WEATHER AND CLIMATE Illinois weather and climate.—Illinois experiences the weather and climate characteristic of middle latitudes in the interior of continents. It lies nearer the equator than the pole. The fortieth parallel passes through the central part of the state. This latitude insures long days and steep sun's rays in summer with abundant warmth and sunshine for the growth of staple food crops. It also insures short days and slanting rays in winter with cold weather, for which provision must be made during the preceding summer. The latitude of Illinois places it throughout the year in the belt of the prevailing westerly winds, whose usual direction is interrupted at inter vals of a few days by the passage of low-pressure areas, or cyclonic storms, of large extent, averaging about 500 miles in diameter. These cyclones carry rainfall from the Gulf of Mexico and the Atlantic Ocean to Illinois and the Mississippi Valley.
Only a few small areas of Illinois have an elevation of more than 1,000 feet above the sea. The state is a plain with an average altitude of about 600 feet. The slight differences of elevation within the state have but little influence upon distri bution of temperature or rainfall, and they permit easy move ment of winds in all directions.
All parts of the state lie more than 500 miles from the Gulf of Mexico, more than 600 miles from the Atlantic, and more than 1,500 miles from the Pacific Ocean. The long distance to the Pacific and the high intervening mountains preclude the possi bility of important influence of the western ocean on the climate of Illinois. The open plains to the Gulf and the moderate altitude of the Appalachians furnish free passage to Illinois of the moisture-bearing winds, which, under cyclonic influences, blow from the Gulf and the Atlantic, bringing the abundant and well-distributed rainfall which enables Illinois to rank as the first agricultural state in the Union.
The location of Illinois, therefore, determines that the weather and climate shall he of the continental type with warm summers, cold winters, and a rainfall exceptionally well adapted to the development of agricultural pursuits.
Changeableness of Illinois weather.—In a region situated as Illinois is, weather changes are frequently rapid and occasionally excessive. The ordinary temperature changes controlled by solar influences whereby the heat of the clay gradually increases until an hour or two after midday, and then gradually decreases during late afternoon and all night, may, under cyclonic influences, be greatly modified or entirely reversed. The following rapid changes in temperature took place at Chicago, but all parts of the state have similar experiences. On May 10, 1011, the temperature rose 27° F. in two hours; on April 11, 1010, the temperature fell 28° F. in one hour; on March 29-30, 1895, the temperature rose 48° F. in a twenty-four-hour period; on November 11-12, 1911, the temperature fell 61° F. in a twenty-four-hour period. This rapid temperature change was general throughout the state. At some stations the fall was from F. to 19° F., or F. in an eighteen-hour period.
While this is the most remarkable change of temperature recorded by the Weather Bureau, more striking results occurred during a sudden drop in temperature on December 20, 1S36, as shown by reports of citizens living at that time in central Illinois. Early in the day, with a temperature of about F., a rain had changed the snow on the ground to slush. Suddenly the temperature grew colder, and in a few minuses the slush became solid ice strong enough to bear the weight of a horse. Chickens were caught in the freezing slush and held fast. Ducks swimming on the pond had ice frozen to their feet and feathers. A man riding to Springfield on horseback through the rain was frozen to his saddle. Arriving at Springfield, he and his saddle were removed from the horse, carried into a warm room and thawed apart. In striking contrast to these unusual changes, the temperature at Chicago on March 24, 1891, remained all day at 32° F.