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Decatur, Illinois climate
Cfa Humid subtropical
Decatur has a strongly seasonal cool-temperate climate (Humid subtropical, Köppen Cfa). The hottest month is July with an average daily high near 29°C, while the coldest is January averaging around −7°C at night. Around 1027 mm of precipitation falls each year, peaking in May (126 mm) and easing off in February (51 mm). Light snow accumulates each winter (35 cm of snow per year on average).
Temperature & precipitation
Daily high–low temperature range with the monthly average, plus monthly precipitation split into rain and snow · 1991–2020 normals · hover a month for the exact figuresClear vs cloudy days
Share of days each month that are clear, partly cloudy or cloudy · 1991–2020 · hover a month for the day countsDecatur climate normals (1991–2020)
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg high (°C) | 1 | 4 | 10 | 17 | 23 | 28 | 29 | 29 | 25 | 19 | 10 | 4 |
| Avg temp (°C) | −3 | 0 | 5 | 12 | 18 | 23 | 24 | 23 | 20 | 13 | 6 | 0 |
| Avg low (°C) | −7 | −5 | 0 | 6 | 12 | 17 | 19 | 18 | 14 | 8 | 1 | −4 |
| Precipitation (mm) | 61 | 51 | 67 | 105 | 126 | 120 | 102 | 89 | 78 | 87 | 82 | 61 |
| Snowfall (cm) | 15 | 6 | 2 | 1 | — | — | — | — | — | — | 1 | 9 |
| Jan – Jun | Jan | Feb | Mar | Apr | May | Jun |
|---|---|---|---|---|---|---|
| Avg high (°C) | 1 | 4 | 10 | 17 | 23 | 28 |
| Avg temp (°C) | −3 | 0 | 5 | 12 | 18 | 23 |
| Avg low (°C) | −7 | −5 | 0 | 6 | 12 | 17 |
| Precipitation (mm) | 61 | 51 | 67 | 105 | 126 | 120 |
| Snowfall (cm) | 15 | 6 | 2 | 1 | — | — |
| Jul – Dec | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|
| Avg high (°C) | 29 | 29 | 25 | 19 | 10 | 4 |
| Avg temp (°C) | 24 | 23 | 20 | 13 | 6 | 0 |
| Avg low (°C) | 19 | 18 | 14 | 8 | 1 | −4 |
| Precipitation (mm) | 102 | 89 | 78 | 87 | 82 | 61 |
| Snowfall (cm) | — | — | — | — | 1 | 9 |
How often — frequency of weather events
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Days with measurable rain (days) | 9 | 8 | 10 | 11 | 13 | 10 | 9 | 7 | 8 | 9 | 9 | 9 |
| Days with high ≥ 32°C (days) | — | — | — | — | 0.7 | 6 | 8 | 6 | 3 | 0.1 | — | — |
| Days with low ≤ 0°C (days) | 26 | 22 | 16 | 4 | 0.1 | — | — | — | 0.1 | 2 | 13 | 24 |
| Days with snowfall ≥ 2.5 cm (days) | 2 | 1 | 0.5 | 0.1 | — | — | — | — | — | 0.0 | 0.2 | 1 |
| Jan – Jun | Jan | Feb | Mar | Apr | May | Jun |
|---|---|---|---|---|---|---|
| Days with measurable rain (days) | 9 | 8 | 10 | 11 | 13 | 10 |
| Days with high ≥ 32°C (days) | — | — | — | — | 0.7 | 6 |
| Days with low ≤ 0°C (days) | 26 | 22 | 16 | 4 | 0.1 | — |
| Days with snowfall ≥ 2.5 cm (days) | 2 | 1 | 0.5 | 0.1 | — | — |
| Jul – Dec | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|
| Days with measurable rain (days) | 9 | 7 | 8 | 9 | 9 | 9 |
| Days with high ≥ 32°C (days) | 8 | 6 | 3 | 0.1 | — | — |
| Days with low ≤ 0°C (days) | — | — | 0.1 | 2 | 13 | 24 |
| Days with snowfall ≥ 2.5 cm (days) | — | — | — | 0.0 | 0.2 | 1 |
Sky & atmosphere
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Cloud cover (%) | 68 | 69 | 67 | 64 | 65 | 57 | 50 | 48 | 46 | 55 | 62 | 69 |
| Clear days (≤ 30% cloud) | 4 | 3 | 5 | 6 | 5 | 7 | 10 | 11 | 12 | 9 | 6 | 5 |
| Partly cloudy days (30–70%) | 11 | 10 | 10 | 9 | 10 | 11 | 12 | 12 | 9 | 10 | 9 | 9 |
| Cloudy days (≥ 70%) | 17 | 16 | 17 | 15 | 16 | 12 | 10 | 9 | 9 | 12 | 15 | 18 |
| Relative humidity (%) | 85 | 82 | 78 | 75 | 77 | 78 | 76 | 73 | 70 | 71 | 78 | 84 |
| Wind speed (km/h) | 13.9 | 13.7 | 14.1 | 14.4 | 11.7 | 9.3 | 7.8 | 7.7 | 8.9 | 11.3 | 13.0 | 12.9 |
| Jan – Jun | Jan | Feb | Mar | Apr | May | Jun |
|---|---|---|---|---|---|---|
| Cloud cover (%) | 68 | 69 | 67 | 64 | 65 | 57 |
| Clear days (≤ 30% cloud) | 4 | 3 | 5 | 6 | 5 | 7 |
| Partly cloudy days (30–70%) | 11 | 10 | 10 | 9 | 10 | 11 |
| Cloudy days (≥ 70%) | 17 | 16 | 17 | 15 | 16 | 12 |
| Relative humidity (%) | 85 | 82 | 78 | 75 | 77 | 78 |
| Wind speed (km/h) | 13.9 | 13.7 | 14.1 | 14.4 | 11.7 | 9.3 |
| Jul – Dec | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|
| Cloud cover (%) | 50 | 48 | 46 | 55 | 62 | 69 |
| Clear days (≤ 30% cloud) | 10 | 11 | 12 | 9 | 6 | 5 |
| Partly cloudy days (30–70%) | 12 | 12 | 9 | 10 | 9 | 9 |
| Cloudy days (≥ 70%) | 10 | 9 | 9 | 12 | 15 | 18 |
| Relative humidity (%) | 76 | 73 | 70 | 71 | 78 | 84 |
| Wind speed (km/h) | 7.8 | 7.7 | 8.9 | 11.3 | 13.0 | 12.9 |
Methodology & sources
Temperature & precipitation — the official 1991–2020 climate normals from NOAA's U.S. Climate Normals, measured at Decatur Wtp (NOAA GHCN station USC00112193), about 1 km from the city centre.
Cloud, humidity, wind & sunshine — modelled estimates from NASA POWER, NASA's satellite-and-reanalysis climatology. This is the standard global source for atmospheric variables, which are not measured at most weather stations.