When the condensation is obvious, but thin, at nightfall, it may not, as I have said, be discernible in the evening. But there are methods by which the incipient storm condensation may be detected. The number of the stars visible, and the distinctness with which they may be seen, indicate the absence or presence of condensation and its density. Virgil, alluding to the indications of fair weather, says:
“Brightly the stars shine forth; Cynthia no more
Glimmers obnoxious to her brother’s rays;
Nor fleecy clouds float lightly through the sky.”
The brightness of the stars and the clear appearance of the moon show the absence of condensation and the dissolution of the fleecy clouds at the close of the day is, as we have seen, always a fair-weather indication.
There is much true philosophy in the allusions of Virgil to the moon. Thus—
“When Luna first her scatter’d fires recalls,
If with blunt horns she holds the dusky air,
Seamen and swains predict th’ abundant shower.”
The horns, or angles of the moon will, of course, appear distinct and sharp or indistinct and blunt, in proportion to the amount of condensation in the atmosphere which impedes the passage of the light. For the same reason, when the moon is new, her entire disk is visible when the atmosphere is very clear, by reason, as is supposed, of light reflected from the earth to the moon and back to us. This double reflection can only take place when the atmosphere is very clear. Hence, Virgil alludes to it, and correctly, as an indication of continued fair weather:
“If (mark the ominous hour!)
The clear fourth night her lucid disk define,
That day, and all that thence successive spring,
E’en to the finished month, are calm and dry.”
Probably Virgil alluded to a month of the summer trade-wind drouth which reaches up on Southern Italy. But that appearance of the moon is occasionally seen here, and the indication is, in degree, philosophically true.
It is somewhat more difficult to determine what will be the result of the condensation seen at the west in the morning, and which is not so far east, or of such a character, as to reflect the rays of the sun; for, although always suspicious, it is sometimes of a foggy character, and disappears between eight and nine o’clock. If it increases in density after ten o’clock, or is of a dense cirro-stratus character, rain may generally be expected. If of a decided cirro-cumulus character, it is certain to disappear. Cirro-cumulus is seen in small patches, with small, distinct, and rounded masses, in summer, in the morning, and sometime, during the day, after high fog has disappeared, and at other times, and is always, when of that distinct character, a fair weather indication. I have seen it thus when the wind was blowing from the N. E., and the scud running toward a storm passing near, but to the south of us, when those who relied upon the existence of the wind and scud as evidences that we were to have the desired rain, were deceived. Thus, the couplet from an old almanac:
“If woolly fleeces strew the heavenly way,
Be sure no rain disturb the summer day.”