Historical weather data, all-time temperature records, and monthly climate averages for Swansea, IL. Explore typical weather conditions by week for trip planning, and see how today compares to the historical record.
April 4th in Swansea typically lands in that pleasant shoulder-season sweet spot, with average highs of 63° and lows around 41°. The date has a 61-degree range in its history — from a balmy 84° in 2011 down to a brutal 23° low in 1987 — so don't put the jacket away just yet. Rain is possible about one in three days, but snow is effectively off the table.
Swansea's weather has real teeth at both ends of the thermometer — 110° in July 1954 and -27° in January 1977 represents a jaw-dropping 137-degree swing in recorded history. A single day in June 1957 dumped 8.15 inches of rain, and a January 1982 snowstorm piled up nearly 15 inches. This is classic Illinois interior climate: no ocean to moderate things, so the extremes are genuinely extreme.
Summers here are hot and humid, with mid-July highs averaging 90° and June bringing the heaviest rainfall of the year. If you're looking for the most comfortable stretch, aim for May (78° avg high) or September (82°), when the heat backs off but the cold hasn't arrived. Winters are cold but not relentless — January highs average just 41° — and things warm up quickly once March arrives.