DUBUQUE, Iowa (KCRG-TV9) - Does it surprise you that fog is the most common in January? On average, January is indeed the foggiest month, and by a decent amount. Using data from the Dubuque Regional Airport, the National Weather Service found dense fog being observed an average of 11 days per January. Dense fog is considered a visibility of a quarter-mile or less. How long this dense fog lasted was not considered, just whether it happened.
The main reason behind this is the fact that we get snow in January, and this tends to be the coldest month of the year, too. Combine this with snowmelt events and a low sun angle, and you have a good setup for fog to occur.