Weather in Castro Valley, California
Fog and sun in daily rhythm. Day 58 of spring. Read this microseason across nine climate regions →
- TodayMay 16Mostly Clear—70°50°
- SundayMay 17Overcast—74°48°
- MondayMay 18Clear—80°59°
- TuesdayMay 19Clear—85°57°
- WednesdayMay 20Overcast—78°59°
- ThursdayMay 21Overcast—76°57°
- FridayMay 22Partly Cloudy—74°57°
- PM 2.5
- 6.4 μg/m³
- PM 10
- 9.5 μg/m³
- NO₂
- 1.8 μg/m³
- Ozone
- 95.0 μg/m³
- UV Index
- 4.1 Moderate
Flowers blooming in late autumn, a sure sign of a bad winter coming.
- Moonrise
- 12:25 PM
- Moonset
- 2:30 AM
- In sign
- ♉︎ Taurus
Fog and sun in daily rhythm
Castro Valley at a glance
- Today vs. normal: 7°F below the seasonal normal for this latitude
- Last frost: April 2 (climatological average for this latitude)
- Microseason: 28 of 72, May 16–20
- Planting window: Direct sow okra, melons, and southern peas. Thin carrot rows.
Right now in the garden
Warm-season window is open
As of May 16, the last spring frost has passed for most years. Transplant tomatoes, peppers, eggplants, basil, and cucurbits (cucumbers, squash, melons) now. Direct-sow beans and corn into warm soil.
Planting calendar
| Month | Plant | Harvest |
|---|---|---|
| January | — | — |
| February | — | — |
| March | — | — |
| April | lettuce, peas, spinach, radishes | — |
| May | lettuce, peas, spinach, radishes, tomatoes, peppers, beans, squash | lettuce, peas, radishes |
| June | tomatoes, peppers, beans, squash | lettuce, peas, radishes |
| July | tomatoes, peppers, beans, squash | tomatoes, beans, summer squash |
| August | — | tomatoes, beans, summer squash |
| September | fall brassicas, garlic (overwinter), carrots | tomatoes, beans, summer squash |
| October | fall brassicas, garlic (overwinter), carrots | winter squash, tomatoes (last) |
| November | — | — |
| December | — | — |
A year in weather
Castro Valley's warmest month is September (~67°F mean) and its coldest is December (~51°F). Rainfall peaks in December (3.2 inches) and bottoms out in July (0.0 inches).
| Month | Mean temp | Precip | Rainy days |
|---|---|---|---|
| January | 51° | 2.9″ | 5 |
| February | 54° | 3.1″ | 7 |
| March | 56° | 2.4″ | 6 |
| April | 58° | 1.2″ | 3 |
| May | 61° | 0.6″ | 1 |
| June | 65° | 0.1″ | 0 |
| July | 66° | 0.0″ | 0 |
| August | 67° | 0.1″ | 0 |
| September | 67° | 0.1″ | 0 |
| October | 64° | 0.8″ | 2 |
| November | 56° | 1.5″ | 4 |
| December | 51° | 3.2″ | 6 |
Regional context
Castro Valley sits within a hot-summer Mediterranean climate region — a cohort that shares similar temperature ranges, seasonal patterns, and growing conditions across nearby cities.
Similar climates: Los Angeles, CA, San Francisco, CA, San Diego, CA, Riverside, CA, Sacramento, CA.
Frequently asked
- When does it freeze in Castro Valley?
- Castro Valley's last spring frost typically falls around mid-April, and the first fall frost arrives around mid-November.
- What is the rainy season in Castro Valley?
- December is the wettest month with about 3.2 inches of rain on average; the city receives roughly 16 inches annually.
- What is the warmest month in Castro Valley?
- September is typically warmest, averaging about 67°F.
- What is the coldest month in Castro Valley?
- December is typically coldest, averaging about 51°F.
- When can I start a vegetable garden in Castro Valley?
- Cool-season crops (peas, lettuce) can be sown around the last spring frost (mid-April); warm-season crops (tomatoes, peppers) wait until 1–2 weeks after.
- How many rainy days does Castro Valley get?
- Castro Valley averages about 35 rainy days per year.
- What hardiness zone is Castro Valley?
- Castro Valley's USDA hardiness zone is determined by its lowest average winter temperature; check the USDA's online lookup with the city ZIP for the current zone designation.
Climate
Castro Valley, California sits in a hot-summer Mediterranean climate zone. January means hover near 51°F while July averages 66°F — a 15°F seasonal swing.
Across the year, Castro Valley receives about 16 inches of precipitation spread over roughly 35 rainy days.
The rhythm of the year is set by latitude (37.7°N), proximity to large water bodies, and elevation — all of which shape what grows here, when frost is likely, and what the weather story looks like day to day.