Raphael Semmes, Memoirs of Service Afloat During the War Between the States

[606] The normal condition of the weather is that of a clear sky, with passing trade-clouds, white and fleecy, and with moderate breezes. If the reader has watched his barometer narrowly, he has observed a very remarkable phenomenon, which is not known to prevail outside of the trade-wind belts— an atmospheric tide. The atmosphere ebbs and flows as regularly as the sea. This atmospheric tide is due, no doubt, to the same cause that produces the aqueous tides—the attraction of the moon. It occurs twice in twenty-four hours, just like the aqueous tides, and there is no other cause to which we can attribute it.

The needle has a like semi-diurnal—indeed, hourly variation—showing the normal, electrical condition of the atmosphere. The atmospherical, tidal wave, as it ebbs and flows, seems to carry the needle backward and forward with it. The average barometer being but a very little under thirty, there is an agreeable elasticity in the atmosphere, and officers, and crew are generally in fine spirits. The sailors enjoy their evening dances, and story-tellings, and when the night-watches are set, sleep with impunity about the decks—guarded, however, by those woollen garments, of which I spoke, when describing our routine life. But observe, now, what a change will take place, as we approach the equator. We are approaching not only the calm-belt, which has been before described, but the Cloud Ring, for the latter is the concomitant of the former. The winds die away, the muttering of thunder is heard, and a pall of black clouds, along which dart frequent streaks of lightning, is seen hanging on the verge of the horizon, ahead of the ship. As she advances, fanned along by puffs of wind from various quarters, she loses sight of the sun altogether, and enters beneath the belt of clouds, where she is at once deluged with rain. She is at once in the equatorial calm-belt, and under the Equatorial Cloud Ring.

The north-east and south-east trade-winds, as they came sweeping along, charged to saturation with the vapors which they have licked up from a torrid sea, have ascended as they met, and when they have reached the proper dew-point, or point of the wet-bulb of the thermometer, precipitation has commenced. The barometer falls another tenth of an inch, or