Winds may blow steadily from one direction for an entire day or for several days; they may change direction with such rapidity that they blow from all quarters of the compass within a few hours; they may change velocity in a short time from a gentle breeze to a strong gale. While the average wind velocity at Chicago is 13 miles per hour, or 312 miles per day, on February 12, 1894, during a severe storm, the wind move ment for the twenty-four hours was 1,347 miles, an average of 56 miles per hour. For a five-minute period the rate was 54 miles per hour, and the fastest mile was at the rate of 115 miles per hour.
The rainfall for each month in Illinois averages more than 2 inches and less than 5 inches, yet the monthly precipitation may vary from 0.00 as at Bushnell, McDonough County, and at Yorkville, Kendall County, in November, 1904, to 20.03 inches as at Monmouth, Warren County, in September, 1911.
The rainfall resulting from the passage of a single cyclonic disturbance may vary from a mere sprinkle to a heavy down pour of more than 8 inches, which resulted from the passage of the Galveston hurricane across the state August 17-18, 1915. Rain may fall so gently that dry ground is barely moist after an hour or more, or it may fall at the rate of 1 inch in 5 minutes as at Springfield on July 23, 1917. The former record was 1 inch in 12 minutes.
Unchangeableness of Illinois climate.—Since climate deals with averages of the various weather elements, sudden changes of temperature, unexpected shifting of wind direction, violent wind storms, unusual drought, or excessive rainfall may not modify to any appreciable extent the averages based on thousands of observations extending over periods of 20 to 40 years. Climatic maps thus take on a character of permanence while weather maps for a day, or a month, or a year may present a wide variation from the climatic conditions established on averages of 20 or more records of weather conditions at a station for a certain date, a certain month, or a certain year.
An average temperature established by observations over a long period of years is known as the normal temperature.
Even when marked departures from the normal occur, the excess is reduced to very slight changes on the average. Thus, while July, 1901, holds the record of the state as the hottest month with an average of F., an excess of 6.3° above
the normal for July, the average temperature of the year for the state was 0.1° below the normal for the state. While December, 1917, was S colder than the normal for Decem ber, the year 1917 was but colder than the normal annual temperature. These departures, when combined with all previous records, change the averages only slightly. In fact, December, 191S, was S.2° warmer than normal, thus offsetting completely in the average for December the unusually cold temperature of the previous December. While January, 191S, holds the record of the state as the coldest month, with an average of or 14.3° colder than the normal for January, the average for 1918 was warmer than normal. The normal annual rainfall of the state, 36.51 inches, would be changed only a fraction of an inch by the 47-inch rainfall of 1898, or by the 25-inch rainfall of 1901. In fact, when both these extremes of rainfall are used in the computation their influence on the average entirely disappears.
This unchangeable character of the state's climate does not indicate uniformity of climate throughout the year. Illinois has a climate of the continental type and therefore experiences great seasonal changes of temperature with smaller seasonal variations in rainfall. The average temperature for the coldest month, January, is 20.9°, and that of the warmest month, July, December has the smallest average precipitation, 2.16 inches, and May has the largest, 4.07 inches.
Weather Bureau stations. United States Weather Bureau Service was established in 1870. It has been enlarged and extended until there arc now 200 regular stations, fully equipped, and in charge of trained observers. Four of these stations are in Illinois. The one at Chicago is in the Federal Building; the one at Peoria is in the Weather Bureau Building on the campus of Bradley Polytechnic Institute; the one at Springfield is in the Weather Bureau Building at 107 Monroe Street; and the one at Cairo is in the Federal Building. Each of these stations is equipped with a full set of instruments for measuring temperature; pressure; direction and velocity of wind; rainfall; snowfall; humidity; sunshine and cloudiness.