Marion, Illinois, located in Williamson County, experiences a humid subtropical climate. Summers are typically hot and humid, with temperatures ranging from the mid-80s to low 90s Fahrenheit, while winters are mild to cool, with average temperatures in the mid-30s to low 40s Fahrenheit. Rainfall is evenly distributed throughout the year, with an average annual precipitation of around 45 inches.

Hydrologically, Marion is part of the Big Muddy River watershed, which is characterized by numerous streams and lakes. The region is primarily agricultural, with corn and soybeans being the predominant crops. The hydrology constituents in Marion are influenced by agricultural runoff, which can introduce nutrients and sediment into the local water bodies.

Outdoor recreation enthusiasts in Marion can explore a variety of activities. One popular destination is the Crab Orchard National Wildlife Refuge, which offers opportunities for bird-watching, hiking, fishing, and hunting. The Shawnee National Forest is also nearby, providing scenic trails for hiking and camping. Additionally, locals and visitors can enjoy water activities such as boating and fishing on the many lakes and rivers in the area, including the Rend Lake Conservation Area.