The state of Illinois, extending as it does through five and a half degrees of latitude, has considerable variation in its climate. It has no mountains, and though undulating, it cannot be called hilly. Its extensive prairies, and level surface, give greater scope to the winds, especially in winter. In the southern part of the State, during the three winter months, snow frequently falls, but seldom lies long. In the northern part, the winters are as cold, but not so much snow falls, as in the same latitudes in the Atlantic States.
The Mississippi at St. Louis is frequently frozen over, and is crossed on the ice, and occasionally for several weeks. The hot season is longer, though not more intense, than occasionally for a day or two in New England.
During the years 1817-18-19, the Rev. Mr. Giddings, at St. Louis, made a series of observations upon Fahrenheit's thermometer.
Deg. Hund. Mean temperature for 1817 55 52 Do. do. from the beginning of May, 1818, to the end of April, 1819 56 98 Mean temperature for 1820 56 18
The mean of these results is about fifty-six degrees and a quarter.
The mean temperature of each month during the above years, is as follows:
Deg. Hund. January 30 62 February 38 65 March 43 13 April 58 47 May 62 66 June 74 47 July 78 66 August 72 88 September 70 10 October 59 00 November 53 13 December 34 33
The mean temperature of the different seasons is as follows:
Winter, 34.53--Spring, 54.74--Summer, 74.34--Autumn, 60.77.
The greatest extremes of heat and cold during my residence of eighteen years, in the vicinity of St. Louis, is as follows:
Greatest heat in July 1820, and July 1833, 100 degrees. Greatest cold January 3d, 1834, 18 degrees below zero,--February 8th, 1835, 22 degrees below zero.
The foregoing facts will doubtless apply to about one half of Illinois. This climate also is subject to sudden changes from heat to cold; from wet to dry, especially from November to May. The heat of the summer below the 40° of latitude is more enervating, and the system becomes more easily debilitated than in the bracing atmosphere of a more northerly region.
At Marietta, Ohio, in lat. 39° 25' N. and at the junction of the Muskingum river with the Ohio, the mean temperature for 1834, was 52 degrees, four-tenths; highest in August, 95 degrees,--lowest, January, at zero. Fair days 225,--cloudy days 110.