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equinoctial storm

equinoctial. 1. Pertaining to the equinoxes; designating an equal length of day and night; as, the equinoctial line. 2. Pertaining to the regions or climate of the equinoctial line or equator; in or near that line; as, equinoctial heat; an equinoctial sun; equinoctial wind. 3. Pertaining to the time when the sun enters the equinoctial points; as, an equinoctial gale or storm; that is, one happening at or near the equinox, in any part of the world. — Webster, 1882

For three days and nights the rain fell steadily, slow, weepy rain, running down the windowpanes and pattering on the roof. “Well, we must expect it,” Ma said. “It’s the equinoctial storm.” – The Long Winter, Chapter 3, “Fall of the Year”

An equinoctial storm is a storm of violent winds and heavy rain that occurs at or near the time of an equinox and popularly, but erroneously, believed to be physically associated with the sun’s passing across the equator. The storm can be at the time of the vernal equinox (in March) or at the time of the autumnal equinox (in September). In The Long Winter, the storms Ma refers to were in September, so they would have occurred near the 20th of September, the date of the autumnal equinox.


It appears to be useless to tell people that there is no scientific basis whatever for predicting a storm at the time the sun crosses the line. It is a superstition that has been handed down from time immemorial, and the fact that the sun crosses the line quite as often without a storm as with one, makes no difference to those who believe in the equinoctial myth. It is true, that at the season, when winter is changing to spring, rains are usually abundant and the popular notion about an equinoctial storm undoubtedly has this fact for a basis. — The Waukesha (Wisconsin) Freeman, March 27, 1884, page 6.

The recurrence of what was called an equinoctial storm at or about the time the sun crosses the equator, has ceased to be (if in fact it ever was) a thing to be depended on. — Ainsworth R. Spofford, An American Almanac and Treasury of Fact (New York: The American News Company, 1883): 15.

The examination of weather-records for sixty-four years shows that no particular day can be pointed out in the month of September (when the “equinoctial storm” is said to occur) upon which there was, or ever will be, a so-called equinoctial storm. The fact, however, should not be concealed, that taking the average of five days embracing the equinox for the period above stated, the amount of rain is greater than for any other five days, by three per cent, throughout the month. — David Ames Wells, The Science of Common Things (New York: Ivison, Blakeman, Taylor & Co., 1872): 112.

Autumnal equinox. The earth is tilted at a constant 23.5° on its axis to the earth-sun plane as it orbits the sun. It is this tilt which causes the change of seasons. In the northern hemisphere (where De Smet lies), when the north pole is tilted toward the sun, it is summertime; when the south pole is tilted toward the sun, it is wintertime. When the earth’s axis is pointed neither toward nor away from the sun, it is the equinox, from the Latin æquuas, equal, and nox, night. At the equinox, the day and night are equal in length. On earth, this happens twice each year, once in March (the Vernal Equinox) and again in September (the Autumnal Equinox).

The drawing at left shows the elliptical obit of the earth around the sun, with the earth in representative position at four points along this orbit. At the Autumnal Equinox (as well as the Vernal Equinox), the sun’s rays strike the earth’ equator directly as they strike the poles equally. The Summer Solstice is that point when the north pole is closest to the sun in June; the Winter Solstice is that point when the south pole is closest to the sun in December.

Harvest moon. In Farmer Boy, Laura Ingalls Wilder describes the harvest moon (see Chapter 20, “Late Harvest). This is always the full moon occurring nearest the Autumnal Equinox. In one out of three years, it comes in October; the other two years, it is in September.


     storm (TLW 3)