| Month | High | Low | Rain | Wet days | Score |
|---|---|---|---|---|---|
| January | 14.9° | 6.3° | 66.8mm | 9.5 | 65 |
| February | 16.2° | 7.1° | 66.6mm | 9 | 70 |
| March | 17.6° | 8.4° | 57.1mm | 9.7 | 73 |
| April | 18.8° | 9.6° | 30.6mm | 5.7 | 85 |
| May | 20.6° | 11.5° | 9.5mm | 2.7 | 95 |
| June | 22.8° | 13.1° | 3.1mm | 0.8 | 98 |
| July | 23.8° | 14.3° | 0mm | 0.1 | 100 |
| August | 24.2° | 14.9° | 0.4mm | 0.4 | 99 |
| September | 24.8° | 14.4° | 2.6mm | 0.9 | 98 |
| October | 22.8° | 12.3° | 22.1mm | 2.9 | 94 |
| November | 18.1° | 8.6° | 36.7mm | 7.2 | 80 |
| December | 14.9° | 6.4° | 88mm | 10.6 | 62 |
January in Castro Valley averages a daily high of 14.9°C with nights around 6.3°C. Expect roughly 9.5 wet days totalling 66.8mm. Generally workable, with some weather to plan around.
February in Castro Valley averages a daily high of 16.2°C with nights around 7.1°C. Expect roughly 9 wet days totalling 66.6mm. Generally workable, with some weather to plan around.
March in Castro Valley averages a daily high of 17.6°C with nights around 8.4°C. Expect roughly 9.7 wet days totalling 57.1mm. Generally workable, with some weather to plan around.
April in Castro Valley averages a daily high of 18.8°C with nights around 9.6°C. Expect roughly 5.7 wet days totalling 30.6mm. Comfortable conditions for being outdoors all day.
May in Castro Valley averages a daily high of 20.6°C with nights around 11.5°C. Rain is rare — about 2.7 wet days and 9.5mm across the month. Comfortable conditions for being outdoors all day.
June in Castro Valley averages a daily high of 22.8°C with nights around 13.1°C. Rain is rare — about 0.8 wet days and 3.1mm across the month. Comfortable conditions for being outdoors all day.
July in Castro Valley averages a daily high of 23.8°C with nights around 14.3°C. Rain is rare — about 0.1 wet days and 0mm across the month. Comfortable conditions for being outdoors all day.
August in Castro Valley averages a daily high of 24.2°C with nights around 14.9°C. Rain is rare — about 0.4 wet days and 0.4mm across the month. Comfortable conditions for being outdoors all day.
September in Castro Valley averages a daily high of 24.8°C with nights around 14.4°C. Rain is rare — about 0.9 wet days and 2.6mm across the month. Comfortable conditions for being outdoors all day.
October in Castro Valley averages a daily high of 22.8°C with nights around 12.3°C. Rain is rare — about 2.9 wet days and 22.1mm across the month. Comfortable conditions for being outdoors all day.
November in Castro Valley averages a daily high of 18.1°C with nights around 8.6°C. Expect roughly 7.2 wet days totalling 36.7mm. Comfortable conditions for being outdoors all day.
December in Castro Valley averages a daily high of 14.9°C with nights around 6.4°C. Expect roughly 10.6 wet days totalling 88mm. Generally workable, with some weather to plan around.
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. Castro Valley peaks at 100.
Based on 30 years of records, May–October scores highest for comfort in Castro Valley, balancing daytime temperature against rainfall.
September is the warmest, averaging a daily high of 24.8°C.
December is the wettest, averaging 88mm of rain across roughly 10.6 wet days.
January is the coolest month, with average daily highs around 14.9°C.