Decatur weather in September

In September, Decatur sees average daytime highs of 78°F and overnight lows near 57°F, with 3.2 in of rain across 7.4 wet days and about 12.3 hours of daylight. It is the 4th-warmest and 8th-wettest month of the year here.

Highs ease over the month, from about 82°F in the first days to 74°F by the end. The sky is clear or mostly clear about 47% of the time in September, and the air most often feels dry.

Daylight stretches from about 12 h 54 min at the start of September to 11 h 36 min by month's end. Set against the rest of the year, September is Decatur's 1st-clearest and 9th-windiest month. For the whole year in context, see Decatur's year-round climate.

Daytime high
78°
4th warmest
Overnight low
57°
9th coldest
Rainfall
3.2 in
7.4 wet days
Daylight
12 h 18 min
5:42 AM – 5:58 PM
Cloud cover
46%
1st clearest
Wind
7 mph
9th windiest
Solar energy
4.72 kWh/m²
Travel score
7.3
out of 10

Decatur is city in Macon County, Illinois, United States.

Density
602/km²
people per km²
Founded
1823

Air quality: WHO Ambient Air Quality Database. Density & description: Wikidata (CC0), GeoNames.

Temperature in September

Average highAverage lowShaded bands: 10–90% of days

Highs ease over the month, from about 82°F in the first days to 74°F by the end.

A typical September day in Decatur reaches about 78°F in the afternoon and slips back to 57°F overnight — a 21°F spread between the day's warmth and the night's chill. On most days the high lands between 67°F and 88°F. Set against its neighbours, September runs about 5°F cooler than August and about 12°F warmer than October.

40°60°80°151015202530
Avg high (°F)Avg low (°F)
18263
28263
38262
48262
58162
68161
78161
88060
98060
108060
118059
127959
137958
147858
157858
167857
177757
187756
197756
207656
217655
227655
237554
247554
257454
267453
277453
287452
297352
307352

How September really feels in Decatur

Once humidity and wind are folded in, September averages 0 days that feel above 90°F and 0 that feel at or below 32°F — the comfort signal a plain temperature reading hides.

Days feeling ≥ 90°F
0
heat index over the day's high
Days feeling ≤ 32°F
0
wind chill below the day's low

September sits comfortably in between — no days that feel dangerously hot or drop to a hard freeze once humidity and wind are folded in.

Decatur weather by month

How September compares with the rest of the year at a glance. September is the 4th-warmest and 8th-wettest month here — the current month is highlighted below.

MonthHigh / Low (°F)Rainfall (inches)Wet daysSnow daysDaylight (hours)Travel score
Jan36° / 21°2.78.549.50.0
Feb40° / 24°2.37.53.410.40.2
Mar52° / 34°3.39.71.511.72.7
Apr64° / 44°4.611.80.3135.3
May73° / 55°4.813.2014.26.2
Jun82° / 64°4.310.9014.86.0
Jul84° / 67°3.610.3014.55.5
Aug83° / 65°3.08.5013.66.1
Sep78° / 57°3.27.4012.37.3
Oct65° / 46°3.58.60.110.96.2
Nov51° / 35°3.58.70.89.82.7
Dec40° / 26°2.58.72.99.20.2

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 47% of the time in September, the clearest month of the year.

Beyond those clear spells, partly cloudy skies cover about 14% of September, with mostly cloudy or overcast conditions the remaining 38%.

0%25%50%75%100%151015202530
Clearer (%)Cloudier (%)
16436
26436
36535
46535
56535
66535
76535
86535
96535
106436
116436
126337
136337
146238
156040
166040
175941
185941
195941
205941
215941
225941
235941
245941
256040
266040
276040
286040
296139
306139

Chance of precipitation in September

Any precipitation

Decatur gets around 3.2 in of rain over 7.4 wet days in September. The line is the day-by-day chance of measurable precipitation.

Day to day, the chance of measurable rain averages around 25% and peaks near 26% on the wettest days.

0%10%20%30%40%151015202530
Rain chance (%)
126
226
326
426
526
626
726
826
926
1026
1126
1226
1326
1426
1526
1626
1726
1826
1926
2025
2125
2225
2324
2424
2524
2624
2723
2823
2923
3023

Humidity in September

DryComfortableHumidMuggyOppressiveMiserable

Most of September feels dry — the stacked bands show each day's mix of comfort levels, read off the dew point.

By dew point, September lands in the dry band about 45% of days. Sticky, muggy-or-worse air turns up on roughly 18% of them.

0%25%50%75%100%151015202530
Dry (%)Comfortable (%)Humid (%)Muggy (%)Oppressive (%)
11919242512
22020242511
32220232410
4242123239
5262123228
6272223217
7292222206
8312222195
9332222185
10352221174
11372221174
12382221163
13402220153
14422120152
15432120142
16452019132
17472019131
18491918121
19511917121
20531817111
21551716111
22581715100
2361161490
2461161490
2564151380
2664151380
2767141270
2867141270
2969131170
3071121160

Wind in September

Winds average 7 mph in September.

Across September, daily wind ranges from about 7 mph on the calmest days to 8 mph at its breeziest. It stays fairly steady from day to day.

051015151015202530
Wind speed (mph)Feels
17calm
27calm
37calm
47calm
57calm
67calm
77calm
87calm
97calm
107calm
117calm
127calm
137calm
147calm
157calm
167calm
177calm
187calm
197light breeze
208light breeze
218light breeze
228light breeze
238light breeze
248light breeze
258light breeze
268light breeze
278light breeze
288light breeze
298light breeze
308light breeze

Sunrise & sunset in September

DaylightNight

Daylight runs from about 12 h 54 min at the start of September to 11 h 36 min by month's end. The lit band spans sunrise to sunset each day.

Days shrink by about 78 minutes over September. Sunrise moves from 5:28 AM to 5:56 AM, and sunset from 6:23 PM to 5:34 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202530
Daylight (h)Sunrise (AM)Sunset (PM)
112.95:286:23
212.95:296:22
312.85:306:20
412.85:316:18
512.75:326:17
612.75:336:15
712.75:346:13
812.65:356:12
912.65:366:10
1012.55:376:08
1112.55:386:06
1212.45:396:05
1312.45:406:03
1412.35:406:01
1512.35:416:00
1612.35:425:58
1712.25:435:56
1812.25:445:54
1912.15:455:53
2012.15:465:51
2112.05:475:49
2212.05:485:48
2311.95:495:46
2411.95:505:44
2511.95:515:42
2611.85:525:41
2711.85:535:39
2811.75:545:37
2911.75:555:36
3011.65:565:34

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 4.72 kWh/m².

For context, solar energy here swings from a low near 1.8 kWh/m² to a high of 6.44 kWh/m² in June across the year — September's 4.72 kWh/m² falls in the middle.

0246151015202530
Solar energy (kWh/m²)Level
15.2Bright
25.2Bright
35.2Bright
45.1Bright
55.1Bright
65.1Bright
75.0Bright
85.0Bright
94.9Bright
104.9Bright
114.9Bright
124.8Bright
134.8Bright
144.7Bright
154.7Bright
164.7Bright
174.6Bright
184.6Bright
194.5Bright
204.5Bright
214.5Bright
224.4Bright
234.4Bright
244.4Bright
254.3Bright
264.3Bright
274.3Bright
284.2Bright
294.2Bright
304.1Bright
Planning a trip in September

Hazard calendar for Decatur

In seasonPeak

In September, Decatur sits inside its atlantic hurricane season. The full year is below, with each hazard's peak months called out.

Decatur sits in the Atlantic hurricane belt; the season peaks August–October.

Atlantic hurricane
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec

Is September a good time to visit Decatur?

Our two travel scores rate September for general sightseeing and for beach or pool weather — 0 (poor) to 10 (ideal).

7.3
Sightseeing
1st-best month of the year
6.6
Beach & pool
3rd-best month of the year

September is one of the better times to visit Decatur for general sightseeing. The water and warmth also suit a beach or pool day.

What to pack for Decatur in September

From September's temperature range, rain chance, sun and wind — what's worth putting in the bag.

In September, Decatur sees daytime highs around 78°F and nights near 57°F. Worth packing: light clothes and sun protection. About 7 days see rain.

light clothessun protection

UV index in Decatur in September

LowModerateHighVery highExtreme

Clear-sky midday UV in September peaks around 6 — enough to warrant sun protection. The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in Decatur peaks around June 22 at about 9 (very high) — sunscreen, a hat and midday shade are essential. It bottoms out near January 1 at about 2 (low).

The index reaches 3 or more — the level where the WHO recommends sun protection — in 9 months of the year.

025710JanFebMarAprMayJunJulAugSepOctNovDecModerateHighVery highHighModerateJun 22UV 9Jan 1UV 2
UV index (midday)Level
Jan2Low
Feb3Moderate
Mar5Moderate
Apr6High
May8Very high
Jun9Very high
Jul9Very high
Aug8Very high
Sep6High
Oct4Moderate
Nov3Moderate
Dec2Low

Air quality in Decatur in September

GoodModeratePoorUnhealthyVery unhealthy

Average fine-particle pollution (PM2.5) through the year — so you can see where September falls, not just the annual average.

Air quality in Decatur is worst in July — around 17 µg/m³ PM2.5 (moderate), about 2.1× the cleanest month (October, 8 µg/m³).

Modeled monthly averages (CAMS reanalysis, 2015–2024).

01020JanFebMarAprMayJunJulAugSepOctNovDecGoodModerateGoodModerateGoodJul 1716 µg/m³Oct 228 µg/m³
PM2.5 (µg/m³)Level
Jan10Good
Feb13Moderate
Mar12Moderate
Apr11Good
May14Moderate
Jun14Moderate
Jul17Moderate
Aug15Moderate
Sep12Good
Oct8Good
Nov8Good
Dec9Good
Location & terrain

Where is Decatur?

Decatur sits at 39.8°N, 89.0°W, 673 ft above sea level, in United States. The nearest listed city is Springfield, 37 mi away.

Topography around Decatur

How much the land rises and falls, and what covers it, within 2, 10 and 50 miles of Decatur.

Within 2 miles, the terrain around Decatur has only modest variations in elevation — a maximum elevation change of 164 ft around an average of 666 ft above sea level; within 10 miles, only modest variations in elevation — a maximum elevation change of 197 ft; within 50 miles, only modest variations in elevation — a maximum elevation change of 489 ft.

The land around Decatur is covered by trees (43%) and artificial surfaces (34%) within 2 miles, cropland (68%) within 10 miles, and cropland (83%) within 50 miles.

2 miles
164 ft
10 miles
197 ft
50 miles
489 ft
treesgrasslandcroplandartificial surfacesbare terrainwater
September weather nearby: Springfield 37 mi · Mattoon 40 mi · Champaign 42 mi · Urbana 43 mi · Bloomington 45 mi · Normal 47 mi · Charleston 48 mi · Morton 60 mi

United States travel facts

Power plugs
Type A / B
120 V · 60 Hz
Prices
≈ 100% of US
price level, 2025
GDP per capita
$90,027
2025
Happiness
6.8 / 10
World Happiness, 2025
Safe drinking water
98%
of population, 2024
Homicide rate
5.8
per 100k, 2023
Road deaths
12.7
per 100k/yr, 2019
Income group
High
World Bank
Religion
Protestant 47%unaffiliated 23%Roman Catholic 21%Jewish 2%

Sources: World Bank, US State Department, World Happiness Report, WHO/UNICEF, EF EPI, CIA World Factbook.

Data sources

Temperature, precipitation, cloud, humidity, wind and solar figures are a 1991–2020 climatology for Decatur (39.8403°, -88.9548°, elevation 673 ft), computed from hourly ERA5 reanalysis — contains modified Copernicus Climate Change Service information. Sun figures are astronomical calculations, not observations. City data © GeoNames, CC BY 4.0. Map outlines from Natural Earth (public domain); interactive map © Google. Tropical-cyclone seasons are the published climatological basin seasons (NOAA National Hurricane Center / regional warning centres). UV index is derived from CAMS (Copernicus Atmosphere Monitoring Service) clear-sky UV data — contains modified Copernicus Atmosphere Monitoring Service information. Monthly air-quality (PM2.5) seasonality is from the CAMS EAC4 reanalysis (Copernicus Atmosphere Monitoring Service), scaled to the WHO annual mean where available. Topography is sampled from the Copernicus DEM GLO-90 elevation model (© ESA/Airbus) and ESA WorldCover 2021 land cover (CC BY 4.0).