Page:Popular Science Monthly Volume 86.djvu/145

Rh has sometimes led people to conclude that the hailstones must have necessarily come from the far north, falling as they do on days when the heat has been oppressive. True hailstones occur only with storms of the thunderstorm type, where violent convection extends to heights of five miles or more above the ground. Here the ascending currents are occasionally so strong that they carry aloft, far beyond the level of permanent freezing temperature, particles of moisture already condensed into raindrops. In the average, there is a fall of temperature of 1° F. for every 300 feet of height, so that even in midsummer, when the temperature at the ground is 90° F., one has to ascend but 3½ miles to encounter a freezing temperature. The water droplets, solidifying upon entering the freezing stratum of air, later fall to lower levels, where they may again be caught up by ascending currents to the colder strata above. This process may be repeated a number of times, with the result that the hailstones, upon finally reaching the ground, will show concentric layers of ice and snow. The moisture content more probably came from the Atlantic Ocean, to the east, or the Gulf of Mexico, to the south, rather than from the far north.

The development of meteorology and climatology has been so recent that the general public has not kept pace with the progress. While there are thousands of weather proverbs which are correct generalizations of weather observations extending over many years, a number of traditions have persisted which are apparently without scientific foundation. A few of these, originating in European countries, and doubtless true in their native environment, have proved inapplicable when imported to America. Others are inadequate as they make no distinction between the real and the apparent—between the objective and the subjective. Still others are found wanting because they are based upon fallacious ideas. Instrumental observations, laboratory experiments, and the exploration of the free air have exposed many more misconceptions. Though we have made but a small beginning in a systematic science of the weather, we have advanced far enough to make it possible to eliminate some of the earlier preconceived notions.