Page:Popular Science Monthly Volume 78.djvu/448

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Many proverbs foretelling rain and bad weather are based on the appearance of solar and lunar haloes and coronas, and as these form only when there is much moisture in the air and some condensation the proverbs of this class are well founded.

Coronas are the small colored rings of light that encircle any bright object when seen through a mist, though the term commonly is used to designate only the colored rings around the sun and moon. They are due to diffraction (the bending of light at the boundary of an object into its geometric shadow) caused by water globules, and have one or another angular diameter depending on the size of the droplets that produce them, in the sense that the larger the droplets the smaller the corona. Hence a decreasing corona implies growing drops and the probability of an early rain.

Haloes, on the other hand, are the rings of large diameter, usually colorless or nearly so, due to reflection and refraction by ice spicules, and are often seen in the high cirrus clouds that have been caught up from the tops of storms and carried forward by the swiftly moving air currents that always prevail at such elevations. It is this usual position of haloes relative to storm centers, that is, in front of them, that makes them the good indicators they are of approaching bad weather.

Typical of such proverbs is that of the Zuñi Indians, who say:

"When the sun is in his house it will rain soon."

Several others refer to the apparent diameter of the circle. Thus we have:

"Far burr, near rain."

"The bigger the ring, the nearer the wet."

"When the wheel is far the storm is n'ar;
When the wheel is n'ar the storm is far."

These latter can not refer to the corona, which actually does change in angular size, because in that case just the reverse is true; the bigger the ring the farther off the storm. Clearly then they apply only to the halo, and as the apparent size of an object of constant angular diameter depends upon its seeming distance away, it follows that the supposed changes referred to are optical illusions, due to erroneous impressions of distance.

A good illustration of this kind of illusion is furnished by the moon as seen by different people, or as seen by the same person at different elevations above the horizon. When high in the heavens, where it appears to be comparatively near, it looks smaller than it does when close to the horizon where it seems to be farther away; and yet careful measurements show but little change in its angular diameter, and that little just the reverse of appearances.

Hence, when the actual distance to a halo is less than it seems to be.