The chief interest and value of the instrumental work in meteorology are to be found not only in the taking of the daily observations at stated hours, but in the working out of numerous simple problems, such as may readily be undertaken with the help of the instruments already described. Thus, the temperature of the air (obtained by the sling thermometer, supplemented by maximum and minimum thermometers, and by the thermograph if available) can be determined under a variety of conditions, e.g., close to the ground, and at different heights above the ground; at different hours, by day and night; in different seasons; in sunshine and in shade; during wind and calms; in clear and cloudy weather; in woods and in the open; over bare ground, grass, snow, or ice; on hills and in valleys. Observations may also be made of the temperature of the ground and of a snow cover, at the surface and at slight depths beneath the surface, in different seasons and under different weather conditions. Among the problems which may be worked out by means of such observations as these are the following:—
A. The Diurnal Range of Temperature under Different Conditions and at Different Heights above the Ground.—Under the influence of the sun the regular normal variation of temperature during
24 hours is as follows: A gradual increase, with the increasing altitude of the sun, from sunrise until shortly after noon, and a gradual decrease, with decreasing altitude of the sun, from the maximum, shortly after noon, until the minimum, about sunrise. This variation is known as the diurnal variation of temperature. Curve a in Fig. 12 illustrates well the normal diurnal variation of temperature, as recorded by the thermograph during a period of clear, warm spring weather (April 27-30, 1889, Nashua, N. H.). The diurnal range of temperature is the difference between the maximum and minimum of the diurnal oscillation. The regular normal diurnal variation in temperature is often much interfered with by other controlling causes than the sun, e.g., cyclonic winds, clouds, etc.
I. Study and compare the diurnal ranges of temperature as indicated by the maximum and minimum thermometers, or the thermograph, in the instrument shelter, in clear, fair, cloudy, and stormy weather, during winds and in calms, in different months. Summarize your results by grouping them according to the general weather conditions, and according to the months or seasons in which the observations were made. For example, group together and average the ranges observed on clear, calm days in winter; on similar days in early summer or autumn; on clear days with brisk northwest winds in winter; on similar days in early summer or autumn; on calm days with overcast sky in the different seasons; on stormy days with strong winds, etc. Study carefully the weather maps for the days on which your observations are made. Pay special attention to the relation between the diurnal ranges and the control exercised over these ranges by cyclones and anticyclones through their winds and general weather conditions.
II. Observations of diurnal ranges of temperature at different heights above the ground may be made by means of maximum and minimum thermometers fastened (temporarily) outside
of the windows of different stories of the school or of some other building. These observations should be made out of windows facing north, and care should be taken to check, so far as possible, any draft from within the building out through the window during the taking of the observation. If a fire escape is provided on the building, the instruments may often be conveniently fastened to that.
Study the ranges under different conditions of wind and weather at various heights above the ground, and compare these results with those obtained under I. Notice the relations of all your results to the cyclonic and anticyclonic areas of the weather maps.
The diurnal range of temperature in the air over the open ocean from the equator to latitude 40° has been found to average only 2° to 3°. In southeastern California and the adjacent portion of Arizona the average diurnal temperature range in summer is 40° or 45°. Over other arid regions, such as the Sahara, Arabia, and the interior of Australia, the range also often amounts to 40°. Observations of temperature above the earth’s surface, in the free air, made on mountains, in balloons, and by means of instruments elevated by kites, indicate very clearly that the diurnal range of temperature decreases with increasing elevation above sea level. The results obtained at Blue Hill Observatory, Massachusetts, by means of kites, show that the diurnal range of temperature almost disappears, on the average, at 3300 feet (1000 meters).
B. Changes of Temperature in the Lower Air, and their Control by the Condition of the Ground, the Movement of the Air, and Other Factors.—Determine the changes of the temperature in the lower air by making frequent readings of the ordinary thermometer in the instrument shelter, of the sling thermometer, or by an examination of the thermograph record. Group these changes, as in Problem A, so far as possible according to the weather conditions under which they occurred, and try to classify the kinds of change roughly into types. Study the control of these
various types by the wind and other weather conditions accompanying them, as illustrated on the daily weather maps. The control exercised by different conditions of the earth’s surface may be studied by means of observations made with the sling thermometer over different surfaces, such as grass, bare ground, snow, etc.