THE CLEAR DAY
We owe our fair weather to that department of atmospheric activity called
anticyclone by the weatherman. The anticyclone is an accumulation of air
which has become colder than the air surrounding it. This accumulation
oftener than not has an area near the center where the air is coldest.
About this coldest area the air currents revolve in the direction of a
clock's hands. And since this cold air is contracted and denser than its
warmer environment it has a perpetual tendency to whirl outward from the
center into this warmer environment.
One comes to think, therefore, of the anticyclone as a huge pyramid of
cold air moving slowly across the country from west to east and all the
while melting down on all sides, like a plate of ice-cream, into the
surrounding territory. It is such an immense accumulation that often while
its head is reared over Montana the first shivers of its approach are
beginning to be felt in Texas and Pennsylvania. It does not extend
equally far, however, to the north and west of its head, which is really
sometimes where its tail ought to be. That is, a long slope of increasing
pressure and cold will sweep in a gentle gradient from Pennsylvania to
Montana and will then decrease by a very steep gradient to the Pacific
Coast.
The anticyclone draws its power from the inexhaustible supplies of cold
air from the upper levels. This air is very dry and accounts for the
almost invariably clear skies of the anticyclone.
In winter when the intensity of all the atmospheric activities is greatly
increased, the anticyclone develops into the cold wave. The rapidly rising
pressure rears its head and rushes along upon the heels of a storm like a
vast tidal wave at sixty miles an hour, tumbling the mercury thirty,
forty, fifty degrees.
These cold waves first appear in the northwest. They cannot well originate
over either ocean and a high-pressure area building up over the southern
half of the country will not attain the sufficient degree of frigidity to
earn the title, for even cold waves have been standardized by the
Government. But although nearly all the cold waves choose Montana or the
Dakotas as a base, they have at least two definite lines of action. Those
which are born amid the mountains or on the great plains of Montana have a
curious habit of bombarding the Texas coast before starting on their
eastward march. It is not unusual for us to read of zero weather in the
Panhandle and freezing on the Gulf while the mercury may still be standing
as high as fifty in New York City.
It is this rapid onslaught from Montana to Texas that produces those
notorious blizzards of that section called northers, during which the
cattle used to be frozen on the hoof. The record time for a drive of this
extent is about twelve hours and the normal about twenty-four which gives
scant time for the Weather Bureau to warn the vast interests of the
impending assault. When the cold wave, after following this path, does
swing toward the Atlantic Coast, as most of them do, it has lost interest
and usually produces only seasonably cold weather along the Appalachians.
Those cold waves that recruit their strength in Canada and enter the
United States through Minnesota or, rarely, this side of the Lakes move
along the border and supply intensely cold weather for a night or two to
New England and the Middle Atlantic States.
Cold waves almost always follow a storm. The storm, being an area of low
pressure makes a fit receptacle for the surplus of the high pressure, and
since the whole business of the weather is to seek peace and pursue it,
the greater the discrepancies the more violent the pursuit. Consequently
we have the spectacle of a ridge of cold dry air following and trying to
level up a fleeing hollow of warm moist air--but rarely succeeding. This
principle of action and reaction is almost the sole principle of the
weather and is nowhere more clearly demonstrated than in the winter's
succession of storm and cold wave.
In summer the anticyclones are not only actually but relatively more
moderate than in winter. But their influence is still the same,--clear
skies, cooler nights, dry, westerly winds. During the year the anticyclone
furnishes us with about sixty per cent. of our weather. The cyclone is
responsible for the remaining forty per cent. The weather depends on the
cyclone for its variety and upon the anticyclone for its reputation. So it
is well to be able to recognize an anticyclone when one appears.
The first and most reliable symptom of the approach of an anticyclone is
the west wind. This sign is valid the country over, and is one of the
very few signs that hold true for most of the North Temperate Zone. In
summer over our country the west wind comes from the southwest, to be
Irish, and in winter from the northwest. But for nearly all of our
forty-eight states for nearly all of the year the westerly winds are those
that bring us fair days and nights. And it is these crisp, clear days and
cloudless, brilliant nights which we have in mind when we boast to English
friends of our American weather.
The west wind is so popular because it has a slight downward flowing
tendency. It also blows from land to sea over all America except the
narrow Pacific coast. These downward, outward directions allow it to
gather only enough moisture to keep it from becoming seriously dry. Its
upper sources supply it with ozone. Its density gives it weight and by its
superior weight it prevails. It dries roads faster than a brace of suns
could do it. It is tonic. And curiously enough, although the anticyclone
loads half a ton excess weight upon us we like it. The greater the burden
the more we feel like leaping and shouting. Our good cheer seems to be
ground out of us, like street pianos.
The reverse holds, too. For when the anticyclone moves off us and the
cyclone hovers over us, removing half a ton of pressure, instead of
feeling relieved we feel depressed, out of spirit. The animals share this
reaction with us. In fact barnyards antedated barometers as forecasters,
because all the domestic creatures, with pigs in particular, evidenced the
disagreeable leniency of the low pressure areas upon their persons.
"Grumphie smells the weather
An' Grumphie smells the wun'
He kens when clouds will gather
An' smoor the blinkin' sun."
The only trouble about this rather extravagant tribute to the pig,
versatile though he is, is that he can tell only a very few hours ahead
about the coming changes and it takes so much more skill to judge what his
actions mean than to read the face of the sky that the science of
meteorology finally comes to supplant barnyardology.
The coming of the anticyclone is foretold by the shifting of the wind from
any quarter to the west. The course that the center of the anticyclone is
keeping may be watched by the same agency. Since the circulation from the
cone of cold air follows the hour hands of a clock it follows that if the
center is moving north of you the wind, blowing outward from the center,
will work from west to northwest and from northwest to north and slightly
east of north.
If the wind has shifted into the west on a Wednesday, it will likely be
cold by Wednesday night and colder on Thursday. By Friday morning the wind
will be coming from the north, likely, with the lowest temperature of all.
By Saturday the cold will moderate, the wind will tire and gradually die
to a calm or become weakly variable. The four day supremacy of the
anticyclone will be over. But, mind you, there are a dozen variations of
this routine. I am only suggesting a usual one.
If after blowing two or three days from the west the wind shifts to the
southwest and south, you may know that the central cold area is passing
south of you and that its intensity will not be great. While these
anticyclones that float down and to the right of their normal path linger
longer, they are never so severely cold, nor, alas, so uniformly clear as
the others. It is a profound law of anticyclones and even more
particularly of cyclones, that if they deviate to the right they weaken,
if they are pushed by an obstacle to the left they increase greatly in
intensity.
Occasionally the central portion of an anticyclone passes over your
locality. Then the wind will fall. The frost will be keen and the cold
will be notably dry and invigorating. In summer although the sunlight may
be powerfully bright and the heat great, yet the air will have a buoyant
effect, the body a resilience. And the nights will cool swiftly. Soon
after the center passes from the locality a wind will spring up from the
east with rapidly rising temperature and increased humidity.
The coldest part of the anticyclone is not, as one would suppose, at the
center, but in advance of it; and its authority, like a schoolmaster's, is
rapidly dissipated after its back is turned upon a place.
The intensity of an anticyclone is measured by its wind velocity and by
the degree of cold obtaining under its influence. But the greatest cold
occurs rarely in conjunction with the greatest velocity of the wind. The
calms that occur at sunrise enable radiation to take an extra spurt which
pushes the mercury lower by a degree or so than happens when the wind is
blowing. But, windy or calm, the period about sunrise is normally the
coldest of the day, even extending in midwinter for as much as half an
hour after sunrise, so slow are the feeble rays at restoring the balance
of loss and gain of heat.
The greatest falls occur at the advent of the cold wave, no matter whether
it arrives at ten in the morning or at midnight. If the temperature
starts to decline gradually during the day, a further and decided fall may
be expected at nightfall if the sky is clear. And if the temperature rises
gradually during the night the normal processes are being displaced and a
change from fair to foul is a surety. In summer the hottest time of day is
not at noon, any more than the coldest part of the winter day was at
midnight, for the reason that the sun can pour in its heat faster than the
earth can radiate it, and the hour for the maximum temperature is pushed
as far along toward evening as four or five or even six o'clock.
The average anticyclone continues its influence for clearness for about
four days. Some, however, hurry the whole thing through in two. Others are
interrupted by a more vigorous cyclone and are put to rout. Others are
held up by an inherent weakness and are forced to mark time over one
locality until strengthened or dissipated. And a few great ones hold sway
over the country for a week. These choose the north-center of the country
in which to locate. There they pile up the cold air until its very weight
causes it to move majestically on. Its skirts sweep the Gulf coast where
they are a bit bedraggled by invading cyclones. It gives the New
Englanders a fortnight of nipping, brisk days and the mercury in
Minnesota and the Dakotas does not emerge above zero. Once, in Montana,
one of these refrigerating systems established the record of sixty-three
degrees below zero. But in Siberia where the immense extent of the land
surface collaborates with a prolonged night, an anticyclone built up an
area of superior chilliness that left a world's record of ninety-one
below.
In summer a succession of these highs causes the frequent droughts of
weeks which harass the West and New England. The air becomes so dry that
it parches and then shrivels the green leaves. Any little cyclones that,
under ordinary conditions, would suck in moist air from the Gulf and
relieve the situation with a rain are dried out and frustrated by the
unclouded sun. It requires a cyclone of great depth to overthrow the
supremacy of these summer anticyclones.
While the anticyclone furnishes fair weather the sky is not necessarily or
even usually free from clouds under its influence. In summer the
evaporation during the long days overloads the air for the time being.
Normally about eleven in the morning little balls and patches of white
clouds dot the blue. These increase in number and size until about three
in the afternoon when they will have grown little black bellies and
fluffy white tops. By five they will have dwindled and by eight entirely
vanished. These heaped clouds, known as cumulus, are a guarantee of a
normal atmosphere and continued fair weather. They mean that currents of
warm, moist air have risen until they have struck a level so cool as to
cause them to condense part of their moisture. This condensation sinks
until it enters a warmer stratum and the cloud is dissipated. The total
movement is a reasonable exchange that preserves the equilibrium of the
air, very much as a person bends one way and then another to maintain his
balance.
In winter there is not such an opportunity offered and the few clouds that
form because of the daily variation in temperature are flatter and are
called stratus clouds. Sometimes these stratus clouds may cover the sky at
midday, but in thin platings and not leadenly. In winter as in summer they
tend to disappear toward evening. They are often accompanied by an
unpleasant wind, but rarely by the snow flurry which is the "April shower"
of the winter months.
But when the snow flurry does come there is no better sign for the
woodsman of coming cold; it never fails. The morning will have begun
brilliantly, but soon great summery puffs of cloud form and increase and
darken on their under sides. Their tops are vague and wear a veil. It is
the snow. The reason is simple. The coming anticyclone strikes the upper
air before it hits the earth's surface. The sudden cold causes rapid
condensation. Hence the flurries. But the anticyclone is an agent of
dryness, hence their short duration. Sometimes the veil of snow does not
reach the earth. Sometimes it blots out everything in a spirited squall.
But it never lasts long, except in the northwest states. And it is
invariably followed by a period of colder weather.
In summer local evaporation may be so long-continued or so vigorous that
the cumulus clouds cannot hold all their moisture content when cooled. A
shower is the result, usually a trifling one and mostly without thunder.
The great thunderstorms are always in connection with the passing of a
cyclone. The small heat thunderstorms are only the indulgences of a spell
of fair weather. These tiny showers are daily and sometimes hourly
accompaniments of clear weather in the mountains. The air warms rapidly in
the valleys and is speedily cooled on rushing up a mountain side and a
threat and a sprinkle are the result. When a performance of this sort is
going on nobody need fear unpleasant weather of long duration.
Another pledge of a clear day that does not appear too credible on the
face of it is the morning fog in summer. In winter it is a different
matter. In August and September particularly the rapidly lengthening
nights allow so much heat to evaporate that the surplus moisture in the
air is condensed to the depth of several hundred feet. By ten o'clock the
sun has eaten into this lowest stratum, heated it and yet begins to
decline in power before the balance swings the other way, so that a
cloudless day often follows a fog in those months. About three mornings of
fog, however, are enough to discourage the sun and a rain follows. Of
course this is because the anticyclone with its special properties has
been losing power.
When these conditions of clear nights with no wind follow the first two or
three windy days of the anticyclone, particularly in autumn and spring,
frost results. In winter the chances that a fog will be dissipated are
rather slim. But if it shows a tendency to rise all may yet be well.
[Illustration: CIRRO-STRATUS WITH CIRRO-CUMULUS BENEATH
_Courtesy of Richard F. Warren_
The fine-spun lines of the cirrus proper drag this veil of whitish cloud
over the sky. The sun sometimes is surrounded by a colored halo due to the
refraction of the light by the ice crystals. But more often it vanishes
behind the veil. The mottled clouds below the veil show that a rather
rapid condensation of the moisture in the air is taking place. This sky is
distinctly threatening, although the direction and force of the wind will
more accurately foretell the severity of the coming storm. With this sky
expect rain or snow within 12 hours.]
An excellent sign of clear weather is this fact of the morning mist rising
from ravines in the mountains. And even if you haven't any mountain
ravines at command the altitude of clouds can be observed. It is safer to
have them lessen in number rather than increase, scatter rather than
combine. The higher the clouds the finer the weather. And if the sky
through the rifts is a clear untarnished blue the prospects of settled
weather are much better than with fewer clouds and a milky blue sky
beyond.
After the direction of the wind and the shapes of the clouds the colors of
the sky are a great help in the reading of the morrow's promise. And the
best time to read this promise is in the morning or evening when the half
lights emphasize the coloring.
Soon after the close observation of cloud colors has commenced the amazing
discovery is made that the same color at sunrise means exactly the reverse
of its meaning at sunset.
"Sky red in the morning
A sailor's sure warning,
Sky red at night
A sailor's delight."
Christ seized upon this phenomenon to throw confusion into the Pharisees
and Sadducees when they asked that He would show them a sign from Heaven.
As Matthew reports it:--"He answered and said unto them, When it is
evening ye say, It will be fair weather for the sky is red. And in the
morning It will be foul weather to-day for the sky is red and lowering. O
ye hypocrites, ye can discern the face of the sky; but ye cannot discern
the signs of the times."
The reasons for this contradictory evidence of color are not nearly so
obvious as the fact itself. Taking the scientist's word for it need not
stretch one's credulity overmuch if he can be followed step by step. He
says that sunlight is white light, and white is the sublime combination of
every color. If no atmosphere existed about us the light would all come
through, leaving the sky black. The atmosphere, however, which is full of
dust and water particles, breaks up these rays, these white sheaves of
light, into their various colors. The longest vibrations, which are the
red, and the shortest, which are the violet, get by and the rest are
turned back, mixing up into the color which we call our blue sky.
If the dust and water particles grow so large and numerous as to divert
more of the short rays than usual we get a redder glow than usual. This is
most noticeable when the sun and clouds are near the horizon for the air
through which they appear is nearer the earth and consequently dirtier. If
these water globules mass together so as to reflect all the rays alike the
result is a whitish appearance. That is why a fog bank, composed of tiny
droplets, each reflecting with all its might, can make the sky a dull and
uniform gray.
As evening approaches the temperature of the normal day lowers. As the
temperature lowers it is the tendency of the moisture in the air to
condense about the little dust particles in the air. And as these
particles increase in size their tendency is to reflect more and more of
the waning rays of light. Therefore if the sky is gray in the evening it
means that the atmosphere already contains a good deal of condensed
moisture. If the cooling should go on through the night, as it normally
would, condensation would continue with rain as the likely result.
If, on the other hand, after the evening's cooling has progressed and yet
the colors near the horizon are prevailingly red it means that there is so
little moisture in the atmosphere that the further increase due to the
night's condensation will not be sufficient to cause rain. Hence the
natural delight of the sailor.
A gray morning sky implies an atmosphere full of water precisely as an
evening gray does. The difference lies in the ensuing process. By morning
the temperature has reached its lowest point and if this has not been
sufficient to cause cooling to the rainpoint the chance for rain will be
continually lessened by the growing heat of the rising sun. The gray,
therefore, is the normal indication of a clear cool night which has
permitted radiation and therefore condensation to this degree. It is for
this reason that we have the heavy fogs of August and September followed
by cloudless days.
A red morning sky shows, like the red evening sky, that condensation has
not taken place to any extent. But this is abnormal for a clear night
causes condensation. The red therefore means that a layer of heavy moist
air above the surface levels has prevented the normal radiation. Hence
when the day's evaporation adds more moisture to that already at the
higher levels the total humidity is likely to increase beyond the dewpoint
with the resultant rain.
These two color auguries are among the most reliable of all the weather
signs. Unfortunately the sunrises are scarcely ever on hand to be examined
except by milkmen. But a careful scrutiny of the sunset will make one
proficient in shades. In summer when the sun burns round and clear-cut and
red on the rim of the horizon the air contains much dust and smoke, the
accompaniment of dry weather. And as dry weather has a way of perpetuating
itself such a sun makes dry and continued weather a safe prophecy. In
winter the same red and flaming sun setting brilliant as new minted gold
is a sure indication of clear and cold weather. In all seasons the light
tints of the evening sky mean the atmosphere at its best. A golden sunset,
a light breeze from the west, a glowing horizon as the sun goes down, slow
fading colors all constitute a hundred to one bet for continued fair
weather. The sunset colors that are surely followed by storm will be
discussed in the next chapter.
The sky is too little regarded. Architects that do not consider the sky
are behind in their calling. Maxfield Parrish has made himself famous by
allying himself with its seas of color. The hunter can read it and learn
whether he may sleep dry without his tent. Only we who shut ourselves
within rooms and behind newspapers forget that there is a sky--until it
falls and we are taken to a sanitarium.
From the night itself much may be discovered about the continuance of fair
weather. A sky well sown with stars is a good sign. If only a few stars
are visible the clear spell is about over. Stars twinkle because of abrupt
variation in the temperature of the air strata. If the wind is from the
west cold and clear will result no matter how much may twinkle twinkle
little star. But if he twinkle with the wind from the south or east the
cloud will soon fly. That is the way with these weather signs. One sign
does not make a prophecy. It is the combination that has strength and
reliability. Furthermore the eye must be trained by many comparisons.
Of all the conditions that make night forecasting easy the later evenings
of the moon are the best. The moon furnishes just the proper amount of
illumination to betray the air conditions. If she swims clear and
triumphant well and good. If she rides bright while dark bellying clouds
sweep over her in summer, inconsequential showers may follow. But if she
disappears by faint degrees behind a thin but close knit curtain of cloud
the clear weather is being definitely concluded.
A great many changes in the weather take place after three in the morning.
Most campers are accustomed to waking anyway once or twice to replenish
the fire, and a glance at the stars will show the sleepiest what changes
are occurring in the eternal panorama. A man may have gone to bed in
security to get up in a snowstorm, whereas a survey of the skies at three
would have noted the coming change. The habit of waking in the dead of
night,--which isn't really so dead after all,--is not an unpleasant one.
Its compensations are set forth in a beautiful and vivid chapter of
Stewart Edward White's "The Forest." Every camper knows them, and this
added mastery that a knowledge of the skies gives him lends a sense of
power, which lasts until the unexpected happens.
For the unexpected happens to the best regulated of all forecasters, the
Government. Equipped with every instrument and with an army whose business
is nothing else than to hunt down storms and warn the public, the Weather
Bureau is still surprised fifteen times out of a hundred by unforeseeable
changes in atmospheric pressure. It is scarcely likely then that amateurs
without flawless barometers and without reports of the current weather in
three hundred places could hope to foretell with complete accuracy. But
there is a place for the amateur, aside from his own personal
gratification and profit. The Weather Bureau within the limits of the
present appropriation cannot expect to predict for every village and
borough. That the amateur may do and with as great accuracy for the few
hours immediately in advance.
The Weather Bureau may predict with this large percentage of
accuracy--85%--for forty-eight hours in advance because its scope is
country wide. It may even forecast in a general way for seven days and
still maintain a considerable advantage over almanac guesswork. But the
man who is relying upon local signs is limited to ten or at most twelve
hours. Of course he may guess beyond that but it is only a guess. The work
that the Bureau does and that he may do within his limits is not
guesswork. Meteorology is an exact science, and forecasting is an art.
Both may be studied now in classes under professors with degrees in the
same way that any other science and art may be studied. The old sort of
weather wisdom which was a startling compound of wisdom, superstition, and
inanity has passed away, or is passing away as rational weather talk
spreads.
These limits of the layman--ten hours with no instruments--are further
defined by his locality. In mountainous country changes come more quickly
than in level localities, in winter than in summer, so that one's
prophetic time-limit is shortened.
While the best indications of the clear day are the great fundamental
ones, there are many little signs that bolster up one's confidence in
one's own predictions. The lessened humidity coincident with clear weather
is responsible often for many little household prognostics. Salt is dry.
The windows (of your summer cottage) do not stick. The children are less
restless. Smoke ascends, or if the wind is blowing is not flattened to the
ground. Flies are merely insects, for the time being, and not the devil.
Swallows and the other birds that eat insects fly high because that is
where the insects are. Spiders do not hesitate to make their webs on the
lawn. They welcome dew but distrust rain. Cow and sheep feed quietly,
rarely calling to one another as they do before a storm. In short the
general aspect of these is normal and therefore remains unnoticed.
But all these household prognostics may be advertising the most placid
weather while only twelve hours away and coming at sixty miles an hour may
be the severest storm of the season. The Weather Bureau with its maps and
barometers follows its every movement. The man in the woods whose comfort
in summer and whose life in winter may depend upon his preparedness for
the approaching storm does well to read its warnings and know its laws.