A hygrometer is useful in detecting increase of moisture, but it is not wholly essential. Where a hygrometer is available it is apparent that the less the difference between the wet bulb and the dry bulb the greater the moisture content of the air and the greater certainty of unsettled weather
The effects of increasing moisture in the air are so well known that the literature of them is great, and popular sayings concerning them are found in all ages.
When the locks turn damp in the scalp house most surely it will rain.—Indian Tradition.
If metal plates sweat it is a sign of foul weather.—Pliny.
The tightening of cordage on ships is taken by sailors as a sign of approaching rain.
A red sun has water in his eye.—New England Tradition.
When it is evening, ye say it will be fair weather, for the sky is red; and in the morning it will be foul weather, for the sky is red and lowering.—Matthew xvi, 2-3.
Rainbow in the morning, shepherds take warning;
Rainbow at night, shepherds’ delight.
Circles around the sun or the moon indicate increasing moisture.
Salt absorbs moisture quickly. Its becoming coherent is a sign of increasing moisture.
The sunflower lifts its head when the moisture of the air increases.
The perfume of flowers becomes stronger when the air becomes damp; so also does the odor of a tobacco pipe.
It is well to bear in mind that these traditions apply to a more or less sudden change from dry to moist air, and not to the long-continued spells of moisture that come with steady sea winds.
Moist weather of long duration may be clear, as is commonly the case along the Atlantic Coast in summer; but a rapid change from dry to moist air almost always brings hazy conditions, and this is the sort of change that precedes rain.
And if through mists Sol shoots his sullen beams,
Frugal of light in loose and straggling streams,
Suspect a drizzling day and southern rain
Fatal to fruits, and flocks and promised grain.
—Virgil.
The foregoing are only a few of the traditions and folklore sayings concerning the humidity of the air. Nearly all of them may be reduced to one or the other of two general principles—
