| Month | High | Low | Rain | Wet days | Score |
|---|---|---|---|---|---|
| January | -2.2° | — | 135.7mm | 22 | 15 |
| February | -0.9° | — | 95.6mm | 19.8 | 15 |
| March | 4.1° | — | 53.5mm | 14.3 | 21 |
| April | 11.6° | — | 52.9mm | 11.6 | 50 |
| May | 18.3° | — | 78.2mm | 11.6 | 71 |
| June | 22° | — | 85.9mm | 10.1 | 80 |
| July | 25.8° | — | 105.9mm | 10.1 | 80 |
| August | 26.3° | — | 172.5mm | 12.1 | 76 |
| September | 22.8° | — | 136mm | 10.7 | 79 |
| October | 15.7° | — | 113.6mm | 14.5 | 57 |
| November | 7.9° | — | 128.4mm | 19.1 | 31 |
| December | 0.1° | — | 160.2mm | 23.7 | 15 |
January in Iwamizawa averages a daily high of -2.2°C. Expect roughly 22 wet days totalling 135.7mm. A hard month to visit for comfort alone.
February in Iwamizawa averages a daily high of -0.9°C. Expect roughly 19.8 wet days totalling 95.6mm. A hard month to visit for comfort alone.
March in Iwamizawa averages a daily high of 4.1°C. Expect roughly 14.3 wet days totalling 53.5mm. A hard month to visit for comfort alone.
April in Iwamizawa averages a daily high of 11.6°C. Expect roughly 11.6 wet days totalling 52.9mm. Demanding conditions — pack for it and plan indoor options.
May in Iwamizawa averages a daily high of 18.3°C. Expect roughly 11.6 wet days totalling 78.2mm. Generally workable, with some weather to plan around.
June in Iwamizawa averages a daily high of 22°C. Expect roughly 10.1 wet days totalling 85.9mm. Comfortable conditions for being outdoors all day.
July in Iwamizawa averages a daily high of 25.8°C. Expect roughly 10.1 wet days totalling 105.9mm. Comfortable conditions for being outdoors all day.
August in Iwamizawa averages a daily high of 26.3°C. Expect roughly 12.1 wet days totalling 172.5mm. Comfortable conditions for being outdoors all day.
September in Iwamizawa averages a daily high of 22.8°C. Expect roughly 10.7 wet days totalling 136mm. Comfortable conditions for being outdoors all day.
October in Iwamizawa averages a daily high of 15.7°C. Expect roughly 14.5 wet days totalling 113.6mm. Generally workable, with some weather to plan around.
November in Iwamizawa averages a daily high of 7.9°C. Expect roughly 19.1 wet days totalling 128.4mm. A hard month to visit for comfort alone.
December in Iwamizawa averages a daily high of 0.1°C. Expect roughly 23.7 wet days totalling 160.2mm. 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. Iwamizawa peaks at 80.
Based on 30 years of records, June–September scores highest for comfort in Iwamizawa, balancing daytime temperature against rainfall.
August is the warmest, averaging a daily high of 26.3°C.
August is the wettest, averaging 172.5mm of rain across roughly 12.1 wet days.
January is the coolest month, with average daily highs around -2.2°C.