A witch storm is a type of intense Midwest windstorm that can occur in late October and November. The term "witch storm" is used to describe the "Witches of November," which refers to the strong winds that frequently blow across the Great Lakes in autumn. These storms are caused by intense low atmospheric pressure and can have winds as weak as tropical depressions or as strong as hurricanes. Witch storms form in the fall when increasing temperature contrasts from north to south across the U.S. produce stronger low-pressure systems that can then produce intense winds. They can be so severe that their force is equivalent to a low-level hurricane, with winds above 80 miles per hour and towering 20-foot seas. The storms are also a major hazard for shipping on the Great Lakes, and have caused numerous shipwrecks over the years. One of the most well-known witch storms sank a huge iron-ore ship called the Edmund Fitzgerald on Lake Superior in November 1975. These storms often cause tree damage and power outages.