With a population of around 4,500, Sullivan sits in Illinois, United States. The city operates in the Central Time Zone.
In 2025, Sullivan recorded an average annual temperature of 55°F — about 1.1°F above the 30-year normal of 53°F. The hottest day reached 96°F in September; the coldest was -5°F in December. Total precipitation was 32 inches (drier than usual). The wettest month was April.
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