This usually occurs a little while before sunrise. It is important in weather observing to make a record of the highest temperature of the day and the lowest temperature of the night. Continuous observation, as the reader will appreciate, is practically impossible for such a record. THE THERMOMETER FOR HUMIDITY IN THE AIR Moisture or dampness in the air, as shown by an instrument called the hygrometer, increases before rain, fog, or dew. Before describing the hygrometer, a definition of a few of the terms used in conjunction with the instrument will be found useful. ABSOLUTE HUMIDITY The amount of vapor actually present in the atmosphere is termed the absolute humidity, expressed usually either in the expansive force that the vapor exerts or in its weight in grains per cubic foot of air. RELATIVE HUMIDITY The absolute humidity divided by the amount of vapor that might exist if the air were saturated gives a ratio that is called the relative humidity. DEW POINT The temperature at which moisture begins to be condensed on a cold vessel or other container and becomes visible is called the dew point. HOW HYGROMETERS ARE MADE The most generally used hygrometer consists of two ordinary thermometers, the bulb of one being covered with a piece of muslin and kept constantly moistened with water by means of a wick or cotton thread communicating with a container of water. The difference in the readings of the two thermometers, the wet and the dry, is observed, and knowing this, it is very easy to determine the humidity by consulting a table (see table on pages 58–59), which has been prepared for this purpose. These instruments are, according to the increase in price, equipped with a table, and the container is held in a wire frame, as you will see from the Figs. 49–50 showing the standard Weather Bureau station instrument and the Gilbert hygrometer. Fig. 49 shows the U. S. Standard Weather Bureau Station Rain Gauge, Fig. 50 the Gilbert Rain Gauge and Fig. 51 the U. S. Standard Weather Bureau Station Rain Gauge, Tipping Bucket Type. The Gilbert Weather Station is equipped with the Tipping Bucket Type Rain Gauge. Fig. 51 shows the apparatus clearly, complete and mounted ready for use. The brass bucket seen in position through the open door is adjusted to tip for each hundredth inch of rainfall collected in the twelve-inch diameter receiver at the top, and this rainfall is electrically recorded at any convenient distance on a register. After any desired period the water may be drawn off and check measurements made by means of the brass measuring tube and graduated cedar stick shown in the figure. THE GILBERT RAIN GAUGE (Fig. 50). (_a_) Tube. (_b_) Funnel. (_c_) Measuring stick.