Photo: Lights and freedom · CC BY-SA 4.0Castro Valley’s winters are milder and less snowy than most of the U.S.
A typical July afternoon reaches 82°, a January night drops to 40°, and snow is almost unheard of.
A year in Castro Valley at a glance
Each month’s typical high and low, and what it brings.
What stands out about Castro Valley’s climate
Compared with towns across the U.S., with Castro Valley’s place in California beside it.
What a year in Castro Valley feels like
Typical highs and lows by season and by month, from NOAA’s 30-year normals.
A freeze most winters
Frost and day counts come from NOAA’s station at Fremont, CA, 12 miles from the city’s center point, 400 ft lower.
The 2023 USDA map puts Castro Valley’s center in zone 9b: the coldest night of a typical winter, averaged over 1991–2020, reaches about 28°. Plant labels use the zone; frost dates tell you when to plant.
Freezing nights by month
Average nights at or below 32°
90° days by month
Average days at 90° or more
About 20 inches of rain a year
December is the wettest month and July the driest. Rain falls on about 68 days a year. Snow is rare: about 0.7" in a normal year at Mt Diablo Junction, CA, so most winters see none.
Rain + melted snow by month
Inches of water
Heating and cooling a home in Castro Valley
86% of the work is heating, so the furnace matters far more than the air conditioner. Months averaging under 55° run from November through March.
Degree days add up how far each day’s average falls below (heating) or rises above (cooling) 65° over a year. NOAA normals at Fremont, CA.
Living with Castro Valley’s climate
What the numbers mean for a garden, a free afternoon and a bad-weather day.
Zone 9b. Frost comes about 6 winters in 10, so tender plants can often stay out through winter.
Months with highs of 62–84° and lows of 40° or more: March, April, May, June, July, August, September, October and November. 29 miles of trails are waiting.
Parks & trails →Of the yearly building losses FEMA expects in Castro Valley from natural hazards, earthquakes are the largest share (67%); heat waves, cold waves, winter storms and ice storms together are 1%. Normals are averages; the hazards page covers the extremes.
Natural hazards →Castro Valley and its neighbors
Every town next door is within a degree of Castro Valley in January and July, closer than the 5 km grid can tell apart. Tap a town to see its climate.
Climates most like Castro Valley’s, farther away
TownRatings’ own comparison of NOAA 1991–2020 normals, not a NOAA product: places of 5,000+ at least 150 miles away, one per state, closest on January low, July high, precipitation and snow.
| Town | Jan. low | July high | Precip. | Snow |
|---|---|---|---|---|
| Castro Valley, CA | 40° | 82° | 20" | 0.7" |
| Green, OR | 37° | 86° | 32" | 0.0" |
| Mercer Island, WA | 37° | 77° | 37" | 3.8" |
| Alpine, TX | 30° | 90° | 15" | 2.2" |
| Sierra Vista Southeast, AZ | 32° | 91° | 15" | 0.7" |
Temperatures and rain come from NOAA’s 5 km grid, so each town gets its own value. Snow and frost-free days come from the nearest weather station, so neighbors often share them.
Warmer than it used to be
The last 30 years in Castro Valley averaged 1.6° warmer than the 1900s. The biggest change is in January (+2.2°).
A bigger shift than in 63% of U.S. towns.
Change by month
The two periods overlap (1991–2000 is in both), which makes the gap a little smaller than it would be between separate periods.
How Castro Valley compares
Against 12,954 U.S. towns of 2,500 or more (one per 5 km climate cell), with Castro Valley’s place among California’s.
Castro Valley weather by month
Highs, lows and precipitation from NOAA’s grid at Castro Valley’s center; snow, wet days and snow on the ground from Mt Diablo Junction, CA. 1991–2020 normals.
| Month | High | Low | Rain + melt | Snow | Wet days | Snow on ground | 30-yr range |
|---|---|---|---|---|---|---|---|
| January | 58° | 40° | 3.8" | 0.1" | 12 | 0 | 46° – 55° |
| February | 62° | 43° | 3.7" | 0.5" | 11 | 0 | 47° – 58° |
| March | 65° | 45° | 2.9" | 0.1" | 10 | 0 | 50° – 61° |
| April | 69° | 46° | 1.5" | 0.0" | 7 | 0 | 54° – 61° |
| May | 74° | 50° | 0.7" | 0.0" | 4 | 0 | 56° – 67° |
| June | 80° | 53° | 0.2" | 0.0" | 1 | 0 | 63° – 70° |
| July | 82° | 56° | 0.0" | 0.0" | 0 | 0 | 66° – 72° |
| August | 82° | 56° | 0.0" | 0.0" | 0 | 0 | 66° – 73° |
| September | 82° | 54° | 0.1" | 0.0" | 1 | 0 | 65° – 72° |
| October | 76° | 51° | 0.9" | 0.0" | 3 | 0 | 60° – 70° |
| November | 65° | 44° | 2.0" | 0.0" | 7 | 0 | 48° – 60° |
| December | 58° | 40° | 3.9" | 0.0" | 11 | 0 | 46° – 52° |
| Year | 71° | 48° | 19.8" | 0.7" | 68 | 0 |
How much a month can swing
A normal is an average. November swings the most: from 1991 to 2020 its average ranged from 48° to 60°. July is the most reliable, staying within 6°.
Where Castro Valley’s numbers come from
Normals are 30-year averages, not a forecast. How TownRatings reads climate data →
- Temperatures and precipitation: NOAA’s gridded 1991–2020 normals for the 5 km cell at Castro Valley’s center.
- Snow: Mt Diablo Junction, CA.
- Frost, freezing nights, 90° and 100° days, heating and cooling: Fremont, CA, 12 miles from the city’s center point, 400 ft lower; the nearest station that passes our checks for distance, height and agreement with the grid.
- Growing zone: USDA Plant Hardiness Zone Map, 2023. USDA Agricultural Research Service and PRISM Group, Oregon State University, prism.oregonstate.edu/phzm, accessed 8 Oct 2026.
- Then and now: NOAA’s gridded normals for 1901–2000 at the same cell.
- Storms and extremes: FEMA National Risk Index (expected annual losses to buildings).
Common questions about Castro Valley’s climate
What is the weather like in Castro Valley?
A typical July afternoon reaches 82°, a January night drops to 40°, and snow is almost unheard of. Milder January nights than 89% of U.S. towns; cooler July afternoons than 80% of U.S. towns.
How cold does it get in Castro Valley in winter?
December nights average 40°. About 3 nights a year reach 32° or below. In the coldest December of the last 30 years the month averaged 46°.
How much snow does Castro Valley get?
About 0.7 inches in a normal winter at Mt Diablo Junction, CA, so most winters see none.
How hot does it get in Castro Valley?
July, with a typical high of 82° and a low of 56°. About 13 days a year reach 90° (Fremont, CA).
When is the first and last frost in Castro Valley?
There is no reliable frost date: a 32° night comes in about 65% of winters at Fremont, CA, so most years bring a freeze but some pass without one.
What growing zone is Castro Valley in?
Zone 9b on the 2023 USDA Plant Hardiness Zone Map: the coldest night of a typical winter reaches about 28°.
What is the best time of year to visit Castro Valley?
By one simple rule (highs of 62–84° and lows of 40° or more), March, April, May, June, July, August, September, October and November. The rule is ours, not NOAA’s; the month table has the numbers to judge by.
Is Castro Valley getting warmer?
Yes: 1991–2020 averaged 1.6° warmer than 1901–2000 on NOAA’s grid. A bigger shift than 63% of U.S. towns.
Neighbors talk weather
Gardens, snow, and the best days of the year in Castro Valley.
Source: NOAA NCEI U.S. Climate Normals, 1991–2020 and 1901–2000; USDA/PRISM 2023 hardiness zones; FEMA National Risk Index. Today’s forecast: National Weather Service.