Examples of temperature changes in the lower air, under different conditions of weather, recorded on the thermograph, are given in Fig. 12, and are briefly referred to their causes in the text accompanying that figure.
C. Vertical Distribution of Temperature in the Atmosphere.—The vertical distribution of temperature in the lower air may be studied by having ordinary thermometers or thermographs exposed at different heights above the ground, e.g., close to the surface; in an instrument shelter; out of windows on successive stories of some high building; and on the roof of the building. They may also, in cases where there is a hill in the neighborhood, be exposed in a valley at the bottom of the hill and at successive elevations up the side of the hill. It is, however, usually much simpler, as well as more practicable, to take these temperature readings by means of the sling thermometer. In the case of observations made out of the windows of a building, one observer can take the readings at different elevations in succession. When the observations are made at different altitudes on the side of a hill, it is best to have the coöperation of several observers, who shall all read their thermometers at the same moment of time. The results obtained in the previous problems (A and B) may, of course, also be utilized in studying the vertical distribution of temperature in the atmosphere.
Study the vertical distribution of temperature in the lower air under various conditions of weather and season; at various hours of the day, and with varying conditions of surface cover. Make your observations systematically, at regular hours, so that the results may be comparable. Group together observations
made under similar conditions of weather, season, time, and surface cover. Determine the average vertical distribution of temperature in the different cases. Note especially any seeming peculiarities or irregularities in this distribution at certain times. Study carefully, as in the previous problems, the relation of the different types of temperature distribution in the atmosphere to the weather conditions as shown on the daily weather maps.
Observations made in different parts of the world, on mountains and in balloons, have shown that on the average the temperature decreases from the earth’s surface upwards at the rate of about 1° in 300 feet of ascent. The rate of vertical decrease of temperature is known in meteorology as the vertical temperature gradient. When it happens that there is for a time an increase in temperature upwards from the earth’s surface, the condition is known as an inversion of temperature.
As a result of the decrease of temperature with increasing altitude above sea level, the tops of many high mountains even in the Torrid Zone are always covered with snow, while no snow can ever fall at their bases, owing to the high temperatures which prevail there. Balloons sent up without aëronauts, but with self-recording instruments, have given us temperatures of -90° at a height of 10 miles above the earth’s surface. On Dec. 4, 1896, Berson reached a height of 30,000 feet and noted a temperature of -52°. Inversions of temperature are quite common, especially during the clear cold spells of winter. Under such conditions the tops and sides of hills and mountains are often much warmer than the valley bottoms at their bases. A good example of an inversion of temperature occurred in New Hampshire on Dec. 27, 1884. The pressure was above the normal, the sky clear and the wind light. The observer on the summit of Mt. Washington reported a temperature of +16° on the morning of that day, while the thermometers on the neighboring lowlands gave readings of from -10° to -24°. In Switzerland, the villages and cottages are generally built on the mountain sides and not down in the valley bottoms, experience having taught the natives that the greatest cold is found at the lower levels.
CHAPTER XXI.
WINDS.