Page:Handbook of Meteorology.djvu/60

 48 an altitude of 3000 feet it is at the rate of 0.1 inch for 100 feet of ascent. The greater number of weather stations in the United States are 1000 feet or more above sea level, and many of those west of the Denver meridian are more than 5000 feet above sea level.

For purposes of comparison in the preparation of daily weather maps, all pressure observations are reduced to sea level basis. For this purpose such a reduction is necessary, and all reduced pressures within an altitude of a few hundred feet of sea level are sufficiently correct for practical purposes. For altitudes materially greater than 1000 feet, the results when applied to mean pressure, are erroneous. Thus, at Mount Washington, the mean recorded pressure for January, reduced to sea level, is greater than that for July. As a matter of fact, the actual mean pressure is less in January than in July. The following illustration will explain:

A compress 12 feet in height is filled with loose cotton. The pressure of its weight at the bottom is, say, 16 lbs. per square foot. Half way to the top, at the 6-foot level, the pressure is half as much. Now let us assume that the cotton is compressed so that its depth is only 9 feet. The pressure at the bottom remains the same; but at the 6-foot level, there is only half as much cotton as before compression; hence the pressure is half as great. The center of mass has been lowered in the process of compression.

The same principle applies in the case of measurements of the atmosphere. Thus, at Mount Washington, and at other stations of considerable altitude, expansion due to temperature-increase raises the center of mass in summer; mean pressure, therefore, is raised. In winter, low temperature causes contraction, lowering the center of mass and therefore the pressure. In other words, while sea level and also the observer’s station are at fixed altitudes, the center of mass is a varying altitude; it is raised by increasing temperature and lowered by decreasing temperature. Hence its effect on mean pressure.