Autumn, Frost, and Indian Summer in Wisconsin

The leaves are falling from the trees and the temperatures are trending lower. It’s fall in Wisconsin.  For some of us, the first frost has already occurred.  The rest of us will have the experience in the near future.  The good news is that the first frost heralds a time commonly known as Indian Summer.  This is our last respite before Winter.


Frost is the term for several types of coatings or deposits of ice that may form in humid air in cold conditions, usually overnight. In temperate climates such as that of Wisconsin, it most commonly appears as fragile white crystals or frozen dew drops near the ground. In colder climates it occurs in a greater variety of forms.

Frost on the Grass

Fall Frost on the Grass

The definition from the National Weather Service is: Frost describes the formation of thin ice crystals on the ground or other surfaces in the form of scales, needles, feathers, or fans. Frost develops under conditions similar to dew, except the temperatures of the Earth’s surface and earthbound objects falls below 32°F. As with the term “freeze,” this condition is primarily significant during the growing season. If a frost period is sufficiently severe to end the growing season or delay its beginning, it is commonly referred to as a “killing frost.” Because frost is primarily an event that occurs as the result of radiational cooling, it frequently occurs with a thermometer level temperature in the mid-30s.

Frost may damage crops or reduce future crop yields, so farmers in frost-prone regions may spend a great deal of money on measures to prevent it forming.  For they typical gardener, frost signals the end of the outdoor growing season.

According to Wendell Bechtold, a meteorologist with the National Weather Service Forecast Office in St. Louis, MO, “Frost formation is a complex process, and conditions have to be “right” for it to occur. Frost forms on surfaces directly from the vapor state, without condensing as dew. If dew forms, frost formation is unlikely, even if the temperature drops below freezing.

Frost is more likely to form on surfaces above the ground first, such as house roofs, or automobiles, because the air immediately above the ground is usually a few degrees warmer than air a few feet higher. There is some heat transfer from the ground to the air a few centimeters above it. If there is much wind, frost will not form either. (Neither will dew, as both these occurrences require little or no wind, so the atmosphere will not stay mixed.) If the skies are cloudy, usually dew or frost will not form either, as the clouds reflect the radiated heat from the ground, which helps in keeping the lower layers mixed.

So the ideal conditions for frost formation is a night with clear skies, light winds, and a temperature forecast to be near or a little below freezing. Standard temperature measurements are taken from about 2 meters above ground. On a calm night the ground temperature can be as much as 5-7 degrees cooler than the standard temperature reading. If there is some wind, the air stays mixed, and the temperature difference disappears.”

The first frost in Wisconsin usually occurs in the months of September and October depending on location.

rost in Wisconsin - Historic

Median Date of First Occurrence of Fall Frost

Current dates of the first frost in Wisconsin are shown on the following map:

Most current date of first frost in Wisconsin

Most current date of first frost in Wisconsin

Here is the current information for a larger portion of the Midwest:

Current date of first freeze for locations in the US Midwest

Current date of first freeze for locations in the US Midwest

Indian Summer

Indian summer is a period of unseasonably warm, dry weather that sometimes occurs in Autumn.  The US National Weather Service defines this as weather conditions that are sunny and clear with above normal temperatures, occurring late-September to mid-November. It is usually described as occurring after a killing frost. There may be several occurrences of Indian Summer in a fall season or none at all.

Late-19th century Boston lexicographer Albert Matthews made an exhaustive search of early American literature in an attempt to discover who coined the expression. The earliest reference he found dated from 1778, but from the context it was clearly already in widespread use. For a detailed discussion of possible origins for the term, see this article. Although the exact origins of the term are uncertain, it is thought to have been based on the warm and hazy conditions in autumn when native American Indians chose to hunt.

Indian Summer

A Beautiful Indian Summer Day

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