On October 28, 2007, the thermometer at the Twin Cities International Airport dipped down to 31 degrees and was the first minimum temperature of 32 or colder for the season. How rare is this? Frost statistics can be difficult to compare due to the fact that for a good part of the Twin City observing history, the official thermometer was on top of a building in downtown Minneapolis, or on top of the old Wold Chamberlain building at the airport. The heights ranged from 105 feet above the ground in downtown Minneapolis, to 32 feet above the ground at the International Airport. This would in effect cause the official temperature to miss some of the calm clear nights when cooler air would settle closer to the ground. From 1891 to 1959 when the thermometer was on top of a building, there are three times when the first frost date was in November, with the latest on November 7, 1900.
Beginning with January 1, 1960 the thermometers were moved to a location approximately five feet above the ground at the airport, and they have remained at this height ever since. The latest frost during this era is October 30, 1973 with the earliest frost occurring a year later on September 3, 1974. The average (1960-2006) frost date is October 6. To compare with this season, these are the statistics that should be used given the elevation of the thermometer.
One more factor must be mentioned when discussing frost. The urban heat island of the Twin Cities continues to expand outward from the center of the downtown areas, as more of the surrounding landscape turns from fields to blacktop. This may also have an effect on the first date of frost as well.