Reading the Weather. Thomas Morris Longstreth
easterly trades of commerce, geography, and fiction. The direction of the upper currents flowing back to the poles is from southwest to northeast; but in our middle zones this becomes almost from west to east, is constant and is known to the profession as the prevailing westerlies.
Look up some day when wisps of clouds are floating very high. You will notice that their port is in the east, mattering not what wind may be blowing where you are. They are above the petty disturbances of the shallow surface winds. They follow a Gulf Stream of immeasurable grandeur. Onward, always onward, they sail, emblems of a great serenity.
Beneath this vast drift of air, which increases in velocity as it nears the pole, is an undertow from two to three miles thick. It is the movements of this undertow that affect our lives. These movements are influenced by all the changes of temperature and by the configurations of land. They take the form of whirls. These whirls may be small eddies, local in effect, or vast cyclones with diameters of fifteen hundred miles. Small or large they roll along under the Westerlies, translated by friction, and invariably moving for most of their course in an easterly direction, like their tractor above. They circle across the United States every few days. Their courses do not vary a great deal, and yet enough to make each one a matter for conjecture. And all the conjecturing centers upon the condition of the atmosphere—the changing atmosphere which is yet so dependable.
The weather we are used to, the daily weather that catches us unprepared, and yet that does not mistreat us all the time is the product of these little whirls, which are so remotely connected with the grander atmospheric movements of our planet. Remembering this, we can at last come back to earth and set about our real business which is to see why certain kinds of weather come at such uncertain times and how to tell when they will arrive.
CHAPTER II
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.
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