WEATHER
Average annual temperatures ranges from 48°F (north) to 58°F (south),
with highs ranging from 57°F (north) to 67°F (south). Average winter highs range from the 30s
(north) to the mid-40s (south), while average lows range from the teens (north) to the upper 20s
(south). Average summer highs are in the 80s, while lows are in the 60s across the state. Both
spring and fall have more moderate temperatures. Average spring highs range from 57°F (north)
to 67°F (south), while average lows range from 36°F (north) to 48°F (south). Average fall highs
range from 60°F (north) to 70°F (south), while average lows range from 40°F (north) to 48°F
(south).
Illinois averages 10 days at or above 90°F (north) compared to just over 40 days (south).
Days at or above 100°F are quite rare, occurring about every other year (north) and 2 days
annually (south). Illinois averages 140 days at or below 32°F (north) but only 80 such days
(south). Days at or below 0°F range from 16 days annually (north) to 2 days (south).
Average annual heating-degree days range from 7000 days (north) to 4000 days (south).
Average annual cooling-degree days range from 800 days (north) to 1600 days (south). The base
temperature used for both heating-degree and cooling degree days is the same (65°F).
The average length of the frost-free growing season in Illinois ranges from 160 days
(north) to more than 190 days (south). Average dates of the last occurrence of 32°F in spring
range from April 28 (north) to April 7 (south), while the average dates of the first occurrence of
this temperature in fall range fromOctober 7 (north) to October 21 (south), and about October 14
near Lake Michigan, including the Chicago area, due to relatively warm waters of the lake.
The highest and lowest temperatures ever reported in Illinois were 117°F in East St.
Louis on July 14, 1954, and -36°F in Congerville on January 5, 1999.
CLIMATE OF ILLINOIS
Page 3 of 4
_____________________________________________________________________________
Illinois State Water Survey, State Climatologist Office, Copyright 2003
PRECIPITATION - Average precipitation exceeds 48 inches a year (south), compared to less
than 32 inches (north). Snowfall distribution is just the opposite, with averages of 36 inches a
year (north) and less than 10 inches (extreme southern Illinois). Winter snowfall is heaviest in
the Chicago area, enhanced by lake-effect snows from Lake Michigan.
Variability in precipitation also extends over time. There have been major multi-year
droughts in the 1930s and 1950s and major prolonged wet periods during the 1970s and 1980s.
May and June are typically the wettest months, and January and February are the driest. Each
year, Illinois has rainstorms producing 40 or more flash-floods each with 4 to 8 inches of rainfall
in a few hours in localized areas.
The average number of days with measurable precipitation ranges from 110 days (north)
to just under 100 days (south). The number of days with an inch or more of precipitation ranges
from 8 days (north) to 13 days (south), and the number of days with an inch or more of snow
ranges from 12 days (north) to 4 days (south). Days with 6 inches or more of snow are
infrequent, ranging from once per year (north) to once in three years (south). Typical dates of the
first inch of snow range from November 20
th
(Chicago area) to December 20
th
(south). Typical
dates of the last inch of snow range from March 26 (north) to March 5 (south). The average
number of days with a measurable snow depth (1 inch or more) ranges from 60 days (northwest)
to only 10 days (southwest).
The following is a list of precipitation records for Illinois. The greatest 24-hour rainfall
was 16.94 inches at Aurora on July17-18, 1996. The greatest one-year precipitation was 74.58
inches at New Burnside in 1950. The greatest 24-hour snowfall was 37.8 inches at Astoria on
February 27-28, 1900. The greatest winter snowfall was 105.1 inches at Antioch in 1978-1979.
SEVERE WEATHER – Illinois experiences about 29 tornadoes annually. Peak months are
April-June (63 percent of the total), but tornadoes have occurred in all months. Although Illinois
averages four tornado-related fatalities per year, the number varies widely from year to year.
Thunderstorms account for 50-60 percent of annual precipitation and are quite common
in Illinois with an average of 60 storms (far northeast) to 80 storms (southwest). Nearly half of
all thunderstorm days occur during the June-August period. Similarly, the average number of
cloud-to-ground lightning strikes per square mile ranges from 5 strikes (northeast) to more than
11 strikes (southwest). Some thunderstorms produce hail, and annual average hail-days vary
from 3.3 days (southwest) to less than 1.8 days (northeast).
SUMMARY- The climate of Illinois has five unique features:
•
Four distinct seasons, each with different conditions.
•
Major north-south temperature contrasts.
•
An extremely wide variety of types and amounts of precipitation with moderate
variations between monthly and seasonal average values.
•
Extreme variability of weather conditions in different parts of the state and, certainly,
between years.
•
A large number of storms during all seasons.
Source: Illinois Climate Atlas. S. A. Changnon, J. R. Angel, and K. E. Kunkel. Illinois State
Water Survey, Champaign, Illinois (in press).
CLIMATE OF ILLINOIS
Page 4 of 4
_____________________________________________________________________________
Illinois State Water Survey, State Climatologist Office, Copyright 2003
ABOUT THE STATE CLIMATOLOGIST-The Illinois State Climatologist is located in
Champaign, Illinois, at the Illinois State Water Survey (ISWS). The ISWS, a division of the
Illinois Department of Natural Resources Office of Scientific Research & Analysis and an
affiliated agency of the University of Illinois at Urbana-Champaign, is the primary agency in
Illinois for research and information on surface water, groundwater, and the atmosphere.
The State Climatologist promotes the use of climate information in areas crucial to the
economic and environmental benefit of Illinois by disseminating climate information, monitoring
climate events, conducting applied research, and making presentations to community
organizations, individuals, and the media.
Additionally, the State Climatologist conveys information to the public on the impacts of
climate change and variability on climate-sensitive systems such as agriculture and water
resources, and on the characteristics and impacts of severe events (heavy rainstorms, tornadoes,
ice storms, and snowstorms).
For more information, contact:
James R. Angel, Ph.D.
State Climatologist
Illinois State Water Survey
2204 Griffith Drive
Champaign, IL 61820-7495
Phone: (217) 333-0729
Fax: (217) 244-0220
E-mail: jimangel@uiuc.edu
Web: http://www.sws.uiuc.edu/atmos/statecli/
Average annual temperatures ranges from 48°F (north) to 58°F (south),
with highs ranging from 57°F (north) to 67°F (south). Average winter highs range from the 30s
(north) to the mid-40s (south), while average lows range from the teens (north) to the upper 20s
(south). Average summer highs are in the 80s, while lows are in the 60s across the state. Both
spring and fall have more moderate temperatures. Average spring highs range from 57°F (north)
to 67°F (south), while average lows range from 36°F (north) to 48°F (south). Average fall highs
range from 60°F (north) to 70°F (south), while average lows range from 40°F (north) to 48°F
(south).
Illinois averages 10 days at or above 90°F (north) compared to just over 40 days (south).
Days at or above 100°F are quite rare, occurring about every other year (north) and 2 days
annually (south). Illinois averages 140 days at or below 32°F (north) but only 80 such days
(south). Days at or below 0°F range from 16 days annually (north) to 2 days (south).
Average annual heating-degree days range from 7000 days (north) to 4000 days (south).
Average annual cooling-degree days range from 800 days (north) to 1600 days (south). The base
temperature used for both heating-degree and cooling degree days is the same (65°F).
The average length of the frost-free growing season in Illinois ranges from 160 days
(north) to more than 190 days (south). Average dates of the last occurrence of 32°F in spring
range from April 28 (north) to April 7 (south), while the average dates of the first occurrence of
this temperature in fall range fromOctober 7 (north) to October 21 (south), and about October 14
near Lake Michigan, including the Chicago area, due to relatively warm waters of the lake.
The highest and lowest temperatures ever reported in Illinois were 117°F in East St.
Louis on July 14, 1954, and -36°F in Congerville on January 5, 1999.
CLIMATE OF ILLINOIS
Page 3 of 4
_____________________________________________________________________________
Illinois State Water Survey, State Climatologist Office, Copyright 2003
PRECIPITATION - Average precipitation exceeds 48 inches a year (south), compared to less
than 32 inches (north). Snowfall distribution is just the opposite, with averages of 36 inches a
year (north) and less than 10 inches (extreme southern Illinois). Winter snowfall is heaviest in
the Chicago area, enhanced by lake-effect snows from Lake Michigan.
Variability in precipitation also extends over time. There have been major multi-year
droughts in the 1930s and 1950s and major prolonged wet periods during the 1970s and 1980s.
May and June are typically the wettest months, and January and February are the driest. Each
year, Illinois has rainstorms producing 40 or more flash-floods each with 4 to 8 inches of rainfall
in a few hours in localized areas.
The average number of days with measurable precipitation ranges from 110 days (north)
to just under 100 days (south). The number of days with an inch or more of precipitation ranges
from 8 days (north) to 13 days (south), and the number of days with an inch or more of snow
ranges from 12 days (north) to 4 days (south). Days with 6 inches or more of snow are
infrequent, ranging from once per year (north) to once in three years (south). Typical dates of the
first inch of snow range from November 20
th
(Chicago area) to December 20
th
(south). Typical
dates of the last inch of snow range from March 26 (north) to March 5 (south). The average
number of days with a measurable snow depth (1 inch or more) ranges from 60 days (northwest)
to only 10 days (southwest).
The following is a list of precipitation records for Illinois. The greatest 24-hour rainfall
was 16.94 inches at Aurora on July17-18, 1996. The greatest one-year precipitation was 74.58
inches at New Burnside in 1950. The greatest 24-hour snowfall was 37.8 inches at Astoria on
February 27-28, 1900. The greatest winter snowfall was 105.1 inches at Antioch in 1978-1979.
SEVERE WEATHER – Illinois experiences about 29 tornadoes annually. Peak months are
April-June (63 percent of the total), but tornadoes have occurred in all months. Although Illinois
averages four tornado-related fatalities per year, the number varies widely from year to year.
Thunderstorms account for 50-60 percent of annual precipitation and are quite common
in Illinois with an average of 60 storms (far northeast) to 80 storms (southwest). Nearly half of
all thunderstorm days occur during the June-August period. Similarly, the average number of
cloud-to-ground lightning strikes per square mile ranges from 5 strikes (northeast) to more than
11 strikes (southwest). Some thunderstorms produce hail, and annual average hail-days vary
from 3.3 days (southwest) to less than 1.8 days (northeast).
SUMMARY- The climate of Illinois has five unique features:
•
Four distinct seasons, each with different conditions.
•
Major north-south temperature contrasts.
•
An extremely wide variety of types and amounts of precipitation with moderate
variations between monthly and seasonal average values.
•
Extreme variability of weather conditions in different parts of the state and, certainly,
between years.
•
A large number of storms during all seasons.
Source: Illinois Climate Atlas. S. A. Changnon, J. R. Angel, and K. E. Kunkel. Illinois State
Water Survey, Champaign, Illinois (in press).
CLIMATE OF ILLINOIS
Page 4 of 4
_____________________________________________________________________________
Illinois State Water Survey, State Climatologist Office, Copyright 2003
ABOUT THE STATE CLIMATOLOGIST-The Illinois State Climatologist is located in
Champaign, Illinois, at the Illinois State Water Survey (ISWS). The ISWS, a division of the
Illinois Department of Natural Resources Office of Scientific Research & Analysis and an
affiliated agency of the University of Illinois at Urbana-Champaign, is the primary agency in
Illinois for research and information on surface water, groundwater, and the atmosphere.
The State Climatologist promotes the use of climate information in areas crucial to the
economic and environmental benefit of Illinois by disseminating climate information, monitoring
climate events, conducting applied research, and making presentations to community
organizations, individuals, and the media.
Additionally, the State Climatologist conveys information to the public on the impacts of
climate change and variability on climate-sensitive systems such as agriculture and water
resources, and on the characteristics and impacts of severe events (heavy rainstorms, tornadoes,
ice storms, and snowstorms).
For more information, contact:
James R. Angel, Ph.D.
State Climatologist
Illinois State Water Survey
2204 Griffith Drive
Champaign, IL 61820-7495
Phone: (217) 333-0729
Fax: (217) 244-0220
E-mail: jimangel@uiuc.edu
Web: http://www.sws.uiuc.edu/atmos/statecli/
Average annual temperatures ranges from 48°F (north) to 58°F (south),
with highs ranging from 57°F (north) to 67°F (south). Average winter highs range from the 30s
(north) to the mid-40s (south), while average lows range from the teens (north) to the upper 20s
(south). Average summer highs are in the 80s, while lows are in the 60s across the state. Both
spring and fall have more moderate temperatures. Average spring highs range from 57°F (north)
to 67°F (south), while average lows range from 36°F (north) to 48°F (south). Average fall highs
range from 60°F (north) to 70°F (south), while average lows range from 40°F (north) to 48°F
(south).
Illinois averages 10 days at or above 90°F (north) compared to just over 40 days (south).
Days at or above 100°F are quite rare, occurring about every other year (north) and 2 days
annually (south). Illinois averages 140 days at or below 32°F (north) but only 80 such days
(south). Days at or below 0°F range from 16 days annually (north) to 2 days (south).
Average annual heating-degree days range from 7000 days (north) to 4000 days (south).
Average annual cooling-degree days range from 800 days (north) to 1600 days (south). The base
temperature used for both heating-degree and cooling degree days is the same (65°F).
The average length of the frost-free growing season in Illinois ranges from 160 days
(north) to more than 190 days (south). Average dates of the last occurrence of 32°F in spring
range from April 28 (north) to April 7 (south), while the average dates of the first occurrence of
this temperature in fall range fromOctober 7 (north) to October 21 (south), and about October 14
near Lake Michigan, including the Chicago area, due to relatively warm waters of the lake.
The highest and lowest temperatures ever reported in Illinois were 117°F in East St.
Louis on July 14, 1954, and -36°F in Congerville on January 5, 1999.
CLIMATE OF ILLINOIS
Page 3 of 4
_____________________________________________________________________________
Illinois State Water Survey, State Climatologist Office, Copyright 2003
PRECIPITATION - Average precipitation exceeds 48 inches a year (south), compared to less
than 32 inches (north). Snowfall distribution is just the opposite, with averages of 36 inches a
year (north) and less than 10 inches (extreme southern Illinois). Winter snowfall is heaviest in
the Chicago area, enhanced by lake-effect snows from Lake Michigan.
Variability in precipitation also extends over time. There have been major multi-year
droughts in the 1930s and 1950s and major prolonged wet periods during the 1970s and 1980s.
May and June are typically the wettest months, and January and February are the driest. Each
year, Illinois has rainstorms producing 40 or more flash-floods each with 4 to 8 inches of rainfall
in a few hours in localized areas.
The average number of days with measurable precipitation ranges from 110 days (north)
to just under 100 days (south). The number of days with an inch or more of precipitation ranges
from 8 days (north) to 13 days (south), and the number of days with an inch or more of snow
ranges from 12 days (north) to 4 days (south). Days with 6 inches or more of snow are
infrequent, ranging from once per year (north) to once in three years (south). Typical dates of the
first inch of snow range from November 20
th
(Chicago area) to December 20
th
(south). Typical
dates of the last inch of snow range from March 26 (north) to March 5 (south). The average
number of days with a measurable snow depth (1 inch or more) ranges from 60 days (northwest)
to only 10 days (southwest).
The following is a list of precipitation records for Illinois. The greatest 24-hour rainfall
was 16.94 inches at Aurora on July17-18, 1996. The greatest one-year precipitation was 74.58
inches at New Burnside in 1950. The greatest 24-hour snowfall was 37.8 inches at Astoria on
February 27-28, 1900. The greatest winter snowfall was 105.1 inches at Antioch in 1978-1979.
SEVERE WEATHER – Illinois experiences about 29 tornadoes annually. Peak months are
April-June (63 percent of the total), but tornadoes have occurred in all months. Although Illinois
averages four tornado-related fatalities per year, the number varies widely from year to year.
Thunderstorms account for 50-60 percent of annual precipitation and are quite common
in Illinois with an average of 60 storms (far northeast) to 80 storms (southwest). Nearly half of
all thunderstorm days occur during the June-August period. Similarly, the average number of
cloud-to-ground lightning strikes per square mile ranges from 5 strikes (northeast) to more than
11 strikes (southwest). Some thunderstorms produce hail, and annual average hail-days vary
from 3.3 days (southwest) to less than 1.8 days (northeast).
SUMMARY- The climate of Illinois has five unique features:
•
Four distinct seasons, each with different conditions.
•
Major north-south temperature contrasts.
•
An extremely wide variety of types and amounts of precipitation with moderate
variations between monthly and seasonal average values.
•
Extreme variability of weather conditions in different parts of the state and, certainly,
between years.
•
A large number of storms during all seasons.
Source: Illinois Climate Atlas. S. A. Changnon, J. R. Angel, and K. E. Kunkel. Illinois State
Water Survey, Champaign, Illinois (in press).
CLIMATE OF ILLINOIS
Page 4 of 4
_____________________________________________________________________________
Illinois State Water Survey, State Climatologist Office, Copyright 2003
ABOUT THE STATE CLIMATOLOGIST-The Illinois State Climatologist is located in
Champaign, Illinois, at the Illinois State Water Survey (ISWS). The ISWS, a division of the
Illinois Department of Natural Resources Office of Scientific Research & Analysis and an
affiliated agency of the University of Illinois at Urbana-Champaign, is the primary agency in
Illinois for research and information on surface water, groundwater, and the atmosphere.
The State Climatologist promotes the use of climate information in areas crucial to the
economic and environmental benefit of Illinois by disseminating climate information, monitoring
climate events, conducting applied research, and making presentations to community
organizations, individuals, and the media.
Additionally, the State Climatologist conveys information to the public on the impacts of
climate change and variability on climate-sensitive systems such as agriculture and water
resources, and on the characteristics and impacts of severe events (heavy rainstorms, tornadoes,
ice storms, and snowstorms).
For more information, contact:
James R. Angel, Ph.D.
State Climatologist
Illinois State Water Survey
2204 Griffith Drive
Champaign, IL 61820-7495
Phone: (217) 333-0729
Fax: (217) 244-0220
E-mail: jimangel@uiuc.edu
Web: http://www.sws.uiuc.edu/atmos/statecli/
Southern Illinois has four distinct seasons with moderate temperatures and precipitation.
Average annual temperatures in Southern Illinois is 58°F with the average high of 67°F. Average winter high is in the mid-40s, while the average low is in the upper 20s. Average summer highs are in the 80s, while lows are in the 60s. Both spring and fall have more moderate temperatures. Average spring temperatures range from 67°F to 48°F. Average fall temperatures range 70°F to 48°F. Southern Illinois averages 40 days at or above 90°F. We average 80 days at or below 32°F.
The average length of the frost-free growing season is more than 190 days. Average dates of the last occurrence of 32°F in spring is April 7, while the average date of the first occurrence of this temperature in fall is October 21. We are in climate region #6.
Average precipitation exceeds 48 inches a year. Snowfall is less than 10 inches.
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