A wind comes along; that is, the air in which the cloud is floating moves. The wind carries the cloud along with it. More rising air, full of evaporated water from the ocean, joins the cloud and cools, and the water forms into more tiny droplets. The droplets get so close together that they shut out the sun's light from the earth, and people say that the sky is darkening.
Meanwhile some of the droplets begin to touch each other and to stick together. Little by little the drops grow bigger by joining together. Pretty soon they get so big and heavy that they can no longer float high in the air, and they fall to the ground as rain.
Part of the rain soaks into the ground. Some of it gradually seeps down through the ground to an underground stream. This has its outlet in a spring or well, or in an open lake or the ocean. But the rain does not all soak in. After the storm, some of the water again evaporates from the top of the ground and mixes with the warm air, and it goes through the same round. Other raindrops join on the ground to form rivulets that trickle along until they meet and join other rivulets; and all go on together as a brook. The brook joins others until the brooks form a river; and the river flows into a lake or into the ocean.
Then again the sun warms the surface of the ocean or lake; the water evaporates and mixes with the air, which rises, expands, and cools; the droplets form and make clouds; the droplets join, forming big drops, and they fall once more as rain. The rain soaks into the ground or runs off in rivulets, and sooner or later it is once more evaporated. And so the cycle is repeated again and again.
And all this is accounted for by the simple fact that when water evaporates its vapor mingles with the air; and when this vapor is sufficiently cooled it condenses and forms droplets of water.
The barometer. In predicting the weather a great deal of use is made of an instrument called the barometer. The barometer shows how hard the air around it is pressing. If the air is pressing hard, the mercury in the barometer rises. If the air is not pressing hard the mercury sinks. Just before a storm, the air usually does not press so hard on things as at other times; so usually, just before a storm, the mercury in the barometer is lower than in clear weather. You will understand the barometer better after you make one. Here are the directions for making a barometer:
Experiment 87. To be done by the class with the aid of the teacher. Use a piece of glass tubing not less than 32 inches long, sealed at one end. Fill this tube to the brim with mercury (quicksilver), by pouring the mercury into it through a paper funnel. Have the sealed end of the tube in a cup, to catch any mercury that spills.[7] When the tube is full, pour mercury into the cup until there is at least half an inch of it at the bottom. Now put your forefinger very tightly over the open end of the tube, take hold of the sealed end with your other hand, and turn the tube over. Lower the open end, with your finger over it, into the cup. When the mercury in the cup completely covers your finger and the end of the tube, remove your finger carefully so that no air can get up into the tube of mercury. Let the open end of the tube rest gently on the bottom of the cup, and hold the tube upright with your hand or by clamping it to a ring stand. Hold a yardstick or meter stick beside the tube, remembering to keep the tube straight up and down. Measure accurately the height of the mercury column from the surface of the mercury in the cup. Then go to the regular barometer hanging on the wall, and read it.
Fig. 152. Filling the barometer tube with mercury.
The reason your barometer may not read exactly the same as the expensive laboratory instrument is that a little air and water vapor stick to the inside of the tube and rise into the "vacuum" above the mercury; also, the tube may not be quite straight up and down. Otherwise the readings would be the same.
[Footnote 7]: If mercury spills on the floor or table during this experiment, gather it all into a piece of paper by brushing even the tiny droplets together with a soft brush; squeeze it through a towel into a cup to clean it. It is expensive; so try not to lose any of it.
Of course you understand what holds the mercury up in the tube. If you could put the cup of mercury into a vacuum, the mercury in the tube would sink down into the cup. But the pressure of the air on the surface of the mercury in the cup keeps the mercury from flowing out of the tube and so leaving a vacuum in there. If the air pushes down hard on the mercury in the cup, the mercury will stand high in the tube. This is called high pressure. If the air does not press hard on the mercury in the cup, the mercury stands low in the tube. This is called low pressure.
