern continent on which to bestow
their blessings of rain and active, purified air; or, as it often may
happen, shuddering for the fate of the mariner who is caught in their
fierce vortical whirls, and for the land areas that may be laid waste
by their gyrating force.
=How the Weather Map Is Made.= At 8 A.M. to-day Washington time,
which, by the way, is about seven o’clock at Chicago, six at Denver,
and five at San Francisco, the observers at some two hundred stations
in the United States and contiguous territory were taking their
observations and from carefully standardized instruments noting
the conditions of the atmosphere. By 8:20 A.M. the barometers at
each station have been reduced to sea level, that is to say, they
have been made to read what they would if they were located at the
level of the ocean. Thus differences in air pressure that are due to
differences in elevation are eliminated, so that they may not obscure
those due to storm conditions. Then, for purposes of brevity and
accuracy, the observations are reduced to cipher form, and each
filed at the local telegraph office. During the next thirty or forty
minutes the observations, with the right of way over all lines, are
speeding to their destinations, each station contributing its own
report, and receiving in return such observations from other stations
as it may require. The observations from all stations are received at
such important centers as Washington, New York, Chicago, and other
large cities having Weather Bureau stations, and from these centers
daily weather maps are printed and issued at 11 A.M. each day.
[Illustration: CHART 3.—WINTER STORM, DECEMBER 15, 1893, 8 A.M.
Black lines connect places having equal barometric pressure; arrows
point in direction wind is blowing; figures at end of arrows show
wind velocity, when it is more than light.
○ clear; ◓ partly cloudy; ● cloudy; R rain; S snow.
HIGH indicates center of anti-cyclone, or high-pressure area; LOW
indicates center of cyclone, or low-pressure area.
Large figures show average temperature in each quadrant of cyclone.]
Now turn to Chart 3. Heavy black lines (isobars, meaning equal
pressure) are drawn through places having the same barometric
reading. The readings are omitted from the printed Chart. By drawing
lines for each difference of one tenth of an inch, the high and the
low-pressure areas (called Highs and Lows) are soon inclosed in their
proper circles. These lines run in oval or circular form, indicating
that storms operate in the form of great atmospheric eddies; that
there are central places of attraction _towards_ which the air is
drawn if the disturbance be a low-pressure area, with its usual
accompaniments of warm, moist, and often rainy weather, and _from_
which the air is driven if it be a high-pressure area, with cool,
settled weather.
The word “High” is written inside the isobar marked 30.6, located
in southern Oregon, and the same word is written inside the isobar
marked 30.4, located on the South Atlantic coast, and also inside the
isobar 30.04, which traverses Nova Scotia. These are the regions of
great air pressure. The word “Low” is written at the center of the
area inclosed by the isobar 29.6, which is situated in the State of
Iowa. The latter is the region of least pressure. Sometimes there are
several such regions shown on the weather map.
=Why the Wind Blows.= Under the pull of gravity the atmosphere
presses downward and outward, thus causing it to flow from the
several regions of great pressure towards regions of less pressure.
Observe the arrows, which fly with the wind, and it will be seen how
generally this law is obeyed. The velocity with which the wind moves
from the High toward the Low depends on differences in air pressure,
modified in the lower stratum by the friction offered in passing over
surfaces of varying degrees of roughness, the speed being greater
over a water surface with the same difference in air pressure than
over a level unwooded prairie, and greater over the open prairie than
over an irregular wooded area. To illustrate:
If the barometer were 30.5 at Bismark, Dakota, and 29.5 at
Chicago, it would press upon the earth with a force of about seventy
pounds greater per square foot at the first place than at the second.
This difference in pressure would cause the air to flow from Bismark
towards Chicago so rapidly that after allowing for the resistance
due to friction on the earth there would remain a velocity of some
fifty miles per hour, and Lake Michigan would experience a severe
“Northwester”; and if the wind continued from the same direction for
twenty-four hours a mighty sea would beat upon the eastern shore of
the lake, and mariners and marine property would be at the mercy of a
destructive tempest unless the Weather Bureau forecaster were alert
and gave warning as soon as he saw such a juxtaposition of pressure
distribution in the process of formation.
We will give careful attention to this chart, for when its details
are understood, others will be easily read.
The chart shows a winter storm central in Iowa on December 15, 1893.
The word “Low” marks the storm center. It is the one place in all the
United States where the barometer reading is the lowest. The heavy
black lines, oval and nearly concentric, about the Low, show the
gradation of air pressure as it increases quite uniformly in all
directions from the center of the storm outward.
The arrows fly with the wind, and, as will be seen, almost without
exception are moving towards the Low, or storm center, clearly
demonstrating the effect of gravity in causing the air to flow from
the several regions marked “High”, where the air is abnormally heavy,
toward the Low, where the air is lighter. As the velocity of water
flowing down an inclined plane depends both upon the slope of the
plane and the roughness of its surface, so the velocity of the wind,
as it flows along the surface of the earth towards the storm center,
depends on the amount of the depression of the barometer at the
center and the resistance offered by surfaces of varying degrees of
roughness.
=Storms and Cold Waves Simply Great Eddies in the Atmosphere.= Now
picture in your mind that all the air inside the 30.2 isobar, as it
flows inward, is rotating about the Low in a direction contrary to
the movements of the hands of a watch, and you have a fair conception
of an immense atmospheric eddy. Have you ever watched the placid
waters of a deep-flowing brook and observed that where the waters
encountered a projecting rock little eddies formed and went spinning
down the stream? Well, our storms are somewhat similar eddies in the
atmosphere, more or less perfect, that are carried along by the
general easterly movement of the atmosphere in the middle latitudes
of both hemispheres. But they are not deep eddies; the Low marks the
center of an atmospheric circulation of vast horizontal extent as
compared with its thickness or extension in a vertical direction.
Thus a storm area extends from Washington, D. C., to Denver,
Colorado, and yet extends upward only about six miles. The whole disk
of whirling air, six miles thick and two thousand miles in diameter,
is called a cyclone, or low-pressure area. It is important that a
proper understanding be had of this fundamental idea, since the
weather experienced from day to day depends almost wholly upon the
movement of these migrating cyclones, or areas of low pressure, and
the anti-cyclones, or areas of high pressure.
The temperature readings are omitted from each station, but the
average temperature of each quadrant of the Low is shown by the
large black figures. The greatest difference in temperature is seen
to be between the southeast and the northwest sections. This is due
in part to the fact that in the southeast quadrant the air is drawn
northward from warmer latitudes, and in the northwest quadrant it is
drawn southward from colder latitudes, and to the further fact that
winds blowing into the west side of a Low have a downward component
of motion, and those blowing in on the front, or east side, have an
upward component.
One should gain a clear idea of the difference between the movements
of the air in the cyclone and the movement of the cyclone itself,
or its translation from place to place; how the wind must blow into
the front of the storm in a direction partly or wholly contrary to
the movement of the storm itself, and into the rear of the storm as
it passes away; how the wind increases in velocity as it spirally
gyrates about the center and approaches nearer and nearer the region
where it must ascend; how the higher layers of air move spirally away
from the center and thus cause an accumulation of air about and over
the outer periphery of the Low, which in turn presses downward and
impels the surface air inward. This whole complex system of motion
moves eastward. Think of the sun drifting in space, while at the
same time each of the planets maintains its respective orbit, and it
will help one to visualize the phenomena of a migrating cyclone or
anti-cyclone.
[Illustration: CHART 4.—WINTER STORM, DECEMBER 15, 1893, 8 P.M.
Black lines connect places having equal barometric pressure; red
lines connect places having equal temperature; arrows point in
direction wind is blowing; figures at end of arrows show wind
velocity when it is more than light.
○ clear; ◓ partly cloudy; ● cloudy; R rain; S snow.
HIGH indicates center of anti-cyclone, or high-pressure area; LOW
indicates center of cyclone, or low-pressure area.
Large figures show average temperature in each quadrant of cyclone.
Shading shows precipitation area of last 24 hours.]
Chart 4, constructed from observations taken twelve hours later,
shows that the Low has moved from central Iowa since 8 A.M., and is
now, at 8 P.M., central over the southern point of Lake Michigan.
The shaded portion of the chart shows that rain has fallen during
the past twelve hours throughout nearly the entire region covered
by the cyclone. This was due to the mixing of the air as the storm
progressed, to the cooling by expansion as the air ascended, to the
more rapid rotation about the storm center, because of the further
lowering of the barometer at the center of the disturbance since
the preceding chart was made, and especially to the more humid air
encountered as the storm moved eastward and came nearer to the supply
of moist winds,—the Atlantic Ocean.
[Illustration: CHART 5.—WINTER STORM, DECEMBER 16, 1893, 8 A.M.
Black lines connect places having equal barometric pressure; red
lines connect places having equal temperature; arrows point in
direction wind is blowing; figures at end of arrows show wind
velocity, when it is more than light.
○ clear; ◓ partly cloudy; ● cloudy; R rain; S snow.
HIGH indicates center of anti-cyclone, or high-pressure area; LOW
indicates center of cyclone, or low-pressure area.
Large figures show average temperature in each quadrant of cyclone.
Shading shows precipitation area of last 24 hours.]
On Chart 5 a line of arrows extends from the storm center westward
to Wyoming, where the storm originated. A small cross inclosed by
a circle marks its western extremity. Another cross located near
Cheyenne shows where the storm center was located twelve hours
after its origin. A third cross gives it location near Des Moines
twenty-four hours after it started eastward. It was here that we
began the study of this storm on Chart 3. A cross near Chicago
indicates the distance traveled by the center during the third twelve
hours, and Chart 5 shows its progress during the fourth twelve-hour
period. When the storm was central at Cheyenne the danger warnings
for mariners were displayed at all ports of the Great Lakes, as
the forecaster knew that in accordance with general laws the storm
must move toward the east. When it was centered at Chicago, danger
warnings were displayed on the Atlantic coast from North Carolina to
Maine, as it was known that long before the storm reached the ocean
the in-rush of wind toward the storm center would cause a dangerous
on-shore gale and the breaking of heavy seas on the shore line. All
craft that could be reached with the danger signals made safe in
port, except the great ocean liners, which are of such strength as
to safely withstand almost any storm. A special set of observations
ordered by the Washington office of the Weather Bureau from its
stations in the region of the storm, and well in advance of it, kept
the chief forecaster informed as to the progress of the cyclone,
and before the storm center reached the coast the danger signals
communicated to mariners the fact that the winds would soon shift to
northwest as the center of the disturbance passed out to sea.
The reader’s attention will now be directed to the red lines on Chart
5; they pass through places having the same temperature, but for
simplicity the readings of temperature, whereby these lines were
located, are omitted from the printed chart. Observe the line marked
40°; it passes across southern New England to western New York,
but when it reaches the center of the storm it encounters the cold
northwest winds blowing into the storm on its west side and is forced
southward to Texas.
Charts 3, 4, and 5 give a graphic history of one severe winter storm.
In summer such general storms do not often occur. They are frequent
in spring and fall, but of higher temperature and less severity than
in winter. In summer Lows drift sluggishly across the continent; the
barometer at the center of the cyclone is usually not more than two
to four tenths of an inch below the pressure of the Highs, and the
rain, instead of falling in a broad sheet, as shown by the shading of
charts 4 and 5, falls in numerous sporadic outbursts, each of which
is but a few square miles in area, their combined surfaces usually
covering only a part of the region over which passes the Low.
=Cold Waves and the Speed of Storm Movement.= Highs and Lows drift
across the continent from the west towards the east at the average
rate of about six hundred miles per day, or about thirty-seven miles
per hour in winter and twenty-two miles in summer, the first at
about the rate of an express train, and the second approximating
the speed of a freight. The Highs are attended by dry, cool, and
settled weather. By a vortical action at their centers they draw down
the cold air from great altitudes above the clouds. In winter, when
vortical action is vigorous, they may reach upward to an altitude of
seven miles. Air starting downward from this region has a temperature
of some 70° below zero. We know this from the records secured
by sending aloft free balloons carrying automatic thermometers.
(Chapters II and III.) This air heats by compression because in its
downward movement it is continually leaving more and more air above
it to exercise pressure upon it. It gains about twenty degrees with
each mile of descent, and if there were no other factors to the
problem it would be hot air when it reached the surface of the earth
instead of cold air. But early in its descent it gains such heat
as to melt and evaporate the ice spiculæ floating at the height of
the fleecy cirrus clouds; then it evaporates and clears away the
moist clouds lower down and finally creates such _diathermancy_ (the
capacity to transmit heat without absorption; see Chapter V) that the
heat lost by radiation to a clear sky causes what we call a “cold
wave”, and this notwithstanding the heat of compression.
The forecaster first observes a cold wave in the northern Rocky
Mountain region, in the form of an intense High. It will travel
southeastward to the center of the continent, and often to the Gulf
if it is preceded by an active Low that is located on a low latitude,
as the latter will draw southward the frosty air of the High; after
that the course of the storm will be more nearly eastward. Now
it is of rare occurrence that a cold wave gains entrance to any
considerable area of our territory without warning, but in the early
days of the Weather Bureau they too often reach Iowa, or States
farther east, without any notice whatever. It was then discovered
that a certain type of weather map preceded such failures of the
forecaster. One who is interested in gaining early knowledge of
the approach of a cold wave to the United States should watch not
only for the appearance of abnormally high barometer readings, from
the stations of the Canadian Northwest, or from Montana and North
Dakota, but especially for a crescent-shaped Low, with one horn of
the crescent touching Lake Superior and the other extending into the
middle Rocky Mountain region, at about Colorado. This Low will appear
to be an innocent affair; there may be a small secondary Low in each
end of the crescent, and no High of any importance in the northwest,
for which one ordinarily would look in anticipating a cold wave.
But when this crescent-shaped Low appears on the morning weather
map, a High of marked intensity invariably will develop with great
suddenness over Montana and North Dakota and bring a cold wave to the
Middle Mississippi Valley before the next morning, if the time of
year be winter.
Do not forget that the Low is as important as the High in causing a
cold wave, for the High that brings the cold air must follow in the
track of the Low and will be attracted by the latter in proportion
to its lowness, as indicated by the isobar inclosing the center of
the Low. A cold wave will reach the Gulf only if the preceding Low
originate in Texas; it will be confined to the Ohio Valley as the
limit of its southern influence if the preceding Low originate in
Colorado; and it will only skirt the northern border of the United
States and the Lake region if the Low begin in Montana.
More and more is man applying science to commerce and industry.
When the weather map, which was unknown but little more than half a
century ago, indicates the formation of a heavy body of cold air in
the extreme northwest, the chief official forecaster at Washington
is on the alert; he orders special observations every few hours
from the Weather Bureau stations directly within and well in advance
of the cold area, and as soon as he becomes satisfied that a cold
wave is on its way, the previously arranged system of disseminating
warnings is brought into action, and by telegraph, telephone, flags,
whistles, bulletins, and other agencies, the people in every city,
town and hamlet, and many in the stock and farming regions, are
notified of the advancing cold twelve to twenty-four hours before it
reaches them.
[Illustration: CHART 6.—COLD WAVE ZONES, MARCH TO NOVEMBER. AMOUNT OF
FALL AND VERIFYING LIMIT.]
Charts 6 and 7 show how the Weather Bureau defines a cold wave.
There must be a fall of sixteen degrees, eighteen degrees, or twenty
degrees within thirty-six hours and a certain degree of coldness
must be reached. The charts show that what is a cold wave in the Gulf
region is far from one in the northwest.
[Illustration: CHART 7.—COLD WAVE ZONES, DECEMBER, JANUARY, AND
FEBRUARY. AMOUNT OF FALL AND VERIFYING LIMIT.]
[Illustration: CHART 8.—LOWEST TEMPERATURES IN THE UNITED STATES,
1871-1913.]
Chart 8 shows the lowest temperatures experienced in the United
States since the founding of the Weather Bureau, 1871 to 1913. Note
the influence of the Pacific Ocean in forcing the zero line from
Arizona northward to British Columbia.
[Illustration: CHART 9.—NUMBER OF COLD WAVES, 1904-1914, INCLUSIVE.]
Chart 9 shows the number of times that a cold wave occurred at each
station of the Weather Bureau for a period of ten years. The number
is greater for northern New England than for the Red River of
the North Valley, because practically all the cold waves that cross
Minnesota reach New England; and the latter also receives fierce
boreal visitors that come to it from the Hudson Bay region lying
directly northeast, which do not visit any portion of Minnesota or
the region farther west. During the period not a single technical
cold wave occurred at the coast stations of California, Oregon,
or Washington, while Red Bluff and Sacramento were the only two
places in California west of the Sierras, and Roseburg, Oregon, the
only station west of the Cascade Range that had any, the numbers
being one, two, and five respectively. In the Florida peninsula
south of Jacksonville, Tampa had two, while none occurred at Miami.
Sometimes the temperature falls lower than that required for a cold
wave, but not within the period of twenty-four hours required by
the regulations. A notable case in point is the severe cold wave in
California in January, 1913, the lowest temperature ever observed
being recorded at San Diego on the 7th, when the minimum fell to 25°.
=Cold Waves Tempered by Great Lakes.= The severity of cold waves is
markedly modified by the Great Lakes, especially in the fall and the
first part of winter, before much of the water surface is covered
with ice and snow. Not only is the number of cold waves much less at
stations of the Lakes than at near-by places in the interior, but
there is a marked variation in the number that occur at the Lake
stations, depending upon which side of the lake and how close to the
water the station is located. The most striking differences are noted
in the Lake Michigan region, the number on the west shore being five
or six times as great as on the east side. Milwaukee shows a count of
forty-seven as compared with nine at Grand Haven. This lake influence
affects the entire Lower Michigan peninsula, but it is not so great
in the interior and eastern sections as along the west shore, Grand
Haven’s nine standing out against fourteen, fifteen, and twenty-three
for Grand Rapids, Detroit, and Port Huron. A similar condition is
noted in New York State; Buffalo, Rochester, and Oswego, near the
lake shore, had twenty, twenty-seven, and twenty-nine cold waves
respectively, while the interior stations of Ithaca, Binghamton, and
Syracuse had thirty-eight, forty-five, and fifty-two.
=Cold Waves Tempered by the Heat of Cities.= Another reason for
the lack of uniformity in the recorded number of cold waves in the
various sections of the country is the difference between city and
suburban temperatures. Stations located in small villages or in the
open land will show a greater number of recorded cold waves than
those located in large cities, where the heat stored up by pavements
and brick buildings during sunshine each day, and where the heat from
thousands of chimneys, and maybe millions of human beings, holds the
minimum temperature of night much above that of the free air in the
open country. Charles City, where the instruments have open country
exposure had sixty-five cold waves, which far exceeds the number
recorded at any other station in Iowa.
No matter how severe may be the cold wave that appears in the
northwest, it will not extend over Wyoming, Colorado, Utah, and any
region south of them, unless the center of the High extends well over
the Rocky Mountain Divide. Otherwise it will come down the east slope
of the mountains and the cold will not cross them.
In the Lows the conditions of the air and its movements are exactly
the reverse of what they are in the Highs; the air is warmer and
moister, it is drawn spirally inward from all directions instead of
being forced outward as in the High, and it ascends as it approaches
the center of depression, sometimes causing rain or snow as it cools
by expansion during its ascent. While the air cools with ascent in
the Low at the same rate that it warms with descent in the High, the
earth experiences a general warming effect with the passage of the
Lows, because the air falls but little in temperature as it rises
before it reaches its dew point, and then there is a liberation of
the latent heat of condensation (see Chapter V); and what is more
important, there is formed a covering of clouds that checks or wholly
stops radiation outward from the lower air. However there are times
when the passage of Lows produces a cooling effect. This is when
abnormally hot weather has prevailed for some days; then the air may
be mixed, washed, and cooled by thunder-showers.
[Illustration: CHART 10.—STORM TRACKS FOR AUGUST FOR TEN YEARS.]
Highs and Lows alternately drift across the continent in periods of
about three days each. They are a part of the divine economy that
provides for the seedtime and the harvest, for, as previously stated,
the Lows draw the warm, vapor-bearing currents inland from the Gulf
and the ocean and cause them to deposit their moisture far to the
north and west. Four sevenths of all our storms come from the middle
or the north plateau regions of the Rocky Mountains, or at least
enter our field of observation from those regions, and pass from this
arid or sub-arid section of the continent easterly over the Lakes
and New England, producing but little rainfall. The greater part of
the remaining three sevenths are first observed in the arid regions
of our southwestern States; they always move northeastward and can be
depended on to give bountiful rainfall so soon as or a little before
they reach the Mississippi River. Some of them cross the Atlantic and
affect the continent of Europe. Charts 10 and 11 show the courses
of storms in this country, and where they originate, or are first
brought under the survey of our system of observation.
[Illustration: CHART 11.—STORM TRACKS FOR FEBRUARY FOR TEN YEARS.]
=West Indian Hurricanes.= A few of the most severe storms that touch
any portion of our continent originate in the West Indies and travel
in a northwesterly direction until they touch our Gulf or South
Atlantic coast, when, passing from the influence of the northeast
trade winds which carried them westward, they recurve and pass along
our eastern coast, usually with their centers offshore and following
the Gulf Stream. These violent atmospheric convulsions are usually
detected in the process of formation through the effectiveness of
the storm-warning service established by the writer during the
Spanish-American War, under the direction of the President, for
the purpose of giving warning to our fleet before the coming of a
hurricane. The President realized the great part played by storms in
many of the naval battles of the past, and it may be surmised that he
was more afraid of a West Indian hurricane than he was of the Spanish
Navy. But Cervera was beaten and the blockade was raised before the
hurricanes of 1898 began.
=Galveston Hurricane.= The new Weather Service, with a cordon of
stations down the Windward Islands and along the north coast of South
America, surrounding our fleet, and inaugurated as a war measure, so
demonstrated its value in locating and giving warning of the coming
of a hurricane soon after the end of the war that Congress continued
it as a permanent instrument of peace; and when the destructive
Galveston Hurricane occurred in 1900 it detected the storm at its
inception and so fully advised shipping of the storm’s movements that
not a vessel was lost as the storm roared and gyrated across the Gulf
of Mexico and crashed upon the Texas coast, destroying a large part
of the city and drowning six thousand people.
The hurricane is simply a rapidly gyrating cyclone; it usually is
only one to three hundred miles in diameter. The storm that destroyed
Galveston moved across the Caribbean Sea at the rate of only about
eight to ten miles an hour. It increased its rate as it moved
northward, crossing the Gulf at about fifteen miles per hour. The
speed of translation was so slow and the velocity of gyration so
rapid that immense swells were propagated outward from the center of
the storm; they reached the Texas coast some sixteen hours before the
storm itself reached Galveston. As it moved northward to Iowa its
velocity of translation increased and its rate of gyration decreased,
so that it crossed the Lakes with both movements at about sixty miles
per hour. At Galveston the anemometer blew to pieces after recording
one hundred and thirty miles per hour.
=Danger to Atlantic Coast Summer Resorts.= The writer frequently
has been asked as to the possibilities of a populous Atlantic coast
resort being submerged by the waters driven inshore by a hurricane,
or being lifted up in the center of the storm as the result of
decreased air pressure inside the cyclonic whirl. The answer is that
such a catastrophe is possible to any Atlantic coast city (more
especially those south of Norfolk) that is not protected by a heavy
breakwater of ten to twenty feet above sea level, and whose building
foundations and walls are not of brick or concrete for at least ten
feet above the water level. It would be necessary for a West Indian
hurricane of unusual intensity—one similar to that which wrecked
Galveston—to be considerably deflected westward out of its normal
track in order to hit one of our coast cities north of Chesapeake Bay
so that the center of the storm would pass over it, or near enough to
cause destruction. In Galveston there was little damage to strongly
constructed buildings of brick or stone.
=The Breaking of Droughts.= It is most important for the forecaster
to know when and how droughts may be broken. He will observe that
when the great cereal plains are famishing for moisture the Lows
all originate on the middle or north Rocky Mountain plateau, in the
region of Colorado or Montana, and that the drought continues until
the Lows begin to form in the extreme southwest—in Arizona, New
Mexico, or Texas. As previously stated such Lows always bring rain as
they move northeastward.
=Warm Waves.= There come in summer periods of almost stagnation in
the drift of the Highs and the Lows across the continent. At such
times if a High be centered in the South Atlantic Ocean, with its
center at Bermuda, and its western limits extending into the South
Atlantic coast States, there will result what is popularly known as
a warm wave, for the air will slowly and steadily move from the
southeast, where the pressure is greater, towards the northwest,
where it is less; it will receive constant accretions of heat
from the radiating surface of the earth, and finally attain to a
temperature that is extremely uncomfortable to all forms of life,
that lowers the physical stamina, and that largely increases the
death rate. This superheated condition of the lower stratum of air
in which we live continues until a Low develops in the southwest and
a High in the northwest, which relation, as we already know, soon
brings rainfall to the interior of the country.
=V-shaped Lows= are reasonably sure to cause precipitation, and
if the barometer at the center of the Low be five to seven tenths
below the outer limits of the depression, heavy precipitation and
destructive local storms may be expected.
=Thunderstorms.= The thunderstorm is caused by cold and heavy air
from above breaking through into a lighter and superheated stratum
next the earth. Some of them have a horizontal rolling motion which
throws forward the cool air in the direction in which the storm is
moving. It seldom is more than five or ten miles in width and twenty
to thirty miles in length. In general, thunderstorms move from the
west toward some eastern point, more often southwest to northeast.
The frequency of thunderstorms is the greatest with ill-defined Lows
whose pressure is but little below the normal air pressure of thirty
inches. Any depression of the barometer slightly below the level at
surrounding stations—such as occurs when a weak High of only thirty
inches, or thirty and one tenth inches, breaks up into two or more
areas, with slightly lower pressure between them—is fruitful of
thunderstorms. A High of but modest intensity advancing eastward
into a region of slightly lower pressure and much higher temperature
causes thunderstorms along its eastern front. A temperature of 80°
on the morning weather map, with a high humidity, seldom can endure
beyond the second day without a break and the coming of cooling
thunder-showers. Any Low with abnormal heat and humidity in its
southeast quadrant is usually attended with numerous thunder squalls
in the regions of high temperature and moisture.
Of the thunderstorm days in the United States few occur in the
Rocky Mountain regions or in northern New England. The greatest
number is in Florida and the Gulf States and thence northward up the
Mississippi Valley.
=The Moon Has No Influence on the Weather.= The moon used to be the
farmer’s most valued friend as a forecaster of the weather and as a
guide in the planting of crops, but a higher order of intelligence is
causing this fallacy to pass away. The moon’s nearness to the earth
and the fact that its phases occur in about seven days, which is
about twice the period of storm recurrence, in the minds of many have
endowed it with potency in the influencing of our weather. Rain may
occur on the same day of the week for several weeks in succession,
but only occasionally, while the moon is constantly progressing from
one phase to another. The few cases that prove the mistaken theory
are taken as proof conclusive, while the many cases that do not prove
acceptable to the moon forecaster are ignored and not mentioned to
his friends nor even acknowledged to himself. One is reluctant to
have a belief disproved, no matter how ridiculous it may be. In fact,
the more untenable it is, the more tenaciously some adhere to it,
as though they were loyally standing by an old friend who had made
mistakes, but who still was good at heart. The attraction of the
moon, because of its nearness and notwithstanding its small mass, is
far more potent in the raising of the tides of the ocean than is the
sun, but its attraction on our atmosphere produces a tide of only
four thousandths of an inch of the barometer, an influence that is
shadowy and without the least influence in causing storms, or changes
of any kind in the weather; and there is no possible way in which the
moon could influence the germination of seed or the growing of crops.
=Equinoctial Storm.= As the summer wanes the Lows become more
pronounced and the sporadic showers give place to general rain storms
along in September. There is no objection to these storms being
known as “Equinoctial”, except that any date in the latter half of
September is as liable to show a beginning of these storms as is the
21st or the 22d. The equinox simply marks the middle period in the
transition from one type of weather to another.
=Forecasting from Halos.= The halos that sometimes surround the sun
or the moon indicate the coming of precipitation to the extent of
making manifest the presence in the upper air of large quantities of
vapor of water in a congealed state. When the vapor of water cools
quietly in the laboratory it frequently forms minute spheres of
water, which, strange to relate, may remain liquid all the way down
to zero and below; but if touched or jostled they instantly turn to
ice, in the form of spiculæ, or needles; they are simply hexagonal
slender prisms capped by hexagonal pyramids. These needles rotate
or spin about as they fall. The geometrical relations of the facets
of the crystals to the axis of rotation and to the line along which
they fall are a complex problem in optics. Suffice to say that the
observer, looking through a filmy cloud of such crystals, would see
in one part of the sky a halo, in another part an arc of light, and
in other directions bright spots like the sun, all of them arranged
symmetrically with regard to the sun and the observer’s zenith. A
lunar halo is a large ring concentric about the moon. A secondary
halo surrounds the first. Mock suns or mock moons may appear
coincident with solar or lunar halos. The ice prisms through which
one sees the phenomena both refract and diffract the light as it
passes through the cloud and by partly decomposing the rays render
visible a part of their elementary colors. The red is on the inside,
next to which is a little yellow or green, with bluish white on
the outside. In coronas, which are much smaller, the red is on the
outside. A detailed description of these phenomena may be found in
Moore’s “Descriptive Meteorology” (Appleton).
=Tornadoes.= The cyclone has a diameter of a thousand to two
thousand miles, the hurricane about one to three hundred and the
tornado only one to ten hundred _feet_. The hurricane is much more
destructive than the cyclone, and the tornado is incomparably greater
in velocity of gyration and rending force than the hurricane. New
England, Florida, and the wide region including the eastern slope
of the Rocky Mountains westward to the Pacific are nearly free from
the atmospheric convulsions that cause the tornadoes, and they are
infrequent in any Atlantic coast State, but numerous in the States
bordering on the Mississippi River, and in the eastern halves of
Oklahoma, Kansas, and Nebraska. During a year of great frequency of
tornadoes, about ninety storms occurred, while during some other
years the number has been as low as twenty. The direction generally
is toward the northeast. The average rate of movement of the tornado
cloud is about twenty-five miles per hour and the width of its
destructive path only five hundred to one thousand feet; the time of
passage is less than half a minute. It does not come upon one unseen
and unheralded. Many times the advancing funnel-shaped clouds may be
seen, and they always are accompanied by a great roar which may be
heard for miles. Except a tornado cellar, the cellar of a frame house
is the safest place. The writer has examined either the wrecks or the
records of hundreds of tornadoes and does not know of a single case
of a person being killed by a tornado in the cellar of a frame house.
If one is in the open and a tornado approaches, never flee to the
north or to the east, but rather to the northwest, and one needs to
travel but a short distance to pass out of the track of the monster.
The tornado always twists counter clockwise, the same as the cyclone
in whose southeast quadrant it nearly always occurs. On the southeast
side of the path there are indrafts; so that it is safer, unless the
track of the oncoming storm is clearly seen to be well to the north
of the observer, for one to run toward the northwest. Persons have
stood near to the north side of a tornado track during its passage
without suffering injury. If a cave, the cellar of a frame house, or
a narrow ditch cannot be reached, the best thing to do is to lie flat
on the ground as far from buildings and trees as possible.
The tornado is essentially an American storm, doubtless caused by the
running together, in the southeast quadrant of a cyclone, of cold
northwest currents and warm winds from the southeast, at a time when
the latter are saturated with moisture. They are confined almost
entirely to the region between the two great mountain systems of the
continent, none occurring in the Rocky Mountains and but few east of
the Alleghanies. The north and south trend of our mountain systems,
quite different from the systems of Europe and Asia, facilitates
the coming together of conflicting winds of widely different
temperatures in the lower reaches of the atmosphere where there is
an abundance of water vapor; no tornadic whirls probably can occur
without an abundance of water vapor and the energizing effect of
the heat liberated in the whirling cloud as this vapor is suddenly
carried aloft and liberated by condensation right in the center of
the disturbance. Because of the relation of the trend of its great
mountain systems to its oceans, the United States occupies a somewhat
unique position meteorologically in the world. Its atmospheric
conditions are more active than those of any other continent, which
conditions are beneficial to the people of this country.
=When to Watch the Weather Map for Tornadoes.= The four conditions
essential to the formation of tornadoes are as follows:
1. A cyclone, the center of which is to the north or northwest;
2. An isotherm of 70° or over extending from the southeast well
up into the center of the cyclone, and then passing outward
toward the southwest, all inside the southeast quadrant of the
Low;
3. Excessive humidity;
4. Time of year March 15 to June 15.
[Illustration: FIG. 17.—TORNADO CLOUD.]
If any one of the four foregoing conditions be absent, tornadoes are
not liable to occur. The reason why spring and early summer is the
time when tornadoes are most frequent is because the earth and a
thin stratum of air immediately next the earth are heated up rapidly
with the gaining heat of the sun’s rays in the spring, while the air
a short distance aloft still retains much of the cold of winter. At
this time cyclonic action may bring together air masses of widely
different temperatures, especially when the upper layers on the west
side of the Low are drawn down and commingled with the hot and humid
surface winds of the southeast quadrant.
=Tornadoes Not Increasing.= The writer does not indorse the theory
that the number of these storms is increasing; that the breaking of
the virgin soil of the prairie, the planting or the cutting away of
the forests, the drainage of land surfaces by tiles, the stringing
of thousands of miles of wire, or the laying of iron and steel rails
have materially altered the climate or contributed to the frequency
or the intensity of storms. To be sure, as population becomes more
dense greater destruction will ensue with the same number of storms.
=Difficult to Forecast Tornadoes.= It is not possible for the
forecaster to warn the exact cities and towns that will be struck by
tornadoes without unduly alarming many places that will wholly escape
injury. What we know is that tornadoes are almost wholly confined
to the southeast quadrant of a cyclone, and that when the thermal,
hygrometric, and time conditions are favorable, a region about one
or two hundred miles square will be sacrificed by a number of these
atmospheric twisters. One of the most destructive tornadoes of record
devastated St. Louis in the afternoon of May 27, 1896. The abnormal
heat and humidity of a rather small and weak cyclone centered in
eastern Kansas on the morning weather map of that day, caused the
Weather Bureau to distribute tornado forecasts at 10 A.M. throughout
all of Missouri. The schools of St. Louis were dismissed and the
children sent home on receipt of the warning, and although some eight
or ten separate tornadoes touched various parts of the State and the
people were prepared for their coming, so many people were terrorized
by the warning in communities that were not harmed, that the writer,
then Chief of the Weather Bureau, at once issued orders forbidding
the specific forecasting of tornadoes in the future. Under tornadic
conditions the forecast is for “conditions favorable for severe local
storms.”
=Freaks of the Tornado.= The writer was in St. Louis the day after
the storm and spent much time in examining the wreckage. He was
impressed with the fact that some buildings were burst outward and
that all four walls fell away from their bases, indicating that the
tornado cloud must have lifted and dropped down over them in such a
way that the partial vacuum that is created by the rotating cloud
through centrifugal force so reduced the pressure of the air on the
outside of the houses that the normal pressure of fifteen pounds per
square inch exploded them. He saw bricks in a plastered wall that
were neatly cleaned of all plaster by the expansion of the air inside
the brick, as the air pressure from the outside was reduced. He saw a
two by four pine scantling shot through five eighths of solid iron on
the Eads Bridge, the pine stick protruding several feet through the
iron side of the roadway, exemplifying the old principle of shooting
a candle through a board. He saw a six by eight piece of timber
driven four feet almost straight down into the hard compact soil, a
gardener’s spade shot six inches into the tough body of a tree, a
chip driven through the limb of a tree, and wheat straws forced into
the body of a tree to the depth of over half an inch. Such was the
fearful velocity of the wind as it gyrated about the small center
of the tornado,—a velocity exceeding that of any rifle bullet. (See
Figures 17, 18, 19, and 20.)
[Illustration: FIG. 18.—THE ST. LOUIS TORNADO OF MAY 27, 1896, SHOT A
PINE SCANTLING THROUGH THE IRON SIDE OF THE EADS BRIDGE.]
[Illustration: FIG. 19.—THE ST. LOUIS TORNADO OF MAY 27, 1896, SHOT A
SHOVEL SIX INCHES INTO THE BODY OF A TREE.]
Some have advocated the planting of trees to the southwest of cities
in the regions where tornadoes are frequent, so that the tornadoes
may expend their energy in uprooting the trees before they come to
the city, but this storm traveled through several miles of brick
buildings, razing them to the ground and almost pulverizing them
and still left the city apparently with greater force than it had
on entering. The largest trees would offer no more resistance to a
tornado cloud than would so many blades of grass.
When the official forecasts contain the statement that conditions are
favorable for “severe local storms” it would be well to carefully
observe the formation of portentous clouds in the west and southwest,
between 3 and 6 o’clock in the afternoon, and if one with black,
ragged fringes on its lower edge and accompanied with a noise like
several railroad trains makes its appearance, seek safety in the
cellar of a frame house.
[Illustration: FIG. 20.—THE ST. LOUIS TORNADO DROVE STRAWS ONE HALF
INCH INTO WOOD.]
=General Rules for Forecastings.= What has gone before in this
chapter gives an idea of what guides the weather forecaster in
making his deductions. In brief, he studies the developments and the
movements of the Highs and the Lows during the past two or three
days, as shown by preceding weather maps, and from the knowledge
gained forecasts the future course and intensity of the fair and
the foul weather areas for one, two, or three days in advance. By
preserving the weather map each day and noting the movements of the
Highs and the Lows, any intelligent person can make a fairly accurate
forecast for himself, always remembering that the Lows, as they drift
towards him, will bring warmer weather and sometimes rain or snow,
and that as they pass his place of observation the Highs following
in the tracks of the Lows will bring cooler and fair weather, except
during periods of extreme summer heat, when the Lows bring showers
that cool the parched earth; and except in the north Rocky Mountain
plateau, where most of the precipitation occurs after the center of
the Low has passed and northwest winds are blowing.
The amateur weather forecaster can closely anticipate the temperature
of his region by remembering that the weather will be cool and the
humidity low so long as the center of the predominating High (the
High inclosing the greatest area within the thirty-inch isobar) is
north of his latitude, either northeast or northwest, and that it
will be warm so long as the High is south of the parallel of latitude
that passes through his section of country.
He will find that the centers of the Lows will follow closely the
direction indicated by the isotherms that lead eastward out of their
centers, and that they move across the country from the west in quite
regular succession, and that the frequent changes from sunshine to
clouds and from warm to cold are the result of the mixing of the air
by these atmospheric eddies.
Experience will teach him that Lows from the southwest are reasonably
sure of causing precipitation, and that if his temperature be
sufficiently low—anywhere from zero to 20°—the fall will be in the
shape of snow; that Lows that only skirt our northern border will be
deficient in precipitation, even if they cause any at all; that the
slow settling of a High over the South Atlantic States means heat for
all the rest of the country east of the Rocky Mountains in degree
that will be dependent upon the magnitude and the intensity of the
southern High; that the heat will continue, even if temporarily
interrupted by showers, so long as this High retains its location in
the southeast; that tornadoes occur in the spring of the year when
Lows have excessive heat and humidity in their southeast quadrants;
that V-shaped Lows cause violent local storms, if not tornadoes, and
often deluges of rain; and that frosts may be expected in the country
when a minimum temperature of 40° is forecast for the city; and that
the severity of cold waves modifies as they come eastward, and that
they will only flow as far south as the area covered by the Low that
preceded them,—that is to say, by that part of the Low included in
the thirty-inch isobar, or by a close approximation to such area.
National Forecaster E. H. Bowie, known to the writer as one of
the ablest forecasters ever developed by the Weather Bureau, in a
recent most valuable publication by the Bureau, entitled “Weather
Forecasting in the United States”, formulates rules for forecasting
as follows:
1. When there is an area of high pressure over the southeast and
a cold wave in the northwest threatens, there will be a storm
development in the southwest and precipitation will be general.
2. If a storm form in the southwest and be forced to the left of a
normal track (Charts 10 and 11), another storm will immediately begin
to develop in the southwest and it becomes a sure rain producer.
Storms that develop in the southwest and move normally are quickly
followed by clearing weather.
3. Troughs of low pressure moving from the west are of two types—the
narrow and the wide. The former moves eastward slowly and storm
centers develop in the extreme northern and the extreme southern
ends. When the trough is wide, the development of an extensive storm
area is not uncommon, especially if the wide intervening area between
the Highs shows relatively high temperatures.
4. When the northern end of a trough moves eastward faster than the
southern end, the weather conditions in the south and southwest
remain unsettled and the chances are that a storm will form southwest
of the High that follows. When the southern end moves faster than the
northern end, settled weather follows.
5. Storms that start in the northwest and move southeastward do not
gather great intensity until they begin to recurve to the northward.
At the time of recurving they move slowly, as a rule, and care must
be exercised in predicting clearing weather.
6. Marked changes in temperature in the southeast and northwest
quadrants imply an increase in the storm’s intensity. Small
temperature changes do not indicate a further development of the
storm.
7. Abnormally high temperatures northwest of a storm indicate that it
will either retrograde or remain stationary.
8. East of the Rocky Mountains, a storm which moves to the left of
its normal track increases in intensity.
9. Storms with isobars closely crowded on the west and northwest
generally move slowly and to the east or southeast, and the
precipitation and high winds are maintained unusually long in the
northern and western quadrants.
10. Storms with the isobars closely crowded in the south and
southeast quadrants move rapidly northeastward and the weather
quickly clears after the passage of the storm center.
=Rules for Making Local Forecasts.= As an illustration of what may
be done by the local observer or the layman in formulating rules
of weather forecasting for his immediate vicinities, the following
rules, which were evolved by the writer in 1892, while serving as
the Weather Bureau local forecaster for Milwaukee, Wisconsin, are
subjoined:
1. In summer warmer weather occurs after the center of the Low has
passed a little to the east, and southwest winds are blowing, because
the easterly winds, which otherwise would be the warmest winds, are
cooled by passing over the lake.
2. A Low from the northwest that reaches western Minnesota and
western Iowa without precipitation or clouds will pass over Wisconsin
as a dry Low, unless the isobars are closer than five eighths of an
inch.
3. Light frosts will occur on clear, quiet nights in the cranberry
marshes when minimum temperatures at Duluth and La Crosse fall to 40°
and 45° respectively. When these stations record five degrees lower
the frost will be killing in the cranberry marshes and light in the
tobacco fields of the southern counties of the State.
4. No frost will occur in the counties bordering on Lake Michigan
until the temperatures at the Weather Bureau stations fall close to
the freezing point, such is the influence of the lake in storing up
heat and slowly radiating it during the night; and on the eastern
side of the lake its protecting influence is much greater.
5. When the wind sets in from points between south and southeast and
the barometer falls steadily, a storm is approaching from the west
or northwest, and its center will pass near or north of the observer
within twelve to twenty-four hours, with wind shifting to northwest
by way of south and southwest. When the wind sets in from points
between east and northeast and the barometer falls steadily, a storm
is approaching from the south or southwest, and its center will pass
near or to the south of the observer within twelve to twenty-four
hours, with wind shifting to northwest by way of north. The rapidity
of the storm’s approach and its intensity will be indicated by the
rate and the amount of the fall in the barometer.
=Vast Extent of the Area Brought Under Observation.= It is a
wonderful panoramic picture of atmospheric conditions which, by the
aid of the electro-magnetic telegraph and two hundred simultaneously
reporting stations, is presented to the eye of the forecaster. Each
day the kaleidoscope changes and a new graphic picture comes into
view. Nowhere else in the world can the student of the weather find
such opportunities.
Early meteorologists studied only the storm of low levels and humid
airs, where convection only needed to carry the moist air currents to
but a slightly higher elevation before cooling by expansion would
produce condensation and an immediate acceleration of the cyclone by
the liberation of latent heat within the region of the upward-moving
air in its central area. They never had seen the cyclones of the arid
northern Rocky Mountain plateau move down to our Great Lakes with
rapidly increasing energy, notwithstanding the fact that there had
been little condensation, and hence no addition of the latent heat
that Espy supposed was essential to a continuation of storms.
The widely differing elevation, topography, temperature, and moisture
of the broad region under observation by the United States Weather
Bureau present conditions unequaled for the study of every phase
of storm development and translation, or at least such as may be
comprehended from data taken on the bottom of the atmospheric ocean;
and it is but a matter of a short time when the data for extremely
high levels will be added.
Here we see summer cyclones formed under the intense solar radiation
that beats down through a nearly diathermanous atmosphere upon the
wastes of the Rocky Mountain plateaus; cyclones that, if they form
in the northern part of the plateau region, move eastward to our
Lakes and thence eastward to the St. Lawrence with scant rainfall;
cyclones that, if they have their origin farther south in the region
of Colorado, move into the Ohio Valley and thence to New England with
considerably more precipitation; and cyclones that, if they have
their origin anywhere in our southwest States or Texas, or enter
our region of observation from the South Pacific Ocean, can always
be expected to cause general rainfall when they reach the Lower
Mississippi Valley and later as they pass up through the central
portions of the continent.
Here also one may view the great winter cyclones that originate in
the Pacific between Hawaii and the Aleutian Islands and come under
our vision as they successfully surmount the formidable barriers
of the Rocky Mountains with but little diminution of energy, sweep
across our continent with increasing force and heavy precipitation,
and within three days pass beyond our meteorological horizon at the
Atlantic seaboard only to be heard from several days later as boreal
ravagers of Northern Europe.
The great anti-cyclones that constitute the American cold waves drift
into our territory from Canadian Northwest provinces, and are studied
under rapidly changing conditions during three thousand miles of
their course.
West Indian hurricanes, at sea level and in humid air, which are
the most violent of all storms except the American tornado, intrude
themselves into the domain covered by the weather map at Florida or
the East Gulf coast and usually pass off to the northeast with high
winds skirting our southern coast stations.
=Permanent Highs and Lows in the Pacific Are Great Centers of
Action.= Near the end of Chapter XII reference is made to the fact
that there is a barrier in the Pacific Ocean that interferes with the
movement of storms from the Orient, but which does not entirely stop
their progress. Extensive Highs and Lows, sometimes called “Centers
of Action” because they do not migrate like the traveling Highs and
Lows that cause the alternations of weather that we experience from
day to day, are also called Sub-permanent Highs and Lows. They are
the parent systems out of which come many of the Highs and Lows that
cross the North American continent, and they act as a bar to the free
passage of storms from the Far East. As these Sub-permanent areas
shift their centers a little to the north or to the south they change
the character and the line of movement of the storms and cool waves
that come to us, and they alter the general character of the weather
for thousands of miles to the east of them. In the region of Iceland
is the center of an extensive Sub-permanent Low that has much to do
in controlling the weather of Europe, and there is a Sub-permanent
High central at or near Bermuda in the southern part of the North
Atlantic Ocean. Whenever the latter is built up by having a migrating
High from the North American continent join with it, the whole
United States experiences what is called a “hot wave”, and the heat
continues as long as this Sub-permanent High remains unusually high
and extends its western limits to include our South Atlantic States.
The matter in the foregoing paragraph is so important that it will
be restated in slightly different form: Whenever either the High
or the Low Center of Action (Sub-permanent High and Low), out of
which comes nearly all of the migrating Highs and Lows, shifts its
normal seasonal position, then storms are erratic and unusual weather
occurs over the North American continent and farther eastward. The
reason why much the greater number of the storms that cross the
United States, the Atlantic Ocean, and Europe originate either in
our Rockies, the Canadian Northwest, or just off the Alaskan coast
is due to the fact (Chart 1, page 99) that the Low center of action
is normally over the middle and northern Rocky Mountain plateau in
summer, and over the Aleutian Islands (Chart 2, page 100) in winter.
The High that follows the migrating Low in winter either separates
from the center of action central over the Canadian Rockies (Chart
2), or from the one central at Honolulu; if from the latter, the
weather will be simply cooler after the passage of the Low, but if
the High separates from the center of action in the Canadian Rockies
it will constitute a cold wave as it follows a Low southeastward into
the interior of the United States and then eastward to the coast.