"Fog in the winter is normally the result of a strong flow of warm, moist air from the Gulf of Mexico, which normally does not occur in the winter," he said. "Usually that occurs in March and April. It's caused by either low pressure over New Mexico or high pressure over Bermuda. Both are common in the summer. Both are rare in the winter. More than 80 percent of moisture that falls in the Midwest is from the Gulf of Mexico, and the primary cause of it coming here is the Bermuda high pressure. The pressure arrived very early and much stronger than usual by April and May this year. And it was the case in 1993."