Matthew Fontaine Maury, Lieutenant, U.S.N.

“ Familiar with clouds and sunshine, the storm and the calm, and all the phenomena which find expression in the physical geography of the sea, the right-minded mariner, as he contemplates “the cloud without rain,” ceases to regard it as an empty thing; he perceives that it performs many important offices; he regards it as a great moderator of heat and cold — as a “compensation” in the atmospherical mechanism which makes the performance of the grand machine perfect.
350. Marvelous are the offices and wonderful is the constitution of the atmosphere. ”

“ One need not go to sea to perceive the grand work which the clouds perform in collecting moisture from the crystal vaults of the sky, in sprinkling it upon the fields, and making the hills glad with showers of rain. ”

“ Having traveled with the calm belt to the north or south, the cloud-ring leaves the sky about the equator clear; the rays of the torrid sun pour down upon the crust of the earth there, and raise its temperature to a scorching heat. The atmosphere dances (§ 205-6) , and the air is seen trembling in ascending and descending columns, with busy eagerness to conduct the heat off and deliver it to the regions aloft, where it is required to give momentum to the air in its general channels of circulation. The dry season continues; the sun is vertical; and finally the earth becomes parched and dry ”

“ Continuing his course toward the equinoctial line, he observes his thermometer to rise higher and higher as he approaches it; at last, entering the region of equatorial calms and rains, he feels the weather to become singularly close and oppressive; he discovers here that the elasticity of feeling which he breathed from the tradewind air has forsaken him; he has entered the doldrums, and is under the “cloud-ring.” Escaping from this gloomy region, and entering the southeast trades beyond, his spirits revive, and he turns to his log-book to see what changes are recorded there. ”

“ We thus have an insight into the operations which are going on in the equatorial belt of precipitation, and this insight is sufficient to enable us to perceive that exquisite indeed are the arrangements which Nature has provided for supplying this calm belt with heat, and for pushing the snow-line there high up above the clouds, in order that the atmosphere may have room to expand, to rise up, overflow, and course back into its channels of healthful circulation. ”

“ Ceaseless precipitation goes on under this cloudring. Evaporation under it is suspended almost entirely. We know that the trade-winds encircle the earth; that they blow perpetually; that they come from the north and the south, and meet each other near the equator; therefore we infer that this line of meeting extends around the world. By the rainy seasons of the torrid zone we can trace the declination of this cloud-ring stretched like a girdle round about the earth: it travels up and down the ocean as from north to south and back. ”

“ Where do the vapors which form this cloud-ring, and which are here condensed and poured down into the sea as rain, come from? They come from the trade-wind regions (§ 115) ; under the cloud-ring they rise up; as they rise up, they expand; and as they expand, they grow cool, form clouds, and then are condensed into rains; moreover, it requires no mercurial instrument of human device to satisfy us that the air which brings the vapor for these clouds can not take it up and let it down at the same temperature. ”

“ In the mean time, the trade-winds north and south are pouring into this cloud-covered receiver, as the calm and rain-belt of the equator may be called, fresh supplies in the shape of ceaseless volumes of heated air, which loaded to saturation with vapor, has to rise above and get clear of the clouds before it can commence the process of cooling by radiation. ”

“ If we imagine the atmospheirical equator to be always where the calm belt is which separates the northeast from the southeast trade-winds, then the loop in the thermal curve, which should represent the line of perpetual congelation in the air, would be always found to stride this equator; and it may be supposed that a thermometer, kept sliding on the surface of the earth so as always to be in the middle of this rain-belt, would show very nearly the same temperature all the year round ”

“ As the vapor is condensed and formed into drops of rain, a twofold object is accomplished: coming from the cooler regions of the clouds, the rain-drops are cooler than the air and earth below; they descend, and by absorption take up the heat which has been accumulating in the earth’s crust during the dry season, and which can not now escape by radiation. Thus this cloud-ring modifies the climate of all places beneath it; overshadowing, at different seasons, all parallels from 5o south to 15o north.
PAGE 177 THE EQUATORIAL CLOUD-RING. ”

“ But the sailor at sea observes phenomena and witnesses operations in the terrestrial economy which tell him that, in the beautiful and exquisite adjustments of the grand machinery of the atmosphere, the clouds have other important offices to perform besides those merely of dispensing showers, of producing the rains, and of weaving mantles of snow for the protection of our fields in winter. ”

“ As important as are these offices, the philosophical mariner, as he changes his sky, is reminded that the clouds have commandments to fulfill, which, though less obvious, are not therefore the less benign in their influences, or the less worthy of his notice. He beholds them at work in moderating the extremes of heat and cold, and in mitigating climates. At one time they spread themselves out; they cover the earth as with a mantle ”

“ No person who has not crossed this region can form an adequate idea of its unpleasant effects. You feel a degree of lassitude unconquerable, which not even the sea-bathing, which every where else proves so salutary and renovating, can dispel. ”

“ But that the temperature of the air at the surface under this cloudring is lower, is a theoretical deduction as susceptible of demonstration as is the rotation of the earth on its axis. Indeed, Nature herself has hung a thermometer under this cloud-belt that is more perfect than any that man can construct, and its indications are not to be mistaken. ”

“ These are physical laws, and therefore, when we see water dripping from the atmosphere, we need no instrument to tell us that the elasticity of the vapor so condensed, and falling in drops, is less than was its elasticity when it was taken up from the surface of the ocean as water, and went up into the clouds as vapor. ”

“ Every day, and all day long, they operate with ceaseless activity upon the upper surface of the cloud stratum. When they become too powerful, and convey more heat to the cloud vapors than the cloud vapors can reflect and give off to the air above them, then, with a beautiful elasticity of character, the clouds absorb the surplus heat. ”