The Chicago station issues district forecasts for Illinois, Wisconsin, and states west to Montana. It has charge of the storm-warning equipment for a part of Lake Michigan. It is the central office for the corn and wheat regions of the country. The Springfield station is the section center for the state. It receives reports from the co-operative observers and issues monthly and annual reports for the state. It also receives reports from a large number of crop correspondents throughout the state and issues a weekly report of Illinois crop conditions during the crop-growing season. The Peoria and Cairo stations maintain river gauges. The Cairo office also has charge of the river stations on the Ohio from Cairo to the mouth of the Wabash, and on the Tennessee and Cumberland rivers throughout their courses.
In addition to the fully equipped stations, the government has established about 4,500 co-operative stations, about 70 of which are in Illinois. The location of the Weather Bureau stations of Illinois is indicated on the foregoing map. Each co-operative station has a rain gauge and a thermometer shelter with maximum and minimum thermometers. Observa tions are made and recorded daily by co-operative observers. At the end of the month the records of both the regular and the co-operative stations are sent to the section director at Springfield. From these he compiles and publishes a monthly report giving in detail the records of the various stations accompanied by a general description and summary of the weather of the month. Early in the year the section director issues an annual summary for the preceding year. The facts of greatest general interest concerning monthly and annual weather conditions are widely published by the newspapers.
Weather records.—For climatological data the Weather Bureau has divided Illinois into three divisions along county lines. The northern division lies between the Illinois-Wisconsin boundary and the county lines near the forty-first parallel; the central division extends southward to the county lines near the thirty-ninth parallel; the southern division includes the rest of the state.
It is impossible to give here the climatological data for all the stations. Selection has therefore been necessary. Of the 15 stations chosen, 5 are in each division. These are well distributed over the state and within the divisions. Only small sections of the state are more than 50 miles from one of these selected sta tions. The climatologi cal data, therefore, for any point within the state will be very nearly the same as that of the near est station given in the tables. The stations are
arranged in order of lati tude from north to south. For convenience in com puting the north-south distances between sta tions, one minute of lati tude may be given the value of one mile, al though it is somewhat more than a mile in length. While latitude is the chief factor in deter mining differences in temperature; and distance from the Gulf of Mexico and the Atlantic Ocean the principal factor in determining differences in rainfall in Illinois; yet other factors, such as altitude, topography, and the presence of water bodies, have their influences, which may modify slightly the results expected from the main factors alone.
Table I, which deals with temperature, shows many interesting facts, but, owing to the limited number of places, not all extremes are shown. Sycamore in DeKalb County has the lowest annual temperature in the state, 47.1°, and Ca-iro the highest, 57.6°, giving a difference of 10.5°. This is at the rate of 1° to 33 miles, or about 2° of temperature to 1° of latitude. In July, however, the differences are less, and in January more, than for the year. In July the lowest temperature, 71.S°, is at Riley in McHenry County, and the highest, 79.7°, at Carbondale in Jackson County. This gives a difference of 7.9° among the stations of the state. In January, the lowest temperature, 17.9°, is at Freeport in Stephenson County, and the highest, 35.8°, at Equality in Gallatin County, giving a difference of 17.9°, or 23- times as great as for July. All parts of the state have experienced temperatures above and below —15°. The coldest tem perature on record for the state, —32°, occurred in Ashton, Lee County, February 13, 1905, the hottest, at Centralia., in Marion County, July 22, 1901. The extreme range for Illinois is thus 147°. The temperature range for the United States is 1S4°, and for the world 217°.
Table II, which records frost data, indicates the average length of the frost-free season or the "growing season" at the various stations. It also shows the extent to which this period has been shortened at times in the spring and in the fall. The growing season of the state varies from 5 months in the north to 7 months in the south. The longer growing period of southern Illinois and the somewhat less severe winter temperatures give more favorable conditions for fruit growing than are found in the northern part of the state. It also makes possible the successful cultivation of cotton in the lowlands of the extreme southern part of the state.