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The maximum frost depth observed in the contiguous United States ranges from 0 to 8 feet (2.4 m). [1] Below that depth, the temperature varies, but is always above 0 °C (32 °F). Alternatively, in Arctic and Antarctic locations the freezing depth is so deep that it becomes year-round permafrost , and the term " thaw depth " is used instead.
The list of snowiest places in the United States by state shows average annual snowfall totals for the period from mid-1985 to mid-2015. Only places in the official climate database of the National Weather Service, a service of NOAA, are included in this list.
Illinois averages around 50 days of thunderstorm activity a year which put it somewhat above average for number of thunderstorm days for the United States. Illinois is vulnerable to tornadoes with an average of 54 occurring annually, which puts much of the state at around 9.7 tornadoes per 10,000 square miles (30,000 km 2) annually.
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Frost advisory NPW – Issued during the locally defined growing season when minimum sheltered temperatures are forecast to be near or slightly above freezing (33 to 36 °F [1 to 2 °C]) over an extensive area on nights with good radiational cooling conditions (e.g., light winds and clear skies). Widespread frost can be expected.
Most frost quakes are associated with minor snow cover on the ground without a significant amount of snow to insulate the ground (i.e., less than 6 inches (15 cm)) A rapid temperature drop from approximately freezing to near or below −18 °C (0 °F), which ordinarily occurred on a timescale of 16 to 48 hours.