Reading the Weather

by Thomas Morris Longstreth

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.