| Month | High | Low | Rain | Wet days | Score |
|---|---|---|---|---|---|
| January | 0° | -7° | 61.6mm | 14.1 | 3 |
| February | 1.3° | -6.6° | 54.8mm | 10 | 12 |
| March | 6.2° | -2.4° | 59.5mm | 10.1 | 29 |
| April | 12.8° | 3° | 77.5mm | 12.4 | 52 |
| May | 19.4° | 8.9° | 86.1mm | 12.7 | 73 |
| June | 25.1° | 14.8° | 88.8mm | 10.9 | 78 |
| July | 27.7° | 18.1° | 85.7mm | 10.1 | 77 |
| August | 26.8° | 17.6° | 82.2mm | 10.2 | 80 |
| September | 23.4° | 13.8° | 85.3mm | 9.9 | 80 |
| October | 16.4° | 7.6° | 79.8mm | 12.6 | 63 |
| November | 8.8° | 1.5° | 67.3mm | 11.1 | 41 |
| December | 2.4° | -3.5° | 61mm | 12.7 | 11 |
January in Sarnia averages a daily high of 0°C with nights around -7°C. Expect roughly 14.1 wet days totalling 61.6mm. A hard month to visit for comfort alone.
February in Sarnia averages a daily high of 1.3°C with nights around -6.6°C. Expect roughly 10 wet days totalling 54.8mm. A hard month to visit for comfort alone.
March in Sarnia averages a daily high of 6.2°C with nights around -2.4°C. Expect roughly 10.1 wet days totalling 59.5mm. A hard month to visit for comfort alone.
April in Sarnia averages a daily high of 12.8°C with nights around 3°C. Expect roughly 12.4 wet days totalling 77.5mm. Demanding conditions — pack for it and plan indoor options.
May in Sarnia averages a daily high of 19.4°C with nights around 8.9°C. Expect roughly 12.7 wet days totalling 86.1mm. Generally workable, with some weather to plan around.
June in Sarnia averages a daily high of 25.1°C with nights around 14.8°C. Expect roughly 10.9 wet days totalling 88.8mm. Comfortable conditions for being outdoors all day.
July in Sarnia averages a daily high of 27.7°C with nights around 18.1°C. Expect roughly 10.1 wet days totalling 85.7mm. Comfortable conditions for being outdoors all day.
August in Sarnia averages a daily high of 26.8°C with nights around 17.6°C. Expect roughly 10.2 wet days totalling 82.2mm. Comfortable conditions for being outdoors all day.
September in Sarnia averages a daily high of 23.4°C with nights around 13.8°C. Expect roughly 9.9 wet days totalling 85.3mm. Comfortable conditions for being outdoors all day.
October in Sarnia averages a daily high of 16.4°C with nights around 7.6°C. Expect roughly 12.6 wet days totalling 79.8mm. Generally workable, with some weather to plan around.
November in Sarnia averages a daily high of 8.8°C with nights around 1.5°C. Expect roughly 11.1 wet days totalling 67.3mm. Demanding conditions — pack for it and plan indoor options.
December in Sarnia averages a daily high of 2.4°C with nights around -3.5°C. Expect roughly 12.7 wet days totalling 61mm. A hard month to visit for comfort alone.
Each month scores out of 100. It starts at 100 and loses points for daytime highs outside a 20–27°C band, for wet days, and for nights below 2°C. Nothing else is weighted in — no crowd data, no prices, no opinion. Sarnia peaks at 80.
Based on 30 years of records, May–September scores highest for comfort in Sarnia, balancing daytime temperature against rainfall.
July is the warmest, averaging a daily high of 27.7°C.
June is the wettest, averaging 88.8mm of rain across roughly 10.9 wet days.
January is the coolest month, with average daily highs around 0°C.