Castle Hill’s winters are milder and less snowy than most of the U.S.
A typical July afternoon reaches 84°, a January night drops to 40°, and snow is almost unheard of.
A year in Castle Hill at a glance
Each month’s typical high and low, and what it brings.
What stands out about Castle Hill’s climate
Compared with towns across the U.S., with Castle Hill’s place in California beside it.
What a year in Castle Hill 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 Concord Wwtp, CA, 8 miles from the town’s center, 230 ft lower.
The 2023 USDA map puts Castle Hill’s center in zone 9b: the coldest night of a typical winter, averaged over 1991–2020, reaches about 27°. 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 24 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 Castle Hill
65% of the work is heating. 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 Concord Wwtp, CA.
Living with Castle Hill’s climate
What the numbers mean for a garden, a free afternoon and a bad-weather day.
Zone 9b. Frost comes about 8 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, September, October and November.
Parks & trails →Of the yearly building losses FEMA expects in Castle Hill from natural hazards, river flooding is the largest share (69%); heat waves, cold waves, winter storms and ice storms together are 1%. Normals are averages; the hazards page covers the extremes.
Natural hazards →Castle Hill and its neighbors
Walnut Creek has the coldest January nights next door (39°, 1° colder) and Walnut Creek the hottest July afternoons (87°, 2° warmer). Differences under a degree are within what the 5 km grid can tell apart. Tap a town to see its climate.
Climates most like Castle Hill’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 |
|---|---|---|---|---|
| Castle Hill, CA | 40° | 84° | 24" | 0.7" |
| Green, OR | 37° | 86° | 32" | 0.0" |
| Sierra Vista Southeast, AZ | 32° | 91° | 15" | 0.7" |
| Alpine, TX | 30° | 90° | 15" | 2.2" |
| Felida, WA | 35° | 81° | 42" | 0.8" |
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 Castle Hill averaged 1.6° warmer than the 1900s. The biggest change is in January (+2.2°).
A bigger shift than in 68% 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 Castle Hill compares
Against 12,954 U.S. towns of 2,500 or more (one per 5 km climate cell), with Castle Hill’s place among California’s.
Castle Hill weather by month
Highs, lows and precipitation from NOAA’s grid at Castle Hill’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 | 56° | 40° | 4.8" | 0.1" | 12 | 0 | 45° – 55° |
| February | 60° | 42° | 4.5" | 0.5" | 11 | 0 | 45° – 56° |
| March | 64° | 44° | 3.7" | 0.1" | 10 | 0 | 48° – 61° |
| April | 68° | 46° | 1.6" | 0.0" | 7 | 0 | 52° – 61° |
| May | 73° | 50° | 0.8" | 0.0" | 4 | 0 | 55° – 69° |
| June | 80° | 54° | 0.1" | 0.0" | 1 | 0 | 62° – 70° |
| July | 84° | 57° | 0.0" | 0.0" | 0 | 0 | 67° – 74° |
| August | 84° | 57° | 0.0" | 0.0" | 0 | 0 | 67° – 75° |
| September | 83° | 55° | 0.1" | 0.0" | 1 | 0 | 65° – 73° |
| October | 75° | 51° | 1.1" | 0.0" | 3 | 0 | 60° – 71° |
| November | 64° | 44° | 2.4" | 0.0" | 7 | 0 | 47° – 59° |
| December | 56° | 40° | 4.8" | 0.0" | 11 | 0 | 44° – 51° |
| Year | 71° | 48° | 23.9" | 0.7" | 68 | 0 |
How much a month can swing
A normal is an average. May swings the most: from 1991 to 2020 its average ranged from 55° to 69°. December is the most reliable, staying within 7°.
Where Castle Hill’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 Castle Hill’s center.
- Snow: Mt Diablo Junction, CA.
- Frost, freezing nights, 90° and 100° days, heating and cooling: Concord Wwtp, CA, 8 miles from the town’s center, 230 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 Castle Hill’s climate
What is the weather like in Castle Hill?
A typical July afternoon reaches 84°, 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 56% of U.S. towns.
How cold does it get in Castle Hill in winter?
December nights average 40°. About 4 nights a year reach 32° or below. In the coldest December of the last 30 years the month averaged 44°.
How much snow does Castle Hill 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 Castle Hill?
July, with a typical high of 84° and a low of 57°. About 47 days a year reach 90° and 6 reach 100° (Concord Wwtp, CA).
When is the first and last frost in Castle Hill?
There is no reliable frost date: a 32° night comes in about 80% of winters at Concord Wwtp, CA, so most years bring a freeze but some pass without one.
What growing zone is Castle Hill in?
Zone 9b on the 2023 USDA Plant Hardiness Zone Map: the coldest night of a typical winter reaches about 27°.
What is the best time of year to visit Castle Hill?
By one simple rule (highs of 62–84° and lows of 40° or more), March, April, May, June, September, October and November. The rule is ours, not NOAA’s; the month table has the numbers to judge by.
Is Castle Hill getting warmer?
Yes: 1991–2020 averaged 1.6° warmer than 1901–2000 on NOAA’s grid. A bigger shift than 68% of U.S. towns.
Neighbors talk weather
Gardens, snow, and the best days of the year in Castle Hill.
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.