Page:Popular Science Monthly Volume 80.djvu/138

134 over this great region, for on the whole the most rain falls when it is most needed for agriculture. A different seasonal distribution of precipitation would banish agriculture from thousands of acres of land which are to-day giving our farmers good crop returns. The fact did not escape the watchful eye of Captain Lewis that the rainfall of the warmer months over the region which he crossed is essentially spasmodic and "patchy" in character, i. e., is of the shower or thunderstorm type, as contrasted with the more general and widespread rains and clouds of the large storms which characterize the winter months over the country as a whole. It is this very peculiarity of "patchiness" of the warm-season rains which renders them disappointing to the farmers whose crops are suffering from drought. A half-hour shower, covering perhaps a very small portion of a state, is a terribly exasperating occurrence to those whose lands are "screaming for water," but are outside of the limited area covered by the rain. Further, the fact that there is "very little rain or snow either winter or summer," is a sufficient emphasis on the general decrease in the rainfall to the west of the Mississippi River, which is so marked a feature on our mean annual rainfall maps. Captain Lewis paid particular attention to thunderstorms, in which, probably because of their violence, he seems to have been much interested. On April 1, 1805, he wrote, "I have observed that all thunder clouds in the western part of the continent proceed from the westerly quarter, as they do in the Atlantic states." This is perhaps the first specific mention of this important meteorological fact. On May 18, 1805, the record states: "We have had scarcely any thunder and lightning; the clouds are generally white, and accompanied with wind only." This we may take to indicate that the season of maximum thunderstorm activity had not begun, the clouds were doubtless our typical summer cumulus clouds, which, being best developed when the wind is strongest, i. e., in the warmer hours, are often called "wind clouds." A thunderstorm which occurred on June 27, 1805, receives special mention. This storm lasted two hours and a half, and was accompanied by hail about the size of pigeons' eggs, which covered the ground to the depth of 1$1/undefined$ inches. Some of the hail-stones rebounded from the ground to a height of 10 or 12 feet. Several of the men were knocked down and bruised; some got under the canoe for protection, and others covered their heads. One hail-stone weighed 3 ounces and measured 7 inches in circumference. The stones were generally round, and perfectly solid. Captain Lewis adds: "I am convinced that if one of these had struck a man on his naked head it would certainly have fractured his skull." On July 6, 1805, another thunderstorm brought hail which covered the ground and was near the size of musket balls. One blackbird was seen to be killed by the hail,