THE STORM CYCLE
Doubtless those who hope for a Hereafter of unmitigated ease and song,
desire, on this earth, one long, sweet anticyclone. But theirs, in most of
the United States, is disappointment. With an irregularity that seems
perversely regular at times our fair weather is interrupted by a storm
which in turn gives way to some more fair weather or another storm,--there
is no telling which very long in advance. And that is why American weather
ranks high among our speculative interests.
To emphasize this irregularity a seemingly regular succession of events
may be noticed. It will cloud up, let's say, on a Sunday, rain on Monday
and Tuesday, clear on Wednesday, staying clear until Sunday when it will
cloud up for the repeat. During this past season it rained on a dozen
washdays in succession. The newspapers grew jocular about it. And very
often one notices two or three rainy Sundays in a row. By actual
observation this year we enjoyed fifteen clear Thursdays in succession in
a normal spring.
The weather gets into a rut. And if the anticyclones and cyclones were all
of the same intensity it is conceivable that the rainy Sundays might go on
until the Day of Rest was changed by Statute. But the intensities of the
whirls differ. Before long an anticyclone feebler than ordinary is
overtaken by a cyclone and annihilated. Or one stronger than the average
may dominate the situation for several days. Or the great body of cold air
in winter over the interior of Canada may send a succession of moderate
antis across our country making a barrier of dry cold air through which
the lurking cyclone can not push.
Mostly, however, three days of anticyclonic influence and three days of
cyclonic influence with one day in between for rest, the transition
period, make up a normal week of it. Let the American farmer thank his
stars (and clouds) for that. For no other regions of the earth are so
consistently watered and sunned all the year round as the great expanse of
the North American Continent.
The cyclone is that activity of the atmosphere which prevents us from
suffering from an eternal drought. The cyclone is an accumulation of air
which has become warmer than the air about it. This area of air usually
has a central portion that is warmest of all. Since warmth expands, this
air grows lighter and rises. Nature, steadfast in her grudge against a
vacuum, causes the surrounding air to rush it. Since these contending
currents cannot all occupy the central area at once they fall into a vast
ascending spiral that spins faster and faster as it approaches the center.
Imagine an inverted whirlpool. It is a replica on a much smaller scale of
the great polar influx, except that the latter has a descending motion.
The cyclone thus is tails to the anticyclone's heads, the reverse of the
coin. Where the anti's air was cool and dry the cyclone's is warm and
moist. The anti had a downward tendency and a motion, in our hemisphere,
flowing outward from the apex in generous curves in the direction of clock
hands. The cyclone has an upward tendency, flowing inward to the core
contrariwise to clock hands.
From these two great actions and reactions come all the varieties of our
weather. To understand the procession of the cyclones and anticyclones
across our plateaus, our mountains, our plains, and our eastern highland
is to know why, and often when, it will be clear or not. To mentally
visualize the splendid sweep of the elements on their transcontinental run
is to glimpse grandeur in the order of things which will go far to offset
the petty annoyances of fog or sleet. Ignorance may be bliss, but
knowledge is preparedness.
The anticyclone suggests a pyramid of cold, dry air. The cyclone suggests
a shallow circular tank in leisurely whirl. But all comparisons are
misleading and a caution is needed right here. For a storm is not a
watering cart driven across our united skies by Jupiter Tonans Pluvius. It
is NOT a receptacle from which rain drips until the supply is exhausted. A
cyclone is a much more delicate operation than that. It is a process. It
can renew itself and become a driving rain storm after it had all the
appearance of being a sucked orange for a thousand miles.
Suppose that our cyclone, this organization of warm, moist air with its
curving winds, enters the state of Washington on a Wednesday, from the
North Pacific. As early as the Monday afternoon before the wind throughout
all that section of the country would have shifted out of the west and
have started to blow in some easterly direction,--northeast in British
Columbia and southeast in lower Idaho. But since these winds are blowing
from the interior they are dry, and consequently rain does not fall much
before the storm center is near, that is on the Wednesday. If the storm
center passes north of Tacoma the winds, shifting by south and southwest,
bring in the ocean moisture and heavy rain commences which continues until
the rising barometer and westerly winds indicate the approach of another
anticyclone. So much for western Washington.
As the cyclone passes eastward it mounts the Cascades and its temperature
is lowered, its moisture is squeezed out, and it stalks over Montana, the
mere ghost of its former self, as far as energy and rainfall are
concerned. To be sure it preserves its essential characteristics of
relative warmth, and inwhirling winds. But let it continue. As its
influence begins to be felt over Wisconsin and the Lake region the moister
air is sucked into the whirl and rain, evaporated from Superior, falls on
Minnesota. The east winds are the humid ones now, the west ones the dry.
Eastward the center moves, over Indiana, Ohio, New York, the rainfall
steadily increasing as the ocean reservoirs are tapped.
The first time you tell a New Englander that his easterly storms come from
the west you are in danger, unless he be a child, for it is to the
children that one may safely appeal. Indeed it is the increasing number of
children who are learning these fundamental weather facts in the public
schools that the Weather Bureau relies upon for a more intelligent support
in the next generation. They teach their parents. These latter find it
difficult to believe, however, that the storms which hurl the fishing
fleets upon the coast in a blinding northeaster have not originated far
out at sea, but have come across the continent. For the safe handling of
boats knowledge of the rotary motion of storms is necessary that one may
be able to tell by the direction of the winds and the way they are
shifting where lies the center of the storm and its greatest intensity.
In Tacoma when the wind shifted by way of southeast, south, and southwest
that was proof that the storm center was passing north of the city.
Likewise if in New York the winds shift by way of northeast, north, and
northwest the storm center is passing south of that city. As it drifts out
to sea it is gradually dissipated by the changing influences on the North
Atlantic. Very few of our storms ever reach Europe, although some have
been traced to Siberia.
The Government has put its sleuths on the track of every storm that has
crossed the United States in the last thirty years. These weather
detectives with a thousand eyes have made diagrams of their actions,
mapped their courses, computed their speeds, and if we don't know where
all our discarded storms go to, we at least know where most of them came
from and how they acted when with us.
About a hundred and ten areas of low-pressure affect the country during
the normal year. Of these all but seven, speaking in averages, come from
the West so that the Boston mechanic who will not believe that the
nor'easter comes via the Mississippi Valley is right about 7/110 of the
time. But even that small fraction is no exception to the general law,
because those seven storms are not born in Newfoundland but in our East
Gulf States. They come up the Coast, and the wind blows from the northeast
and north into their centers while they are still on the Carolina coast.
The great hurricanes which are cradled in the tropics and march westward
under the influence of the trades are genuine exceptions to the general
westward rule, although they always eventually turn toward the east. They
will be given the prominence they demand later, since the eastbound
schedule must not be sidetracked now.
[Illustration: CIRRO-CUMULUS TO ALTO-STRATUS
_Courtesy of Richard F. Warren_
The wispy edges of the cloud at the brightest part are cirrus, the fleecy
cloud at the extreme top is a thin alto-cumulus, and the dark base of the
sky is stratus. But this stratus is too high for that classification and
so they call it alto-stratus. This sky shows that the temperatures are
moderate, a cold sky being much better packed, and a warm one fluffier.
The fact that a veil of cirrus has not preceded the heavier clouds argues
that the coming storm will not be of much consequence. This sort of cloud
bank arising after a period of cold weather is the best possible
prediction of a thaw. Slight rain might follow within a few hours.]
Three cyclones a year form over the lower Ohio River basin. On account of
their origin over land instead of over water they rarely acquire much
energy. Once in a decade such depressions deepen rapidly. It was one of
these Ohio River storms that increased greatly in energy while moving from
West Virginia to the Jersey Coast that gave Philadelphia her Christmas
Blizzard, a surprise to her citizens and to the Weather Bureau, for most
of the snow fell with the mercury above freezing. The flare-back which
gave Taft his big inaugural snowstorm is another example of the way a
depression may deepen on approaching the coast. Until the upper atmosphere
is as well understood and watched as the lower, or until instruments are
perfected whereby the weather conditions can be made self-announcing such
surprises are absolutely unavoidable. Under conditions that warrant any
suspicion of sudden developments the Bureau at Washington is careful to
order extra observations in the areas likely to be affected, but no
surface observations can quite suffice.
Fifteen storms a year originate over the west Gulf States, or, drifting in
from the Pacific over Arizona and New Mexico begin to acquire energy in
Texas. Twelve are set up over the Colorado mountains. These usually dip
down into Texas before starting their drive toward the northeast. After
both these sets of storms get under way they strike resolutely for the
same locality,--the St. Lawrence Valley. The conformation of the St.
Lawrence region provides an irresistible attraction for American storms.
Occasionally a very strong anticyclone holds that territory and pushes the
cyclone off the coast at Hatteras or even makes them drift across the
country to Florida. But such occasions are exceptional. Give the ordinary
cyclone its head, and, ten to one, you will find it on the way to the St.
Lawrence. The inhabitants will confirm this statement, I am sure. They do
not feel discriminated against in the matter of weather. They get nearly
everything that is going. Since they have to accommodate from seventy to
eighty cyclones in fifty-two weeks they have very little time to brood
over any one variety of weather. With the optimism of that section of the
country they say, "If you don't like our weather, wait a minute."
Ten storms a year originate over the Rocky Mountain Plateau, north of
Colorado. About twenty cross over from the Canadian Provinces of Alberta
and British Columbia. And all our other storms, about forty each year,
enter our country from the North Pacific by way of Washington and Oregon.
Many of these drift across the northern tier of states without any great
display of energy, at least before they reach the Lake region. But the
majority loop down somewhat into the middle west as far south as Kansas,
and then make their turn toward the inevitable St. Lawrence. They usually
require four days to make the trip from coast to coast by this route, as
also by the more direct northern route, because on that they travel more
at leisure. But the storms from Texas, whose energy is greatest because of
greater heat and moisture, occasionally speed from Oklahoma to New York in
thirty-six hours.
In summer all speeds are reduced. This is because the disparities in
temperature are less. In winter where greater extremes of temperature are
brought into conjunction the processes of the storm are all more violent.
And it is a bit disheartening to know that a storm is aggravated to even
greater endeavors by its own exertions. Its energy provides the conditions
to stimulate greater energy, and, like a fire, it increases as it goes. If
it did not run out of the zone which nourished it and proceed into another
zone where conditions were distinctly discouraging the limits of the
storm would be much extended, and vast territories would be devastated by
the self-propelling combination of wind and water.
To the generality of us the word storm means rain. To the scientist it
means wind. In reality the cyclone is rare that crosses our country
without causing rain somewhere along its track. The curiosity of the
Weather Bureau to find out the paths of the storm centers is abundantly
justified because it is along these paths that the heaviest rainfall and
the severest winds occur. But whether or not there is precipitation on the
path of the cyclone it is rated as a storm if there is a lowering of
pressure and consequent wind-shift.
The storm centers are not always well-defined, and quite often the
circulation of the wind about them is not complete. Such cyclones never
amount to much, although there is always the possibility of their closing
in and developing a complete circulation with the attendant increase of
energy. The incomplete cyclones over the desert and plateau regions are
lame affairs, lacking interest and advancing timidly if at all. But once
let them drift into a locality where they can be supplied with moist air,
they pick up energy, keep a definite course, and advance with increasing
speed.
Very often the center will split up, the circulation perfecting itself
around both centers of depression. One of these will likely be over
Minnesota and the other over Texas and the organization will steam-roller
the states to the east in the manner of a gigantic dumb-bell. This
formation is more likely to have been caused by the two centers appearing
simultaneously than by a split in an original center. The weather reports
call this fashion of storm a trough of low pressure. The southern center
is the one that develops the more energy on its turn to the northeast. If
the two centers should unite on reaching the northeast a very heavy
precipitation is the invariable result.
All cyclones have much greater length than breadth. They frequently
stretch from unknown latitudes in Canada into unrecorded distances into
the Gulf, while on the other hand it is a very large storm that rains
simultaneously upon the Mississippi and the Atlantic. Behind a cyclone of
pronounced energy a second whirl, called a secondary depression, often
develops, in which case the period of wet weather is prolonged. Also, more
rarely, an offshoot forms ahead of the main depression.
A sluggish, sulky cyclone either in winter or summer provides more
opportunity to humanity for self-discipline than almost any other feature
of our national environment. In winter when the depression slows up it
settles down upon one community in the guise of fog, and stays by the
locality until an anticyclone blows in and noses it out. Fog is
aggravation, but a hot wave is suffering and the hot wave is caused by a
depression weak in character but generous in dimensions getting held up on
the northern half of our country. By its nature it attracts the air from
all sides, and being in the north, the direction of the wind over most of
the country would be southerly. Air from the west and north has a downward
tendency, but south and east winds are surface currents. Consequently
these winds, blowing over leagues of heated soil, become dry and parching.
If the depression lingers long the entire country to the east, south, and
west soon suffers from superheated air. At last the very intensity of the
heat defeats itself and the reaction to cooler is effected dramatically
through a thunderstorm.
The well-developed cyclone in winter causes what we all know as a three
days' rain, although continuous precipitation rarely lasts over ten
hours. The rest of the time is occupied by general cloudiness with
occasional sprinkles and a final downpour as the wind shifts to the west
and the anticyclone nears. In summer the depressions, being shallower,
rarely cause continuous cloudiness for three days, although their
influence often lasts as long as that in the guise of a series of
thunderstorms. The line of storms extends several hundred miles,
bombarding all the towns from Albany to Richmond. These thunderstorms
sometimes achieve in an hour or two even greater results than their winter
relatives can accomplish in three days in the matter of rainfall, wind
velocity, and general destructiveness. Our wettest months are July and
August and not December and January.
The freedom of the wind has been the subject of much poetic and prosaic
license. As a matter of fact the wind is the veriest slave of all the
elements. It is harried about from cyclone to anticyclone, wound up in
tornadoes, directed hither and thither by changing temperatures. It blows,
not where it listeth, but where it has to. And circuitously at that. For
once the path of duty is not straight. That is another fact that the
Boston mechanic would have been slow to accept,--that the wind blows in
curves. A little consideration, however, of the fact that the wind is
perpetually unwinding in great curves from the anticyclone and winding up
on the cyclone will show that nowhere can it be blowing in a perfectly
straight line.
Thus it becomes the surest indication that a cyclone is to the west of one
if the wind blows from an easterly point. The storm is bound to move
toward the east, therefore the rapidity with which the clouds move and
thicken will signify when the area of precipitation will reach the
observer. The cycle of the storm is normally this: After a cloudless and
windless night a light air springs up from a little north of east. At the
same time strands of thin wavy clouds appear, very high up. They may be
seen to be moving from the southwest or northwest. Their velocity is
great. Their name is cirrus, and they are called mares' tails by the
sailors. They are followed by several hours of clear skies, usually; but
if the storm is smaller and close at hand there is no clear interval.
Before the larger storms these cirrus clouds are sent up as storm signals
twenty-four and even forty-eight hours in advance. The day that intervenes
is very clear, the air feels softer, the temperature is higher. In
midafternoon more cirrus appears, and as condensation follows the quick
cooling the silky lines increase in number. Beneath them a thicker
formation, known as cirro-stratus, forms a dense bank in the west and
southwest. The sun sets in a gray obscurity. If there is a moon it fades
by degrees behind the veil of alto-stratus, and the halo which first was
seen wide enough to enclose several stars narrows until it chokes the moon
in its ever-thickening cocoon of vapor.
There is no value whatever in the old superstition that the number of
stars within the halo foretells the number of days that it will rain or
snow. The same halo that encloses three stars at eight o'clock may have
narrowed down to one by midnight, or none at all, so that the prophetic
circle is bound in the very nature of its increase to contradict itself.
The presence of a halo is a pretty sure sign of some precipitation within
twenty-four or thirty hours. It fails about thirteen times in a hundred.
If the halo is observed around the sun it is an even surer sign, failing
only seven times the hundred.
During the time of cloud-increase the wind will probably lull before a
snow, so that the hour or so before precipitation begins is one of intense
brooding calm. Or if there is no calm the wind, now easterly, will be very
gentle. Soon after the precipitation begins the wind will begin to
freshen and will continue to increase in velocity until the center of the
storm is close to the locality. This will require about eight hours for
the average storm. As storms vary an average is a very misleading thing
and the best way to judge of the length and severity of the storm is by
watching the wind. If it increases gradually the storm will be of long
duration. If the wind rises fitfully and swiftly it will not likely be
long but may be severe. If the wind reaches any considerable velocity
before the rain or snow begins the storm is sure to be short and severe.
The color and formation of the clouds will tell when the precipitation is
about to begin. In summer, no matter how striking and black are the shapes
and shadows of the clouds, rain will not fall until a gray patch, a
uniform veil called nimbus is seen. In the little showers of April this
patch of unicolored cloud is there, as well as behind the great arch of
the onrushing thunderstorm. In winter raindrops are smaller and the
tendency of the clouds is to appear a dull, uniform gray at all times. But
the careful observer can detect a difference between the nature of the
clouds several hours before precipitation and their color immediately
before.
When snow is about to fall no seams are visible. An impenetrable film
obscures all the joints. From such a sky as this snow is sure to fall. But
if seams are visible, if parts of the skyscape are darker than others,
then, no matter whether the temperature on the ground is below freezing a
rain storm will ensue. Very often these winter rains begin in snow or
sleet, but the clouds register the moment when the change from snow to
rain is to be made. The presence of swift-flying low clouds from the east
is a certain sign that the change to a temperature above freezing has been
effected in the upper strata of the atmosphere. This variety of cloud is
called scud, and accompanies rain and wind rather than foretelling it long
in advance.
If the storm is approaching from the southwest the precipitation begins
near the coast about twelve hours after the cirrus clouds commence to
thicken and about twenty-four after they were first seen. In some
localities as much as thirty-six and even forty-eight hours are sometimes
required for the east wind to bring the humidity to the dew-point. Just a
little observation will enable one to gauge the ordinary length of time
required to bring things to the rain-pitch in one's own country. Of course
no two storms in succession make the trip under the same auspices and
with the same speed. The sign of the Universe should be a pendulum.
One period of cyclone, anticyclone, cyclone will traverse the country
rapidly. Then there will be a halt all along the line, and the next
series,--anticyclone, cyclone, anticyclone, will take three days longer to
make the crossing. Otherwise our weather would have a deadening
regularity.
On an average our storms cross the country at the rate of about six
hundred miles a day. This is the average. Some delay, linger, and wait for
days over one locality. Others do a thousand miles in the twenty-four
hours. They thicken up enough to cause rain from two hundred to six
hundred miles in advance of their centers. It stops raining not long after
the actual center has passed.
But for picnic purposes the storm is far from being over. For even though
continuous raining has stopped the low pressure still induces a degenerate
sort of precipitation called showers, or oftener mist for another twelve
hours (usually in winter). Then as the cooling influence of the
anticyclone approaches the rain recommences. This time it is not for long,
however, and is followed by permanent clearing, the wind shifting into the
west. Sometimes the change to blue sky is abrupt. But if the subsequent
anticyclone is not very well defined, cloudy conditions may linger for a
couple of days. Such clouds are usually much broken and show white at the
edges and never cause more than a chilly feeling.
This attempt to outline the customary cycle of the storm,--clear sky,
cirrus cloud, wind-shift to the east, the denser cirro-stratus, the
pavement-like stratus, the woolly nimbus, the first continuous hours of
rain, the misty interval, the windshift to the west, the final shower, and
breaking cloud, the all-blue sky--this storm-schedule is always subject to
change. But the fundamentals are there in disguise every time. They only
have to be looked for and there is some satisfaction in penetrating the
disguise.
When a storm comes up the Atlantic Coast, as happens a few times a winter,
the process is shortened, because the effects of the larger easterly
quadrants are felt only at sea. The most prominent recent illustration of
this type of storm was the severe snowstorm that swept the coast states
from Carolina to Maine the Saturday before Easter, 1915. Its calendar read
as follows: Friday, 8 P. M., cirrus clouds thickening into cirro-stratus.
Midnight, stars faintly visible, wind from northeast, 12 miles an hour.
Sunrise, stratus clouds, wind rising in gusts at Philadelphia to 30
miles; 8 A. M., rapid consolidation of clouds with snow shortly after,
although the temperature at the surface of the earth was as high as seven
degrees above the freezing point. This rapidly dropped to freezing. Flakes
were irregular in size. Until one o'clock in the afternoon the snow
thickened with gusts of wind up to forty miles. Snowfall for five hours
was 14 inches, an unprecedented fall for this locality.
Then the storm waned for five hours more, 5 inches more of snow falling.
Precipitation practically ceased at 6 P. M. By sunrise on Sunday the skies
were free of clouds and the wind blew gently from the northwest.
Occasionally a high pressure area out at sea and beyond the ken of the
Weather Bureau causes one of these coast storms to curve inward to the
surprise of everybody. Occasionally, too, the transcontinental storms are
driven north or south of their accustomed paths. While the divergence may
be slight, it causes a margin of variance from the accuracy of the
Bureau's report. Then arises a second storm,--one of indignation--from all
the people on one side of the strip who carried umbrellas to no purpose,
and from the others,--who didn't.
This pushing aside of the cyclone is caused by pressure variation that
only hourly reports from many localities could detect. Vast hidden
influences shift the weights ever so little and the meteorological express
is wrecked. But this happens, at most, fifteen times in a hundred, and
remembering the unseen agencies to be coped with people are refraining
more and more from the tart criticisms of former times, not in charity but
in justice, although there is small tendency yet to forward eulogies to
the Bureau in recognition of the eighty-five times it is right.