United States · 1991–2020

Best time to visit
Saint Mary.

September leads for comfortable weather in Saint Mary: typically 28.3°C by day, 14.6°C at night, with 75 mm of rain. Based on measured 1991–2020 NOAA station history.

Best comfortSeptember

Typical high28.3°C

Typical low14.6°C

Rain in September75 mm

Comfort

September

September · October · May

77climate match

Beach

August

August · July · September

74climate match

Outdoors

October

October · April · September

79climate match

The weakest month is January, scoring 5 against September’s 77 — days reach only 5.8°C, 18° below the 24°C the score treats as ideal.

Month by month

The whole year,
without the guesswork.

The bars show the comfort score. Day and night temperatures come from daily observed highs and lows; rain is the historical monthly estimate.

Jan

55.8° / -4.6°

Feb

108.5° / -3.2°

Mar

3214.3° / 1.5°

Apr

5720.6° / 6.9°

May

6926° / 13°

Jun

5830.8° / 18°

Jul

4932.7° / 20.2°

Aug

5831.8° / 18.7°

Sep

7728.3° / 14.6°

Oct

7021.6° / 7.8°

Nov

3213.9° / 1.9°

Dec

57.6° / -2.6°

Monthly climate evidence for Saint Mary — daylight is calculated from latitude, everything else is measured
Month	Day	Night	Rain	Wet days	Daylight	Comfort	Coverage
January	5.8°C	-4.6°C	68 mm	5.4	9 h 48	5	Strong coverage
February	8.5°C	-3.2°C	62 mm	5.7	10 h 44	10	Strong coverage
March	14.3°C	1.5°C	87 mm	8.1	11 h 50	32	Strong coverage
April	20.6°C	6.9°C	119 mm	8.8	13 h 07	57	Strong coverage
May	26°C	13°C	133 mm	10.7	14 h 12	69	Strong coverage
June	30.8°C	18°C	104 mm	7.6	14 h 46	58	Strong coverage
July	32.7°C	20.2°C	102 mm	6.9	14 h 33	49	Strong coverage
August	31.8°C	18.7°C	82 mm	6.4	13 h 37	58	Strong coverage
September	28.3°C	14.6°C	75 mm	5.8	12 h 23	77	Good coverage
October	21.6°C	7.8°C	80 mm	6.7	11 h 09	70	Strong coverage
November	13.9°C	1.9°C	89 mm	6.7	10 h 04	32	Strong coverage
December	7.6°C	-2.6°C	76 mm	6.8	9 h 33	5	Strong coverage

What has changed

Saint Mary runs +0.5°C
on the 1961–1990 baseline.

Each bar compares the 2011–2025 daytime average for that month against the same month in 1961–1990, measured at the same station. Months where the older record is too thin to compare are left blank.

Jan

+1.1

Feb

+1.5

Mar

+1.4

Apr

0.0

May

0.0

Jun

+0.5

Jul

-0.9

Aug

-1.1

Sep

+0.8

Oct

+0.9

Nov

-0.2

Dec

+2.4

September daytime highs, 2011–2025. Last recorded: 29.4°C.

Records at KASKASKIA RVR NAVIGATION LOCK, 1974–2026
Month	Record high	Record low	Wettest day	Wettest / driest month	Years
January	23.3°C22 Jan 1986	-27.8°C20 Jan 1985	95 mm9 Jan 1997	189 mm1997 · driest 3 mm in 1986	48
February	28.9°C28 Feb 2024	-25.6°C3 Feb 1985	66 mm7 Feb 1999	181 mm2018 · driest 3 mm in 1996	49
March	33.3°C27 Mar 2026	-20.0°C4 Mar 1978	102 mm19 Mar 2008	202 mm2008 · driest 0 mm in 2001	49
April	37.2°C28 Apr 1989	-5.6°C10 Apr 2007	112 mm5 Apr 2025	339 mm2017 · driest 27 mm in 1988	50
May	36.7°C21 May 1998	-0.6°C3 May 1976	91 mm1 May 1983	234 mm1981 · driest 4 mm in 2012	51
June	42.8°C29 Jun 2012	6.7°C11 Jun 1988	100 mm17 Jun 2015	354 mm2015 · driest 16 mm in 2023	51
July	42.8°C28 Jul 1987	10.6°C29 Jul 2014	101 mm5 Jul 2007	197 mm1981 · driest 3 mm in 2002	48
August	42.8°C17 Aug 1988	5.0°C29 Aug 1986	62 mm29 Aug 2004	207 mm1985 · driest 12 mm in 1983	47
September	44.4°C6 Sep 1990	0.0°C25 Sep 1989	93 mm17 Sep 2016	236 mm1984 · driest 3 mm in 1998	46
October	35.6°C14 Oct 1989	-5.0°C25 Oct 1981	83 mm9 Oct 2009	284 mm2009 · driest 13 mm in 2010	50
November	31.7°C1 Nov 1990	-13.3°C29 Nov 1976	108 mm5 Nov 2024	344 mm1985 · driest 3 mm in 1999	49
December	25.6°C4 Dec 1988	-27.8°C24 Dec 1989	206 mm28 Dec 1990	434 mm1990 · driest 15 mm in 1980	48

Typical days per month at KASKASKIA RVR NAVIGATION LOCK, 1991–2020
Month	Days 25°C+	Days 30°C+	Frost nights	Heavy rain (10 mm+)	Snow days
January	0	0	24	2	1
February	<1	0	21	2	1
March	3	<1	13	3	<1
April	8	2	3	4	0
May	20	7	0	4	0
June	28	20	0	3	0
July	30	24	0	4	0
August	30	22	0	3	0
September	23	12	0	2	0
October	11	2	2	3	0
November	1	<1	12	3	<1
December	0	0	22	2	<1

The best month has moved. On the 1961–1990 numbers the strongest month here was May (scoring 79); on 2011–2025 it is September (81). Both are scored the same way, from the same station — only the climate underneath changed.

How often, not how much — every day in KASKASKIA RVR NAVIGATION LOCK’s record sorted by what it was actually like, rather than averaged
Month	Warm & dry days	Wet days	Washouts	Week with 3+ wet days	Daytime mean has ranged
January	2%	17%	7%	14%	1.8–8.7°C
February	6%	20%	8%	18%	3.2–12.1°C
March	18%	28%	10%	31%	10.1–17.5°C
April	39%	30%	11%	37%	17.9–23.5°C
May	59%	32%	13%	41%	23.3–28.3°C
June	74%	26%	10%	27%	29.1–32.6°C
July	78%	22%	11%	20%	30.3–35.3°C
August	79%	21%	10%	19%	29.8–34.3°C
September	77%	19%	8%	16%	25.9–30.3°C
October	51%	22%	9%	23%	19.3–23.7°C
November	17%	23%	10%	23%	10.8–17.1°C
December	3%	22%	8%	21%	3.8–10.4°C

A warm and dry day reaches 20°C with under 1 mm of rain; a wet day gets 1 mm or more; a washout gets 10 mm or more. The week column is the share of seven-day stretches in that month, across the station’s whole daily record, that contained three or more wet days — the odds a week-long trip is rained on repeatedly, counted rather than modelled. The last column is the 10th to 90th percentile of that month’s daytime average across individual years: a narrow band means the average is what you will get, a wide one means the average is a midpoint between years that felt quite different.

1975Each stripe is one year’s mean temperature against the 39-year average. Warmest: 1998. Coolest: 1979.2025

The evidence behind this page

Nearby, measured,
and named.

Place identity: GeoNames 4407110
NOAA station: USC00114629 · KASKASKIA RVR NAVIGATION LOCK
Station distance: 11.6 km from the place centre
Climate period: 1991–2020
Minimum month coverage: 24 usable years
Daylight: Calculated from latitude 37.88°, mid-month, sunrise to sunset including refraction. Not a station measurement.
Important limit: Historical climate is not a forecast. Nearby stations can miss mountains, coasts, and local microclimates.

These places read the same station, so their numbers are identical — distance from KASKASKIA RVR NAVIGATION LOCK is what differs

Chester→Sainte Genevieve→Red Bud→Evansville→Prairie du Rocher→Bloomsdale→

Or put two places side by side: Saint Mary vs Chester

Best time to visitSaint Mary.