National Geographic : 1890 Apr
Geography of the Air. been as low as 0.09 per centum in a very dry year. In the Mur ray basin the average discharge relative to the rainfall is esti mated to be about 27 per centum from a record of seven years, and has risen as high as 36 per centum in a flood year. In connection with the regimen of rivers, it appears a proper occasion to again refute the popular opinion that the spring and summer floods of the Missouri and Mississippi valleys result from the melting of the winter snows. This is an erroneous impres sion which I have combatted since 1873, when my duties required a study of the floods of the entire Mississippi catchment basin. It is only within the last two years, however, that the meteorological data has been in such condition that the opinion put forth by me could be verified, namely : that the floods of the late spring and early summer owe their origin almost entirely to the heavy rains immediately before and during the flood period. Occasionally a very heavy fall of snow precedes extended gen eral rains ; but in this case the snow is lately fallen and is not the winter precipitation. Referring to the Missouri valley, the section of the country where the winter snowfall has been thought to exercise a domin ating influence in floods, it has elsewhere been shown by me that about one-third of the annual precipitation falls over that valley during the months of May and June. In either of the months named the average precipitation over the Missouri valley is greater than the entire average precipitation for the winter months of December, January and February. Woiekoff thinks that the anomalies of temperatures shown in forest regions, particularly in Brazil-with its abnormally low temperatures, are due to heavy forests promoting evaporation, and by causing the prevalence of accompanying fogs thus pre vent more intense insolation. He considers this an argument for the maintenance of forests to sustain humidity and distribute rain over adjacent cultivated land, as well as to maintain the fer tility of the soil, which diminishes rapidly by washing away of the soil after deforestation. W. Koppen has devised a formula for deriving the true daily temperature from 8 A. M., 2 P. M. and 8 p. M. observations in con nection with the minimum temperature, in which the minimum has a variable weight dependent on place and month. The results of Koppen's formula tested on six stations in widely dif ferent latitudes, indicate that it is of value.