3. 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.
CIRRO-CUMULUS TO ALTO-STRATUSCourtesy 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.