Illinois Climate – July 2026 Update
Top 5 Cities: Chicago, Aurora, Rockford, Joliet, and Naperville
Illinois climate data from NOAA Climate at a Glance
A fresh climate update for Illinois is being generated…
The new monthly summary is being written by Gemini in the background. This usually takes 5–10 seconds. The live climate data below is already up to date.
At a Glance
Temperature – Average
Climate Map – USUnited States
Source: NOAA Climate at a Glance — US states & climate regions (tavg, pcp), plus GPCC (Snow, water-equivalent). Anomalies are vs the 1961–1990 baseline. See methodology.
Typical Climate – Illinois
Köppen Dfa · ContinentalIllinois has a continental climate (Köppen Dfa: Humid continental - hot summer), with distinct warm summers and cool-to-cold winters. July is typically the warmest month, averaging 24.4°C, while January is the coldest at -2.5°C. Annual precipitation averages around 1021mm, spread fairly evenly across the year.
Monthly climate normals (2016–2025) - bar colour and height show average temperature, the shaded area shows average monthly rain, with snow (water-equivalent) shown separately.
Data: NOAA National Centers for Environmental Information (temperature/precipitation; frost: World Bank CCKP; snow: GPCC (DWD)).
Long-Term Trends
Combined Timeline – Illinois
Right axis: everything else standardized to standard deviations (σ) from its own average, since mm/hours/days aren't directly comparable to °C or to each other.
Data: NOAA National Centers for Environmental Information (temperature/precipitation; frost: World Bank CCKP; snow: GPCC (DWD)).
Heatwave Days – Illinois
Warm Spell Duration Index (WSDI) - the WMO/ETCCDI heatwave measure: days in a 6+ day run above the local 90th-percentile temperature for that time of year.
Data: World Bank Climate Change Knowledge Portal (ERA5-derived). Refreshed roughly 1-2x/year, ~7-8 months behind present.
Year-on-Year Trends
The 4byo Climate Helix – Illinois
Data: NOAA National Centers for Environmental Information (temperature/precipitation; frost: World Bank CCKP; snow: GPCC (DWD)).
Monthly Temperature – Illinois
Background lines show the last 40 years (plus the record year and its opposite, wherever they fall).
Data: NOAA National Centers for Environmental Information (temperature/precipitation; frost: World Bank CCKP; snow: GPCC (DWD)).
Records – Illinois
NOAA National Centers for Environmental Information (frost: World Bank CCKP; snow: GPCC (DWD)) - Anomalies vs 1901-2000 mean
Shifting Seasons
Warm / cold seasonsKöppen Dfa · ContinentalHow spring and autumn have shifted in Illinois. Spring is defined as the date monthly temperatures first rise above the long-term annual mean (10.7°C, from 1950–1979); autumn is the date they fall back below it. Temperature swings 28.4°C peak-to-peak across the year - a classic four-seasons rhythm.
Baseline vs recent monthly temperature climatology. Biggest warming: Mar (+2.7°C).
Data: NOAA National Centers for Environmental Information (temperature/precipitation; frost: World Bank CCKP; snow: GPCC (DWD)).
Emissions & Energy
Explore
Explore Climate Data
Data Sources
Data Sources for Illinois
Every figure on this page is sourced from official, openly published climate datasets. Anomalies are calculated against the 1961–1990 baseline (temperature) and 1991–2020 (rainfall, sunshine, frost) - see the Methodology & Sources page for the complete dataset list and update calendar.
Statewide monthly temperature and precipitation (nClimDiv).
Frost Days and Heatwave Days (Warm Spell Duration Index), ERA5-derived. World Bank Climate Change Knowledge Portal.
Monthly snow water-equivalent, Global Precipitation Climatology Centre (DWD).
FAQs
FAQs
How is the climate in Illinois changing?
Illinois is warming in line with the rest of the world. The page above shows the latest monthly temperature anomaly versus the 1961-1990 baseline, the long-term annual trend, and the region's rank in the historical record. The headline panel also shows the long-term trend rate per decade and the warmest and coolest years on file.
What is the climate like in Illinois?
Illinois has a continental climate (Köppen Dfa: Humid continental - hot summer), with distinct warm summers and cool-to-cold winters. July is typically the warmest month, averaging 24.4°C, while January is the coldest at -2.5°C. Annual precipitation averages around 1021mm, spread fairly evenly across the year.
Where does the climate data for Illinois come from?
Climate data for Illinois comes from NOAA Climate at a Glance (temperature and precipitation), refreshed every month, when the upstream temperature and rainfall data are refreshed.
What is the climate baseline used on this page?
Anomalies on this page are calculated against the 1961-1990 climatological baseline, which is the standard reference period used by the Met Office, NOAA, IPCC and most national climate services. Some panels also show the source-native 1901-2000 (NOAA) or 1991-2020 (WMO) baselines for verification. See Methodology & Sources for the full reference.
Which areas does the Illinois climate data cover?
The Illinois climate profile covers Chicago, Aurora, Rockford, Joliet and surrounding areas. Illinois climate data from NOAA Climate at a Glance
How often is the Illinois climate update refreshed?
The Illinois climate update is refreshed monthly, typically a few days after the previous month closes and the upstream provider (Met Office HadUK-Grid, NOAA Climate at a Glance, Copernicus ERA5 or the Global Carbon Project) publishes its update. See the Climate Rankings for cross-region comparisons.
What counts as a Heatwave Day on the Illinois climate page?
Heatwave Days use the Warm Spell Duration Index (WSDI) - the WMO/ETCCDI-standard heatwave measure: days that are part of a run of 6 or more consecutive days above the local 90th-percentile temperature for that calendar day. The 90th-percentile threshold varies by time of year (a hot day in January is judged differently to one in July) but is fixed to the 1961-1990 baseline for every year shown - it does not shift as the climate warms, which is why Heatwave Days trend upward over time.
Does the Illinois climate page include frost and snow data?
Yes. Frost Days (days with a minimum temperature below 0°C, World Bank CCKP) and snow water-equivalent (GPCC) are shown alongside temperature and precipitation, each measured against the 1961-1990 baseline where a like-for-like historical comparison is available.