Page:Weather Facts and Predictions.djvu/42

 or “heat” lightning (unattended by thunder) is in Scotland a prognostic of unsettled weather, it being considered the forerunner of an approaching storm.

Hailstorms are very local in their character, seldom occur during night or in winter, but most frequently in summer and during the hottest part of the day.

Snowflakes vary in size from 1-14th inch to 1 inch. They are largest when the temperature is near 32°, and smallest at very low temperatures. The crystals of the same fall of snow are generally similar to each other, but one set of crystals will differ a good deal from another.

When snow descends in very large flakes with a southerly wind, there will soon be a thaw.

Usually, snow with an E. wind and a falling barometer turns to rain, the wind becoming more southerly.

If after severe cold it begins to snow, and the wind veers from E. to S.E., and the barometer falls and the cold becomes less intense, still the thermometer may remain below 32°. In such a case, when the wind reaches the S., the snow does not turn to rain, and if the southerly current is displaced the snow will continue almost or quite uninterruptedly.

Snow with a W. wind brings more cold, though, perhaps, after an interval. [Because snow is more usual with W. than with E. winds, and W. winds are apt to work round