The weather during the Battle of Antietam was marked by extreme fog, which significantly hindered visibility for both Union and Confederate troops on the morning of September 17, 1862.

This thick fog delayed the start of the battle and complicated troop movements, making it difficult for commanders to coordinate their forces effectively. The lack of visibility contributed to confusion and miscommunication on the battlefield, impacting strategic decisions. Additionally, the fog created an atmosphere of uncertainty, heightening the tension as soldiers engaged in combat, ultimately influencing the battle's dynamics and contributing to the high casualty rate.