O'Fallon weather in August

In August, O'Fallon sees average daytime highs of 86°F and overnight lows near 67°F, with 3.0 in of rain across 9.2 wet days and about 13.5 hours of daylight. It is the 2nd-warmest and 9th-wettest month of the year here.

Day-to-day highs hold fairly steady near 86°F through the month. The sky is clear or mostly clear about 41% of the time in August, and the air most often feels muggy.

Daylight stretches from about 14 hours at the start of August to 12 h 54 min by month's end. Set against the rest of the year, August is O'Fallon's 3rd-clearest and 12th-windiest month. For the whole year in context, see O'Fallon's year-round climate.

Daytime high
86°
2nd warmest
Overnight low
67°
11th coldest
Rainfall
3.0 in
9.2 wet days
Daylight
13 h 30 min
5:21 AM – 6:53 PM
Cloud cover
50%
3rd clearest
Wind
5 mph
12th windiest
Solar energy
5.77 kWh/m²
Travel score
5.2
out of 10

O'Fallon is city in St. Charles County, Missouri, United States.

Density
1,105/km²
people per km²
Founded
1856

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

Temperature in August

Average highAverage lowShaded bands: 10–90% of days

Day-to-day highs hold fairly steady near 86°F through the month.

A typical August day in O'Fallon reaches about 86°F in the afternoon and slips back to 67°F overnight — a 19°F spread between the day's warmth and the night's chill. On most days the high lands between 78°F and 94°F. Set against its neighbours, August runs about 1°F cooler than July and about 7°F warmer than September.

60°80°100°151015202531
Avg high (°F)Avg low (°F)
18769
28769
38769
48769
58769
68769
78769
88668
98668
108668
118668
128668
138668
148668
158668
168668
178667
188667
198667
208667
218667
228667
238667
248667
258667
268567
278566
288566
298566
308566
318465

How August really feels in O'Fallon

Once humidity and wind are folded in, August 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

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

O'Fallon weather by month

How August compares with the rest of the year at a glance. August is the 2nd-warmest and 9th-wettest month here — the current month is highlighted below.

MonthHigh / Low (°F)Rainfall (inches)Wet daysSnow daysDaylight (hours)Travel score
Jan40° / 24°2.57.739.60.0
Feb44° / 28°2.47.62.910.50.8
Mar55° / 37°3.610.61.411.73.5
Apr66° / 47°4.511.30.2135.8
May75° / 57°4.913.3014.16.3
Jun84° / 66°4.011.1014.75.5
Jul87° / 70°3.29.9014.44.6
Aug86° / 67°3.09.2013.55.2
Sep79° / 59°3.27.6012.37.1
Oct67° / 48°3.38.50116.6
Nov54° / 37°3.58.30.89.93.5
Dec43° / 28°2.47.72.29.30.6

Cloud cover in August

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 41% of the time in August.

Beyond those clear spells, partly cloudy skies cover about 21% of August, with mostly cloudy or overcast conditions the remaining 38%. Skies tend to open up as the month goes on.

0%25%50%75%100%151015202531
Clearer (%)Cloudier (%)
15842
25842
35842
45842
55941
65941
75941
86040
96040
106040
116040
126139
136238
146238
156238
166238
176337
186436
196436
206436
216337
226337
236337
246337
256337
266436
276436
286436
296436
306535
316535

Chance of precipitation in August

Any precipitation

O'Fallon gets around 3.0 in of rain over 9.2 wet days in August. The line is the day-by-day chance of measurable precipitation.

Day to day, the chance of measurable rain averages around 29% and peaks near 32% on the wettest days.

0%10%20%30%40%151015202531
Rain chance (%)
132
232
332
432
532
632
731
831
931
1031
1131
1230
1330
1430
1529
1629
1729
1829
1929
2028
2128
2228
2328
2428
2528
2627
2727
2827
2927
3026
3126

Humidity in August

DryComfortableHumidMuggyOppressiveMiserable

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

By dew point, August lands in the muggy band about 32% of days. Sticky, muggy-or-worse air turns up on roughly 60% of them.

0%25%50%75%100%151015202531
Dry (%)Comfortable (%)Humid (%)Muggy (%)Oppressive (%)Miserable (%)
13132129332
23132130322
33132131312
43122132302
53122133291
64122033291
74122034281
84122034281
95131935271
105131935271
115131935271
125141935261
135141934261
145151934261
155151934261
165151933261
175151933261
185152033261
195152032261
205152032261
215152032271
225152132271
235152132271
245142132261
256142232261
266142231251
276142331251
287152331240
297152431230
308152430220
319162530200

Wind in August

Winds average 5 mph in August, the calmest month.

Across August, daily wind ranges from about 5 mph on the calmest days to 5 mph at its breeziest. It stays fairly steady from day to day.

0246810151015202531
Wind speed (mph)Feels
15calm
25calm
35calm
45calm
55calm
65calm
75calm
85calm
95calm
105calm
115calm
125calm
135calm
145calm
155calm
165calm
175calm
185calm
195calm
205calm
215calm
225calm
235calm
245calm
255calm
265calm
275calm
285calm
295calm
305calm
315calm

Sunrise & sunset in August

DaylightNight

Daylight runs from about 14 hours at the start of August to 12 h 54 min by month's end. The lit band spans sunrise to sunset each day.

Days shrink by about 66 minutes over August. Sunrise moves from 5:07 AM to 5:35 AM, and sunset from 7:10 PM to 6:31 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202531
Daylight (h)Sunrise (AM)Sunset (PM)
114.05:077:10
214.05:087:09
314.05:097:08
414.05:107:07
513.95:117:06
613.95:127:05
713.95:137:04
813.85:147:03
913.85:157:02
1013.75:167:00
1113.75:166:59
1213.75:176:58
1313.65:186:57
1413.65:196:55
1513.65:206:54
1613.55:216:53
1713.55:226:51
1813.45:236:50
1913.45:246:49
2013.45:256:47
2113.35:266:46
2213.35:276:44
2313.35:286:43
2413.25:296:41
2513.25:296:40
2613.15:306:38
2713.15:316:37
2813.15:326:35
2913.05:336:34
3013.05:346:32
3112.95:356:31

Solar energy in August

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in August, about 5.77 kWh/m².

For context, solar energy here swings from a low near 1.92 kWh/m² to a high of 6.42 kWh/m² in June across the year — August's 5.77 kWh/m² falls in the middle.

0246151015202531
Solar energy (kWh/m²)Level
16.1Intense
26.1Intense
36.1Intense
46.1Intense
56.0Intense
66.0Intense
76.0Intense
86.0Bright
95.9Bright
105.9Bright
115.9Bright
125.9Bright
135.8Bright
145.8Bright
155.8Bright
165.8Bright
175.7Bright
185.7Bright
195.7Bright
205.7Bright
215.6Bright
225.6Bright
235.6Bright
245.5Bright
255.5Bright
265.5Bright
275.4Bright
285.4Bright
295.4Bright
305.3Bright
315.3Bright
Planning a trip in August

Is August a good time to visit O'Fallon?

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

5.2
Sightseeing
6th-best month of the year
7.1
Beach & pool
1st-best month of the year

August is a reasonable time to visit O'Fallon for sightseeing. The water and warmth also suit a beach or pool day.

What to pack for O'Fallon in August

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

In August, O'Fallon sees daytime highs around 86°F and nights near 67°F. Pack light clothes. Worth having: rain gear and sun protection. About 9 days see rain.

light clothesrain gearsun protection

UV index in O'Fallon in August

LowModerateHighVery highExtreme

Clear-sky midday UV in August peaks around 8 (very high — sun protection essential). The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in O'Fallon 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 10 months of the year.

025710JanFebMarAprMayJunJulAugSepOctNovDecModerateHighVery highHighModerateJun 22UV 9Jan 1UV 2
UV index (midday)Level
Jan2Low
Feb3Moderate
Mar5Moderate
Apr7High
May8Very high
Jun9Very high
Jul9Very high
Aug8Very high
Sep7High
Oct4Moderate
Nov3Moderate
Dec2Low

Air quality in O'Fallon in August

GoodModeratePoorUnhealthyVery unhealthy

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

Air quality in O'Fallon is worst in July — around 17 µg/m³ PM2.5 (moderate), about 2.3× the cleanest month (October, 7 µg/m³).

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

01020JanFebMarAprMayJunJulAugSepOctNovDecGoodModerateGoodJul 1616 µg/m³Oct 237 µg/m³
PM2.5 (µg/m³)Level
Jan9Good
Feb11Good
Mar11Good
Apr10Good
May13Moderate
Jun14Moderate
Jul17Moderate
Aug14Moderate
Sep11Good
Oct7Good
Nov7Good
Dec8Good
Location & terrain

Where is O'Fallon?

O'Fallon sits at 38.8°N, 90.7°W, 541 ft above sea level, in United States. The nearest listed city is Saint Peters, 3.7 mi away.

Topography around O'Fallon

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

Within 2 miles, the terrain around O'Fallon has only modest variations in elevation — a maximum elevation change of 207 ft around an average of 535 ft above sea level; within 10 miles, only modest variations in elevation — a maximum elevation change of 413 ft; within 50 miles, significant variations in elevation — a maximum elevation change of 928 ft.

The land around O'Fallon is covered by trees (43%), artificial surfaces (34%) and grassland (20%) within 2 miles, trees (41%) and cropland (24%) within 10 miles, and trees (52%) and cropland (31%) within 50 miles.

2 miles
207 ft
10 miles
413 ft
50 miles
928 ft
treesgrasslandcroplandartificial surfacesbare terrainwater
August weather nearby: Saint Peters 3.7 mi · Wentzville 8.1 mi · Saint Charles 12 mi · Chesterfield 12 mi · Wildwood 16 mi · Maryland Heights 16 mi · Ballwin 17 mi · Hazelwood 18 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 O'Fallon (38.8106°, -90.6998°, elevation 541 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. 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).