Table III, which records precipitation, shows the normal annual rainfall and the amount received in each of the four seasons, also the amount falling during the six months, April to September, inclusive—the half of the year during which crops make practically all their growth. In recording snowfall as a part of the precipitation, the observer makes three records; one indicates the depth of newly fallen snow on the level; another, the depth of snow including previous snowfalls; and the other, the amount of water obtained by melting the snow which fell during the past 24 hours. This latter result is added to the rain fall of the month, and thus snowfall finds its way into the precipitation records as rain. The table shows clearly the influences of latitude on the amount of snowfall received.
The annual-rainfall map shows that the normal annual rainfall of Illinois is not as regu larly distributed as the normal annual tempera ture. There is, however, a similar change as the state is crossed from north to south. The least normal annual rainfall reported by any station is 31.28 inches at Elgin in Kane County; the heaviest is 47.44 inches among the Ozark Hills at Anna in Union County, making a difference of 16.16 inches.
The normal annual rainfall for the state, 36.54 inches, with its seasonal distribution, is exceptionally favorable for the production of maximum crops. If the average rainfall were the actual rainfall year by year, season by season, and month by month, there would be no crop failures of any kind; there would be neither swollen streams nor flooded farm lands; the larger precipitation of spring and summer would always suffice for the rapid growth of all crops; and the smaller rainfall of autumn and winter would give the best of weather for har vesting and threshing small grains, and for the ripening and gathering of corn.
The normal temperatures of the state and the average length of the growing season are also favorable for the production of large yields of staple farm crops. It is be cause the normal or average climatic conditions of Illinois are especially satisfactory that the state can undergo marked variations from aver age conditions without dis astrous results.
The rainfall of a single year.—A study of the cli matic factors of a single year shows that weather condi tions may vary widely from the climatic averages. The abundant crops of 1915 were produced with an annual rainfall for the state of 41.90
inches, an excess of 4.77 inches. The variations among stations were from 29.46 inches at Joliet, where the deficiency for the year was 3.33 inches, to 59.16 inches at Chester, Ran doph County, where the excess was 16.21 inches. The differ ence between the two stations for this year was 29.70 inches, while the greatest difference between the normal annual rain fall for any two stations is 16.16 inches, and the difference between the six northernmost and the seven southernmost counties is 10.55 inches. During 1915 the varia tion in rainfall by months was from only a trace at Fairfield, Wayne County, in April, to 14.77 inches at Griggs ville, Pike County, in June. There was a de ficiency of rainfall in the months of February, March, April, October, and November amount ing to 6.45 inches. The excess of 11.22 inches during the other seven months was suffi cient to equal the defi ciency of these five months and to add an excess of 4.77 inches for the year.
The table of state averages (Table 1V) shows how the pre cipitation of 1915 varied from the normal by months.
The following is from the annual summary of weather con ditions for Illinois in 1915, published by the Weather Bureau: The year opened with a rather cold month and some extremely low temperatures, but February was almost springlike and there was very little snow. The early spring was dry, with warm sunny weather in April. Crops were planted in good season, A period of cool, cloudy, and wet weather began in May and continued through August, and the summer was the coldest and rainiest in the meteorological history of the state. Destructive local storms were frequent, and much land was inundated. Corn was damaged and its growth retarded, and threshing operations were greatly delayed. The autumn was favorable for farm work, especially in October when the weather was clear and dry. Killing frost injured late corn in the more northern counties. Field work ended December 10 and considerable snow fell after the twenty-third.
The mean temperature was practically normal, but the highest, 101, has been equalled or exceeded in every previous year. Lower minimum temperatures occurred in 1905, 1910, and 1912. There have been but two wetter years since 1884, the number of cloudy days was exceeded only in 1898. The precipitation was above normal at nearly all stations; and in several counties the excess was more than 10 inches.