The first fall freeze, with lows of 32 degrees or colder, typically occurs during the first half of October in central Illinois. Click here to monitor the occurrence of this year's first freeze. (click image to enlarge)
Frost occurs when the surface temperature cools to the dewpoint temperature, and both are at or below 32 degrees. Water vapor in the air turns directly into ice without becoming a liquid first, producing visible ice crystals on surfaces. Freezes occur when the air temperature (measured about 6 feet off the ground) falls below 32 degrees; temperatures of 28 degrees or colder are considered to be a hard freeze.