Illinois lies midway between the Continental Divide and the Atlantic Ocean and some 500 miles north of the Gulf of Mexico. Illinois' climate is continental, i.e., cold, rather dry winters, warm, humid summers, and frequent short duration, but relatively large magnitude changes in temperature, humidity, precipitation, and wind direction. Excellent soil and annual mean temperatures varying from 47F in the north to 58F in the south, and annual rainfall (rather well-distributed through the growing season) from 34 inches in the north to 46 inches in the south are favorable for agriculture. However, year-to-year precipitation varies substantially.

Illinois' weather and climate depends on the frequency and duration of air originating over (1) Canada and the Arctic, (2) the Pacific Ocean, and (3) the Gulf of Mexico. During winter, air from the first two areas dominate Illinois, whereas the latter dominates from late spring through early fall. Spring and fall experience the greatest day-to-day fluctuations.

Since the Gulf is our primary source of water vapor, northern Illinois receives less precipitation than does the south in the mean, as it is further from the source. Summers in northern Illinois are somewhat cooler and less humid than those of the far south, and the duration of "summer" like weather is some four weeks shorter than that of the south. Conversely, winters of northern Illinois are longer and colder than those of the south. Indeed, a winter in southern Illinois will occasionally pass, without the occurrence of one substantial snowfall, and no groundfrost.

Illinois extends about 400 miles north to south. Daily high temperatures in southern Illinois are typically 10F to 12F warmer than those in the north. Sub-zero (F) temperatures occur on an average seven days annually in the far north, and only one day or less in the south. Temperatures greater than 90F occur about 45 days per year in the south, but only 12 days per year in the north.

Except for heavy snowstorms which typically impact relatively small areas, winter precipitation in Illinois tends to continue for several hours and affect most or all of the state with about equal amount of precipitation. Heavy snowstorms tend to affect only narrow bands of some 50 miles north to south, and about 200 miles east to west. In contrast, summer precipitation occurs principally as brief showers or thunderstorms, affecting only a few square miles. The small scale feature of showers almost always results in erratic and uneven precipitation patterns.

Severe weather in Illinois includes heavy snowstorms (average of five somewhere in the state annually), severe windstorms, including tornadoes and straight line winds, and freezing precipitation. Illinois averages 28 tornadoes per year, with an average of 12 days with tornadoes annually. Although tornadoes have occurred in all months in Illinois, June is the month in Illinois exhibiting the greatest tornado frequency (April is a close second.) Straight line winds (associated with thunderstorms or strong cold fronts) also are least frequent in winter, reaching a maximum in July. Central Illinois receives more freezing precipitation storms than either north or south, with an average of five per year.

Annual average snowfall ranges from approximately 30 inches in the north to approximately ten inches in the south, but there is great variation from year to year. Southern Illinois occasionally experiences winters with little "winter" weather during the winter months. For example, during the 1956-57 winter months, Mt. Vernon received only 0.6 inches of snow (average of about ten inches).

Sunshine averages more than 70 percent of that possible during the three summer months, but only 45 percent of that possible during winter months, primarily due to the large aerial coverage and persistence of clouds at that time of the year.

Although Illinois is relatively flat, winds are not particularly strong nor persistent. Wind speeds in Illinois average eight to 12 mph. The strongest winds usually occur during the day and from winter through early spring. The weakest winds usually occur in the summer and early fall.

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