Page:Handbook of Meteorology.djvu/184

 the hygroscopic character of many common substances, or the mistiness of the air which tends to scatter all but the red rays of the sun.

The varying conditions of humidity usually afford indications more or less characteristic. These are more noticeable at morning and evening when the humidity is high. They are apt to be most pronounced on the horizon when vision penetrates a layer of air of greatest density. The following are proverbs of seamen:

This may be correct if the haziness is due to misty air; but it is not true if the hazy or distorted outlines are caused by the refraction of air currents.

This may be true in some localities, but it is not generally true. The appearance is due to the reflection of straggling rays of sunlight from dust motes or from mist.

Cloud Indications.—Cloud matter is the first step in condensation. If precipitation is feeble in energy and slow in process, only cloud is formed; if it is more energetic the cloud matter forms rain or snow.

Because cirro-stratus clouds, higher than others, consist of the first precipitation on the advancing low, they are among the best indications of an approaching storm. In a majority of instances they are the overflow from the upper part of the approaching cyclone; they may be more than one hundred miles in advance of the storm center.

Not all cirrus and cirro-stratus clouds are storm clouds, however. If they rise to a higher altitude, or if they disappear, fine weather is likely to follow. If they accompany a rising barometer, fair weather is likely to continue.