Humidity in the Atmosphere

"A FTER a good rain the fields are muddy; thousands and thousands of threads of water running down the slopes have filled the ditches or collected in puddles and pools; the foliage of the trees shows a glossy green and shines as if the watery coating left by the storm were so much varnish; at the end of each leaf, at the tip of the tiniest blade of grass, trembles a drop of water, flashing like a diamond in the sunlight.

"But wait a few days and if the sun is hot and the wind blows a little, there will not be left a trace of this shower that gladdened the farmer's heart. The earth will have become dusty again, in the woods the little cushions of moss that but lately were so exquisitely fresh will have shriveled up and faded, the leaves of the trees will hang limp and lifeless, the puddles will be dry, and the mud will have turned to dust.

"What has become of the quantities of water discharged by the storm? The soil drank part of it, that is certain, to the benefit of the growing vegetation; but the air—insatiable toper!—also took its share, and a large one. The water that fell in the form of rain from the upper air has by evaporation returned to the atmosphere whence it came; the air has taken back what it gave to the soil for a little while. The drops that sparkled at the ends of the leaves have returned, invisible, to the immensities of the atmosphere without touching the earth. In short, evaporation has dissipated the waters of the storm into space.

"Countless bodies of water, running or stationary, are constantly giving off moisture from their surface by evaporation. Little brooks hastening to unite and form a rivulet, rivulets contributing their waters to make a river, rivers emptying into larger rivers, and these in turn discharging their floods into the sea; lakes, ponds, marshes, stagnant pools—all, absolutely all, even to the smallest puddle no larger than the hollow of one's hand, yield vapor to the atmosphere without a moment's pause.

"To picture to oneself the total volume of water thus rising continually into the air would be beyond the power of the imagination; and yet that is a mere nothing, for we are forgetting the chief source of atmospheric humidity. We forget the sea, the immense sea, covering three quarters of the earth's surface—the prodigious sea, in comparison with which all the rivers combined are as a mere trifle. What is a drop of water compared with a mill-pond? Nothing. It is the same with the waters irrigating the continents when we contrast their combined volume with the vastness of the sea.

"From the oceans and the bodies of water on the various continents there rises unceasingly into the atmosphere an inconceivable quantity of vapor. Now, this does not stay long over the bodies of water that gave it forth; the wind carries it away, to-day in one direction, to-morrow in another, sometimes to immense distances, so that a layer of air saturated with moisture a thousand leagues from here may reach us and furnish the air we breathe.

"Through the agency of storms, hurricanes, and all kinds of winds, gentle or boisterous, that agitate the air, there is brought about an indiscriminate redistribution, in every direction, of the water vapor that rose from any given part of sea or land. In this way the air about us holds moisture, always and everywhere.

"Yes, this air that surrounds us now, this air in which we come and go, contains water in the form of invisible vapor; and it always contains it, sometimes more, sometimes less, every moment and in all seasons. You will be convinced of this if I succeed in making visible to you what is now invisible; if, in short, I succeed in reconverting into water, running water, this vapor which no eye can see.

"As it required only a slight rise in temperature to convert ordinary water into invisible vapor, it will suffice to lower the temperature a little, and thus cool the vapor, to restore it to its former condition, or in other words to reconvert it into water. This cooling process, this taking away of heat, will undo what warming or the application of heat had done in the first place. That is all plain enough, it seems to me.

"Let us go back a moment to the pot of boiling water. Cover it with the lid, first wiping the latter perfectly dry inside; or, better still, hold the lid over the steam escaping from the pot, and at a short distance from the mouth.

"You foresee what will happen. The inside of the lid, perfectly dry at first, will in a few moments be covered with drops of water. Where do these drops come from if not from the steam which, by contact with the cold lid, has lost its heat, to which it owed its subtile form, and has returned to its original state, that of water? So much, then, we have demonstrated, that cold changes vapor to water; which is exactly the opposite of the change wrought by heat.

"For the sake of brevity we will in our future talks use a few terms which I will here define. The return of vapor to the state of water is called condensation. The opposite term is evaporation, which designates the conversion of water into vapor. If we wish to say that vapor becomes water again, we say it condenses.

"How shall we make the invisible vapor that is in the air manifest itself as water? Nothing easier, if we have some ice or snow at our disposal. We fill a water-bottle with small pieces of ice, wipe the outside well to remove any moisture that may already be there, and put the bottle on a perfectly dry plate.

"We do not have to wait long for the result. The glass of the water-bottle, at first perfectly clear, becomes dull and veiled in a kind of fog. Then little drops form, grow bigger, and slowly run down into the plate. Wait a quarter of an hour and we shall have enough water in the plate to pour into a glass and drink if we wish.

"Where does this water come from, if you please? Certainly not from the inside of the water-bottle, for water cannot pass through glass. Then it must come from the surrounding air, which by contact with the ice-cooled glass has itself become so chilled as to make its vaporous contents first appear as a kind of fog, the initial step in condensation, and then run down the glass in the form of drops. In this manner it may be proved at any season of the year that there is a certain amount of moisture in the air.

"We do not always have ice or snow at our disposal, especially in summer. During that season must we, then, forego this interesting and instructive experiment? Certainly not. We merely have to fill the water-bottle with very cold water and we shall see the glass become dull and clouded with moisture.

"There may even be some formation of drops large enough to run down the sides. The result will be the same as when ice is used, only less pronounced, because the cooling effect of cold water is less than that of ice.

"This phenomenon may have been presented to our view many times without attracting our notice. The carafe of cold water placed on our table at dinner very soon loses its transparency and turns dull from the condensation of vapor on the outside. A glass filled with cold water ceases to be transparent, becomes covered with a dull cloud, and looks as if badly washed. That, too, is the vapor from the surrounding air condensing—and collecting on the cold object."