Sarnia swings 32°C across the year, from 25°C highs in July to -7°C lows in January, and September is usually the wettest month in a year that brings about 968 mm of precipitation. Over 10 years of recorded weather it averages 68 days a year at 25°C or above, 15 of them at 30°C or above, 116 frost nights, and rain on 134 days.
Warmest month
July
25°C average high
Coldest month
January
-7°C average low
Wettest month
September
968 mm a year
Comfortable
4 months
May, June, July and August
Residents
72,000
share this forecast
Most accurate here
European
93/100 CheckWeather Score
Warm days
68 days
a year at 25°C or above
Frost nights
116 nights
a year below 0°C
Rain days
134 days
a year with 1 mm or more
Day counts are from the ERA5 reanalysis for Sarnia’s grid square, 2016 to 2025, counted the same way for every city so the numbers compare. A reanalysis square is an area, not a weather station, so it is good for counting days and not for a single day’s record.
The weather map: all three models at once
Env. Canada, European, American side by side around Sarnia and nearby towns.Live rain radar →
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Sarnia hourly forecast: next 14 hours
consensus of the three models
Rain in the next 24 hours, model by model
each model's own hourly precipitation, side by side
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Sarnia 10-day forecast, out to 14 days
where the models agree, and where they split
Today
Cloudy
17°4°
models agree
Tomorrow
Cloudy
19°5°
models agree
Mon
Cloudy
15°9°
models agree
Tue
Cloudy
13°7°
±4°
Wed
Cloudy
18°7°
±3°
Thu
Mostly clear
21°9°
±4°
Fri
Mostly clear
17°9°
models agree
Sat
Cloudy
19°11°
models agree
Sun
Showers
18°13°
models agree
Mon
Light rain
16°6°
±4°
Tue
Drizzle
9°2°
models agree
Wed
Light drizzle
9°0°
models agree
Thu
Light drizzle
12°4°
±6°
Fri
Cloudy
12°3°
±5°
Sarnia, Ontario is at its warmest in Jul (average highs near 25°C) and coldest in Jan (lows around -7°C), with roughly 968 mm of precipitation across the year. Below, compare what Env. Canada, European, American forecast for Sarnia right now, and see exactly where they disagree.
average daytime high and overnight low across a typical year (1991-2020)
Jan
-1°
-7°
Feb
2°
-7°
Mar
5°
-3°
Apr
12°
2°
May
18°
8°
Jun
23°
14°
Jul
25°
17°
Aug
24°
17°
Sep
21°
13°
Oct
15°
7°
Nov
7°
1°
Dec
1°
-4°
Source: 1991-2020 averages, ERA5 for this place adjusted with NASA POWER.
How Sarnia's weather has changed since 1995
30 years of daily data, 1995-2024, shown as change
+0.8°C
Around Sarnia, the difference between 1995-2004 and 2015-2024 is +0.8°C, within the normal swing from year to year.
1995each stripe is one year2024
cooler than averagearound averagewarmer than average
Days above 30°C
no clear change
2015-2024 against 1995-2004. Within the normal year-to-year swing
Frost nights
8 fewer nights a year
2015-2024 against 1995-2004, below 0°C. Within the normal year-to-year swing
Average wind
−3%
2015-2024 against 1995-2004. Within the normal year-to-year swing
Bars: each year against the area's 30-year average. Dashed lines: the average of the first and the last ten years.
See the numbers year by year
Year
Temp vs average
Days >30°C vs avg
Frost nights vs avg
Wind vs avg
1995
−1.1°C
+3
+18
−1%
1996
−1.7°C
−6
+29
+1%
1997
−1.1°C
−5
+17
+1%
1998
+1.5°C
+1
−22
−3%
1999
+0.4°C
+4
−10
+3%
2000
−0.2°C
−5
−9
+3%
2001
+0.1°C
+3
+2
−1%
2002
±0.0°C
+3
+19
+3%
2003
−1.2°C
+0
+8
+1%
2004
−0.3°C
−6
−7
+3%
2005
+0.4°C
+14
+5
−3%
2006
+0.8°C
+2
−17
+1%
2007
+0.3°C
+6
−2
+3%
2008
−0.4°C
−5
+12
+5%
2009
−0.7°C
−6
−7
−1%
2010
+0.8°C
+7
−10
+1%
2011
−0.2°C
−2
−2
−1%
2012
+1.3°C
+4
−16
−3%
2013
−0.6°C
−1
+11
+1%
2014
−1.9°C
−6
+12
−1%
2015
−0.6°C
−4
−8
−3%
2016
+0.6°C
+1
−9
−1%
2017
+0.2°C
−3
−10
+1%
2018
−0.4°C
−2
+21
−3%
2019
−0.7°C
−4
+7
+1%
2020
+0.6°C
+0
+7
−1%
2021
+1.3°C
+4
−4
−3%
2022
+0.2°C
+0
+2
+1%
2023
+0.9°C
−3
−12
−7%
2024
+1.8°C
−3
−27
−5%
Source: NASA POWER daily data from the MERRA-2 reanalysis, 1995-2024. Each value describes a grid square of about 50 km around Sarnia, not a weather station, and a square that takes in sea or a large lake runs milder than the city itself. That is why this section shows change against the square's own 30-year average rather than counts you could compare with a local station. Rain and snow are left out because their long-term record in this dataset is not consistent over time. "Within the normal year-to-year swing" means the change is smaller than the usual differences between single years.
Which forecast is most accurate in Sarnia?
CheckWeather Score, graded against the weather that actually happened
Provider
CheckWeather-score
Temp
Wind
Rain
Run by
European ★ Best
93
/100rank #1
89±0.6°C
95±2.6 km/h
9597% hit
ECMWF - European Centre for Medium-Range Weather ForecastsReading, United Kingdom
EC
Env. Canada
88
/100rank #2
88±0.7°C
96±2.1 km/h
8290% hit
Environment and Climate Change CanadaDorval, Québec, Canada
American
82
/100rank #3
81±1.0°C
97±1.6 km/h
7184% hit
NOAA / NCEP (United States)College Park, Maryland, USA
The accuracy above is the measured record. This is your opinion: which forecast service do you trust most in Canada? Give each 1 to 5 stars.
Env. CanadaGEM model
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EuropeanECMWF
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AmericanNOAA GFS
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One vote per service. Anonymous: we store no personal data, only a one-way hash of your connection for a day to stop double votes.
Where the models disagree
temperature and rain, judged separately
Day
Temperature
Rain
Today
±2°16° to 18°
models agree0% to 0% chance
Tomorrow
±2°18° to 20°
±2%0% to 2% chance
Mon
±1°14° to 15°
±4%0% to 4% chance
Tue
±4°11° to 15°
models agree0% to 0% chance
Wed
±3°17° to 20°
±9%0% to 9% chance
The models often agree on temperature and still split on rain, and rain is usually what decides whether a plan survives. A wide rain range means the shower is genuinely uncertain, not that the forecast is wrong.
Sunrise and sunset in Sarnia
today, local time
Sunrise
07:54
Sunset
18:32
Daylight
10h 38m
Common questions
What's the weather in Sarnia today?
Saturday, October 24: Env. Canada, the European and the American model agree on a high near 17° in Sarnia today. Tomorrow: cloudy, around 19°, with a 2% chance of rain. The forecast on this page refreshes live.
Will it rain in Sarnia today or tomorrow?
Rain looks unlikely in Sarnia on Sunday, October 25: no model gives more than 2% (Env. Canada 0%, European 2%, American 1%). On Saturday, October 24 the highest chance of rain from any model is 0%. The hourly strip and the model-by-model rain panel above update live.
Which forecast is most accurate for Sarnia?
Over the last 30 days, the European model has been the most accurate for Sarnia (CheckWeather Score 93/100), ahead of the Env. Canada at 88. We grade every model against the recorded weather. See the CA accuracy scoreboard for the full method.
What is the warmest month in Sarnia?
July is typically the warmest month in Sarnia, with average highs around 25°C.
How cold does Sarnia get?
The coldest month in Sarnia is January, when overnight lows average about -7°C.
How much rain does Sarnia get?
Sarnia sees roughly 968 mm of precipitation a year, with September usually the wettest month.
Is Sarnia getting warmer?
Not clearly. In NASA's reanalysis for the area around Sarnia, the difference between 1995-2004 and 2015-2024 is 0.8°C, within the normal swing from year to year.
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