Editing Williamson County, Illinois (section)

== Climate and weather ==
{{climate chart
|[[Marion, Illinois]]
|19|38|3.26
|23|44|3.17
|33|55|4.45
|42|66|4.54
|52|75|4.93
|61|83|4.32
|65|87|3.89
|63|86|3.72
|55|79|3.13
|43|68|3.06
|34|55|4.77
|24|43|3.72
| float      = right
| units      = imperial
| clear      = both
| source     = The Weather Channel<ref name="weather">{{cite web
| url        = http://www.weather.com/weather/wxclimatology/monthly/graph/USIL0727
| title      = Monthly Averages for Marion, Illinois
| access-date = January 27, 2011
| publisher  = The Weather Channel
}}</ref>}}
Williamson County lies on the border between [[humid continental climate]] ([[Köppen climate classification]] ''Dfa'') and [[humid subtropical climate]] ([[Köppen climate classification]] ''Cfa''), with neither large mountains nor large bodies of water to moderate its temperature. It is subject to both cold Arctic air and hot, humid tropical air from the [[Gulf of Mexico]] and, along with the rest of the midwestern United States, is home to some of the largest temperature extremes in the world.

The region has four distinct [[seasons]]. Spring is the wettest season and produces erratic severe weather ranging from [[tornado]]es to [[winter storm]]s. Summers are hot and humid, with only occasional and brief respite, and the humidity often makes the [[heat index]] rise to temperatures feeling well above {{convert|100|°F|°C|abbr=on}}. Fall is mild with lower humidity and can produce intermittent bouts of heavy rainfall, with the first [[snow flurry|snow flurries]] usually forming in late November. Winters are cold with periodic snow and temperatures often below freezing; however, thaws are usually frequent. Winter storm systems, such as [[Alberta clipper]]s and [[Panhandle hook]]s, can bring days of heavy [[freezing rain]], [[ice pellets]], and [[snowfall]].

The normal high temperature in July is {{convert|90|°F|°C|abbr=on}}, and the normal low temperature in January is {{convert|19|°F|°C|abbr=on}}, although this varies from year to year. Both {{convert|100|and|0|°F|1}} temperatures can be seen on an average 2 or 3 days per year. In recent years, average temperatures have ranged from a low of {{convert|19|°F}} in January to a high of {{convert|88|°F}} in July, although a record low of {{convert|-25|°F}} was recorded in January 1977 and a record high of {{convert|113|°F}} was recorded in August 1977. Average monthly precipitation ranged from {{convert|3.06|in}} in October to {{convert|4.93|in}} in May.<ref name="weather" />

Williamson County has [[thunderstorm]]s about 50 days a year on average. Thunderstorms contribute over half of the annual precipitation. Especially in the spring, these storms can often be severe, with high winds, large [[hail]] and tornadoes.

Some late autumns feature the warm weather known as [[Indian summer]]; some years see roses in bloom as late as early December.