When accompanied by rain or by snow they are the “storms” of popular tradition. Local disturbances, such as tornadoes, water spouts and thunder-storms, usually affect pressure, and leave each its record on the sheet of the recording barometer. Observers learn quickly to interpret these records.
Isobars and Gradients.—The distribution of pressure is best shown by means of lines drawn on a map through adjacent points having the same pressure. These lines are isobars. The maps, pp. 40 and 42, show midwinter and midsummer isobars. For the sake of comparison, the figures are reduced to the basis of sea level and temperature of 32° F (0° C). The isobars on the daily weather map show the respective positions of highs and lows, and from them the daily forecasts are made.
On the daily weather map the conditions of pressure are interpreted by a study of the relative positions of the isobars, which constitute a contour map of the air. If the isobars of a high, or of a low, are close to one another, the slope of the air wave is steep; if far apart, the slope is gentle. Therefore, in either case, they show the gradient of pressure, and from the gradient of pressure an approximate velocity of the wind may be indicated.
About half a century ago, Whipple, of Kew Observatory, prepared an empiric table based upon isobars 15 nautical miles apart—that is, a 15-mile gradient. If, for instance, the gradient is 0.1 inch, the indicated velocity of the wind will be approximately 9 miles per hour; if 0.2 inch, it will be about 17 miles per hour, etc. The study of the pressure gradient, therefore, is a fairly accurate indication of wind velocity. It also enables the observer to make a reasonably accurate forecast of wind velocity from twenty-four to thirty-six hours in advance.
Actual and Recorded Pressure.—Pressure decreases with altitude, at a varying rate. If the lowest of a pile of ten books is lifted, the weight of the nine books above it must be overcome; but if the fifth book is lifted, the weight of only four books must be overcome. The same principle applies to the atmosphere. At sea level a column of air I square inch in cross-section, presses with a weight of 14.7 lbs., but at a height of 19,000 feet the weight of the column is only half as great. For the first few hundred feet above sea level the pressure decreases at the rate of 0.1 inch for each 90 feet of ascent; at
