Part 2 (1/2)

If it doesn't conditions are abnormal and chances are that mischief is brewing. This ebb and flow of warmer and cooler air is, on a small scale, exactly what is happening on a vastly larger field of operations between cyclone and anticyclone. And it is the dominance of the anticyclone with its prolonged rush of air from the northwest that interrupts the sea breeze for two or three days in winter, as the cyclone prevents the night land breeze from taking place when it is central off the eastern coast.

The exchange of air between mountain side and valley is similar to the land-and-sea breeze. The rarer air on the mountain side heats faster by day and cools faster by night than the denser air in the valley. Therefore during the day it rises and the valley air rushes up to take its place; during the night it cools and sinks into the valley. This is a great help when one is shut up in a secluded valley for several days and cannot get a good view of the skies. The atmosphere is acting properly and will remain settled so long as the air blows up your ravine for most of the day, and turns about sundown and blows out and down the ravine like a flood of refres.h.i.+ng water.

Of course many valleys are so large as to be affected, not by these local causes, but by the larger movements of the anticyclones when the sure-clear west wind may blow up the valley for three days at a time. But, nevertheless, for most mountainous places the logic holds and you may expect rain if the wind does not blow coolly down the ravine at night. Of course watch your clouds for confirmation.

In times of calm prepare for storm. An eminent meteorologist has frowned upon me for saying that. It is not the whole truth, I admit, but there is a certain kind of calm which happens often enough to justify the remark. It happens this way. A severe storm has pa.s.sed. The customary anticyclone with its brisk northwest winds has arrived and is blowing with all the vigor necessary to induce one to believe that the clear weather is to continue for the usual length of time; that is, three or four days. But suddenly in the early afternoon, just when it should be blowing its hardest, the wind drops, lulls, shows a tendency to change its direction. There is only one explanation. Another cyclone has developed off in the west. It has knocked the anticyclone on the flank, taken the teeth out of the gale.

The wind shows this before clouds can. The absence of wind when there ought to be a lot shows it before even the first cirrus swims overhead. The chance is that when the flow of anticyclonic air has been thus rudely cut off and stillness follows, it will be storming by morning. It is best to keep an eye on these abnormal, precipitous calms. In times of peace prepare for rain.

But the eminent meteorologist was eminently right when he said that the statement was misleading unless explained. For there are many kinds of calms that do not portend coming storms. Nearly every day, winter and summer, but particularly in summer, the wind drops to a calm at sunset. That is a time of adjustment. After sunset when the accounts are all in the wind springs up with as much force as it had in the afternoon and continues until dawn. At sunrise, however, there is another truce. If this truce is neglected either at sunrise or at sunset it is a sign that either a cyclone on an anticyclone is very much in the ascendency. These truces are most often observed at the seash.o.r.e when you are out sailing and the smell of supper fills your nostrils but is not sufficient to fill your sails. These calms are normal and the best sign of a fair day on the morrow, provided the other signs agree.

During the great transition period from summer to winter comes that autumnal truce, Indian Summer, which is the chief claim to fame of American weather. For day after day a brooding haze sleeps in the air, sometimes for weeks there is no wind of any strength. Winter advances insidiously in the fall but retreats in commotion, and the cooling off process permits of these still days while they are uncommon in the spring. The wind checks off more mileage in March than in any other month.

While the regular day's end calm and the calm of the year's exhaustion mean continued fair weather, there is one calm that everybody knows, which is the most dramatic moment in the whole repertory of the weather: the foreboding, ten-count wait before the knockout blow of the thunderstorm. But when that calm comes every one is already sitting tight so that it is not much account as a warning. They say that the intense stillness before the hurricane strikes is uncanny.

Whether insh.o.r.e or afloat the wind is to be watched if you would know what weather is to be. It is only another of Nature's paradoxes that the most unstable element should be the most reliable guide of all on the uncertain trail of the next day's weather.

TEMPERATURES.

Considering that the temperature of the sun is 14,072 degrees Fahrenheit and the temperature of s.p.a.ce is absolute zero, 459 degrees below ours, we do very well on earth to be as comfortable as we are.

And we owe it all to the atmosphere which keeps the sun from concentrating upon us. Our place in the sun is so very small that we intercept only one-half of one billionth of the heat which it is giving off night and day. But that is sufficient to do a lot of damage if it could get at us.

But even the paltry range of temperatures so far recorded on our planet,--from 134 degrees above zero one day in California, to 90 degrees below zero one night in Siberia,--is by no means a fair statement of the extremes we are called upon to bear. Only twice a decade in our country does the mercury vary as much as sixty degrees in twenty-four hours, and there are vast areas where the daily change amounts to only a few degrees.

The changes that do come so suddenly to us, particularly in winter and that are known as cold waves, are in reality beneficial. To them we Americans may owe our energy, our vivacity, our changeability of mood. The refrigerated, revivified air sweeping down from the north is tonic. It is heavy, and issuing from antiseptic alt.i.tudes, drives the humid, germ-nursing air from our city streets. If we had arranged a process of refreshment like this at vast expense we should have been intensely proud of it. As it is we are intensely annoyed at it and occasionally a few people are frozen to death. The Weather Bureau warnings and the coal clubs are reducing the loss in property and lives.

If you are sleeping out it is of great importance to know when the mercury is going to take one of these swoops, for sleeping cold means little real rest because one's muscles are tense, and the next day's packing needs all the relaxation one can get. Two generalizations govern pretty much every change of temperature: the mercury will rise before a storm and it will fall after one, winter and summer, but much more conspicuously in winter.

There are two reasons for this. Our cyclones usually cross our country over such a northern track that over most of the country the air drawn into them comes from the southern quarters and is therefore warmer than the air previously flowing from the anticyclone. Also the process of precipitation causes heat. This is true to such an extent on the coast of Ireland where it rains most of the time that a scientist has computed that the inhabitants get from one-third to one-half as much heat from the rainfall as they do directly from the sun. Thus a normal storm is doubly sure to warm up the environment.

In summer the reverse is partially true, for very often the rain does not begin until the actual center of depression has pa.s.sed and the west winds have begun to exercise their cooling influence. So that in summer we have a sultry, sunny day as the first half of the storm area and then a cooling shower. Also after two or three days of warm weather in spring and autumn we have a rainstorm of the winter type which lowers the temperature instead of raising it. This is because the heat produced by the storm is less than that of the sun's rays intercepted by the clouds. The clear skies of the preceding anticyclone had permitted the land to warm up very fast under the midsummer sun, and the clouds of the cyclone, by cutting off the supply, had made a relative chill.

In winter the sunrays are so much feebler because of their slant and radiation proceeds so rapidly under the dry air of the anticyclone that a much greater degree of cold is produced than when the cyclonic clouds prevent the radiation. Therefore the rainy area is the warmest of all. Even in summer the winds from the southeast, south, and southwest are warmer than those from the opposite quarters, not only because they blow from a quarter naturally warmer on account of the sun, but because they are surface winds and have absorbed some of the heat from the soil. Being denser, they absorb it more readily and hold it longer.

The change, then, from the period of fair weather to that of storm brings an increase of temperature. But the rate of increase varies. The faster the storm is approaching the faster the temperature will rise; and the route of the storm's center makes all the difference as to the amount of the rise. If the wind s.h.i.+fts by way of the north and holds in the northeast until precipitation begins the rise in temperature will be very slight. The great snowstorms of the northern half of the country occur under just such a circ.u.mstance. If the wind s.h.i.+fts by way of the north but gets around to the east or even southeast before the precipitation starts the rise in temperature will be more p.r.o.nounced, as much as thirty degrees sometimes in a few hours, and the winter storm that started in as snow soon changes to sleet and rain.

If the wind s.h.i.+fts by way of the south and then into the southeast the rise will be vigorous and the storm will likely be a comparatively warm rain. If the wind s.h.i.+fts only so far as the south the rise will be highest of all and blue sky will often appear between the showers, showing that the air is heated to a considerable height.

The progress of the temperature changes from the maximum of the cyclonic area to the minimum of the anticyclone is also dependent upon the wind. If the storm center is pa.s.sing south and the wind begins to pull into the northeast and north the temperature will fall steadily and slowly. The rain or snow often cease gradually by the time the wind has reached the north, but the temperature continues to fall slowly until it reaches very low levels in mid-winter. If the storm center is pa.s.sing north of you the wind which has brought most of the rain while it was in the southeast with comparatively high temperatures swings into the southwest, the temperature falls somewhat.

There is usually a final downpour and a rapid s.h.i.+ft of the wind into the west or northwest, but almost never directly into the north. The temperature falls several degrees in a few minutes, quite unlike the gradual decline of the northeast-by-north s.h.i.+ft, and clear skies come at once with rapidly diminis.h.i.+ng temperatures. In the vicinity of Philadelphia a fall of twenty-five degrees would be most unusual on the northeast s.h.i.+ft,--such storms reaching 38 degrees and falling to 15, while with the other s.h.i.+ft a fall from 55 degrees to 15 would not be unusual. Of course any one set of figures given could only show the tendency and not the rule or limits.

After the manner of the wind-s.h.i.+ft the intensity of the storm is a good gauge of the temperature change to be expected by the camper. As a rule the greater the intensity of the storm the greater will be the degree of cold that follows it. The storms that have a complete wind circulation about them are always more severe than those with incomplete circulation and are invariably followed up by some reduction in temperature. If the decrease is not proportionately great and the subsequent wind has only a moderate clearing quality look out for another cyclone.

In such a case the temperature is the best witness of the contemplated change. For instance, after a summer thunderstorm a decided coolness is de rigeur. If this does not occur it means nearly every time that there is another thunderstorm in process of construction. There may be not a cloud in the sky, there may be no wind (although there should be) so that the course of the thermometer is the only means of telling what is to be the next event. Anybody can take a thermometer with him although a barometer--the most accurate forecaster of all--may be thought too much expense and bother.

At some future date the Weather Bureau will be able to predict the temperature of seasons in advance. This, together with the amount of rain scheduled to fall, will be an invaluable aid to everybody and to the farmers most of all. At present mild seasons that have severe storms without the appropriate degree of cold after them cannot be entirely explained, let alone being prediscovered. They all hinge upon the more or less permanent areas of high and low air pressure over the oceans and international meteorological service has not progressed far enough to support many ocean stations as yet.

Sometimes clear weather may intensify, growing brighter, stiller, colder. This is because the pressure is increasing. Cold seasons are distinguished usually by a succession of anticyclones. There is no way of telling how long a certain spell of cold weather is to last, but I have noticed that the same characteristics rarely predominate for longer than a month at a time. In other words, if December has been warm and rainy, January will likely be cold and dry. Of course, that is precisely the unscientific sort of generalization which the Bureau very rightly frowns upon, but which one may nurse privately until science has provided a subst.i.tute as she already has in so many instances.

With a little practice it is an easy matter to estimate the temperature to within a very few degrees. Try guessing for a few mornings and then look at the thermometer. You will hit within three degrees every time after a week of this.

Allowance must be made for the amount of moisture in the air and for the force of the wind. Damp air feels colder by several degrees than crisp, dry air, and a breeze increases the difference still more. Air in motion is not necessarily colder than calm air. As a matter of fact the lowest temperatures of all are recorded about sunrise after a still, clear night. The amount of radiation accomplished during the last hours of the night is amazing, and the downward impetus of the thermometer is often carried on for an hour or more after the sun has appeared above the horizon. A self-recording thermometer is an amusing toy which will show this and becomes a valuable instrument if one raises fruit.

In winter three o'clock of an afternoon sees the highest temperature usually, and in summer this maximum occurs as late as half-past five, due to the fact that the sun can pour in its heat faster than the earth can radiate it off. For the half hour before and after sunset, particularly in winter, the loss of heat is relatively greatest; then the pace slackens till three or four in the morning, when the plunge of the mercury is accelerated until the rays of the rising sun counteract the radiation.

If the mercury does not rise appreciably on a clear winter's day it is a sign that a cold wave is stealing in, due, doubtless, to a gradual increase in pressure without its customary bl.u.s.ter. Very often snow flurries predict its approach, but this may be so gradual that only the restriction of the daily thermal rise may indicate it. By the next morning the temperature will likely be twenty degrees colder.

If the mercury does not fall on a clear winter's night it is a sign that a layer of moist air not far above the surface of the earth is checking the normal night radiation. Unsettled weather is almost sure to follow unless this wet blanket is itself dissipated and the mercury takes its customary tumble before morning.

If the temperature falls while the sky is still covered with clouds clearing, possibly after a little precipitation, will soon follow.

Hot waves approach insidiously. A night will not cool off as it properly should, the sun will rise coppery, and while the day is yet young everybody begins to realize that all is not exactly right. But the heat increases usually for several days, not only by reason of steadily lowering pressure, but also by acc.u.mulation. Finally when a climax is reached it departs abruptly on the toe of a thunderstorm.

A cold wave reverses the process. It arrives abruptly on the heels of a departing cyclone and, after losing power, steals away without any commotion whatever. Its rate of progress is in close relation to the cyclone ahead of it.

Our mountains play a great part in our weather. They are a right arm of Providence to our agricultural communities. Due to their north and south trend a cold wave of any severity reaches the Pacific Coast only once a generation. Just once has snow been observed to fall at San Diego and it is so rare south of San Francisco that many people never have seen a flake. East of the mountains the belt of desert makes natural crops impossible for a thousand miles, but if they crossed the continent all the territory north of them would have such a cold climate that none of the present enormous crops of Canada and our northern states could possibly be grown. It is also due to the wide insweep of winds from the Gulf that the plains states are so well watered.

[Ill.u.s.tration: c.u.mULUS.

Courtesy of Richard F. Warren.

The tops of c.u.mulus are irregular, looking like wool-packs; the bases are flat. The true c.u.mulus shows a sharp outline all the way round. Its shape is in constant change due to the strong winds it is encountering. It is caused by the swift uprush of warm air on a sunny day. This cloud is a sign of fair weather, because the base is not large, compact, or dark enough to threaten rain and its comrades are also disjointed. If the c.u.mulus grow darker toward the horizon and increase toward evening a squall is likely.]

In lesser fas.h.i.+on the Appalachians protect the Atlantic seaboard. They withstand the impact of the cold waves to a great extent, although they are not high enough to divert the flow of cold air entirely toward the south and it is not desirable that they should. As things are the cold strikes Alabama before it hits New Jersey, and is often more severe there.

Comparative cold is often registered by the green color of the sky. A fiery red continues the prevailing heat.

The day that is ushered in by a fog, in summer, will likely be warm, providing the fog lifts by ten o'clock.

The temperature of a night with even a thin covering of clouds will be a good deal higher than if the sky is clear. In the British Isles the whole difference between freezing and no freezing lies with the fairness of the heavens. Everywhere frost will not form while the sky is covered, although the temperature may be below the freezing point. In summer radiation on a still clear night may be so rapid that frost may follow a temperature of fifty degrees at nightfall.

The temperature at the surface of the earth may easily deceive, as a colder or warmer stratum of air may overlie that immediately next to the ground. I have seen water particles fall when the temperature was as low as 16 degrees above zero, showing that the stratum of cold air was very thin. Our sleet storms in which immense damage is done to trees and telegraph wires occurs from just such a situation,--a cold, shallow layer of air close to the earth, with the warm moisture-bearing air flowing over it. The reverse of this situation is not uncommon--the sight of a snowstorm proceeding merrily along with the ground temperature at 35 or even 40 degrees.

Coming warmth may be noticed by the increase in size of snow flakes, with finally hail and rain. Coming cold is foreshadowed by hail mixed with the rain and lastly snow flakes which have a tendency to decrease in size. Colors of the clouds predict temperature changes, but it takes much practice to distinguish the cold, hard grays from the soft, warm ones. A warm sky is always less uniform in color than a cold one. The colors of winter sunsets are, as a rule, much brighter than those of summer skies.

The stars seem brighter on a night that is to be cold. If they twinkle it is because of rus.h.i.+ng air currents, and if the wind is from the northwest the result may be a subsequent lowering of temperatures.

The whole question of whether it will be colder and how much is vital to the camper and if the signs of change are taken along with the look of the clouds and the direction of the wind he need never be wrong as to the direction the mercury is going, and will soon be able to guess the distance pretty fairly.

RAIN AND SNOW.

East of the Mississippi River rain falls with the utmost impartiality upon every locality. Thirty to fifty inches are delivered at intervals of three or four days throughout the year. And if there is a slight irregularity in delivery one can be sure that from 125 to 150 of the 365 days will be rainy. Occasionally there is a more or less serious hold up of supplies, but this rarely happens in the spring of the year and never happens to all sections at once. And if there is a desire to make amends for the drought, we have what we call a flood and blame it on the weather instead of on our precipitous denudation of the watersheds.

West of the Mississippi particular people have to go to particular places for their rain. If they like a lot of it they must go to the coast districts of Was.h.i.+ngton or Oregon where they can have it almost every day. It rains a good deal at Eastport, Maine,--about 45 inches a year; that is, nearly an inch a week,--but at Neal Bay, Was.h.i.+ngton, at about the same lat.i.tude, in one year it rained 140 inches, and it never stops short of 100 inches any year.