Reading the Weather

by Thomas Morris Longstreth

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.

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.