Page:Handbook of Meteorology.djvu/232

 Experience will teach the observer to look for the unusual in comparing the daily records, and also to interpret it.

The dew-point may be found without the use of the hygrometer by a very practical method. A thermometer, a polished tin cup—a “shaker” is better—and ice water are required. The cup must be absolutely free from grease. The cup is half-filled with water at about the temperature of the air. Ice water is added little by little and stirred with the thermometer until mist forms on the outside of the cup. The water is then at the temperature of the dew-point, and this is shown by the thermometer.

The Measurement of Evaporation; Evaporimeters.—The rate of evaporation depends on the amount of moisture in the air. If the humidity is low, evaporation is more rapid than when it is high. With the humidity above 95 per cent a piece of wet muslin in the open air may require more than an hour to dry; with the humidity at 100 per cent it does not dry at all. A high temperature also favors evaporation—chiefly from the fact that, with rising temperature, the relative humidity decreases without any change in absolute humidity. The rate of evaporation is therefore roughly an indication of the degree of humidity.

Evaporimeters vary in form but not in principle. In every case the evaporimeter is a device for measuring the depth of water lost by evaporation from an open surface. A very common form consists of a graduated glass tube filled with water, inverted in a vessel of water. A pin-hole aperture about half an inch from the lower end of the tube admits air to the top of the tube when evaporation lowers the water in the level of the pan. The level of the water in the pan is constant; the loss is in the tube. If the area of the surface of the container is 0.01 that of the section of the tube, a loss of I inch of water in the tube is equivalent to a loss of 0.01 inch by evaporation.

The Piche evaporimeter is a type of the best sort of instrument. A collar fastened around the tube at its mouth carries a disk which presses against and covers the mouth of the tube. A circular piece of filter paper, about twice the diameter of the tube, between the disk and the mouth of the tube allows a sufficient flow of water to keep the paper wet. By this device, loss of water by accident is avoided, and evaporation is recorded