What Causes Frost?

Typically at the beginning and ending of the growing seasons, the possibility of frost/freezes threaten sensitive vegetation and agriculture. For La Crosse, the average last freeze occurs on April 26th and the average first freeze occurs on October 10th. For Rochester, the average last/first freeze is May 7th and September 30th, respectively.

The following list are some meteorological conditions that can lead to frost conditions:

Clear skies lead to radiational cooling, allowing the greatest amount of heat to exit into the atmosphere.
Calm to light winds prevent stirring of the atmosphere, which allows a thin layer of super-cooled temperatures to develop at the surface. These super-cooled temperatures can be up to 10 degrees cooler than 4-5 feet above the surface, where observations are typically taken. For example, if conditions are favorable, air temperatures could be 36 F, but the air in contact with the surface could be 30 degrees or colder.
Cool temperatures, with some moisture, that promote ice crystal development. If the super-cooled, freezing temperatures can cool to the dew point (the temperature at which, when cooled to at constant pressure, condensation occurs; moisture will have to come out of the atmosphere as fog, frost, etc) frost could develop on exposed surfaces.
A local study done on frost formation relating temperature to dew point has these guidelines for frost: temperatures from 38 to 42 F can lead to patchy frost, 33 to 37 areas of frost, and 32 and below widespread frost/freeze. Note that the study did not factor in other considerations to frost, such as sky cover and wind speeds.
Local topography has a large role in determining if and where frost develops. Cold air will settle in the valleys since it is heavier than warm air, therefore frost conditions are more prone in these regions. Valleys also shelter the area from stronger winds, enhancing the potential for frost.

Other local effects, such as soil moisture/temperature and stage of vegetation "greenness" are factors that can affect the possibility of frost forming.