Castle Hill, CA climate
Castle Hill has warm winters and warm summers. A typical July afternoon reaches 84°, a January night drops to 40°, and snow is almost unheard of.
NOAA 1991-2020 climate normals Updated once a decadeWhat a year in Castle Hill feels like
Winter
Spring
Summer
Fall
Castle Hill weather by month
Average highs, lows, rain and snow for every month, from NOAA’s 1991–2020 normals.
| Month | High | Low | Rain + melt | Snow | Wet days | Snow on ground | 30-yr range |
|---|---|---|---|---|---|---|---|
| January | 56° | 40° | 4.8" | 0 | 11 | 0 | 45° – 55° |
| February | 60° | 42° | 4.5" | 0 | 11 | 0 | 45° – 56° |
| March | 64° | 44° | 3.7" | 0 | 11 | 0 | 48° – 61° |
| April | 68° | 46° | 1.6" | 0 | 7 | 0 | 52° – 61° |
| May | 73° | 50° | 0.8" | 0 | 4 | 0 | 55° – 69° |
| June | 80° | 54° | 0.1" | 0 | 1 | 0 | 62° – 70° |
| July | 84° | 57° | 0.0" | 0 | 0 | 0 | 67° – 74° |
| August | 84° | 57° | 0.0" | 0 | 0 | 0 | 67° – 75° |
| September | 83° | 55° | 0.1" | 0 | 1 | 0 | 65° – 73° |
| October | 75° | 51° | 1.1" | 0 | 3 | 0 | 60° – 71° |
| November | 64° | 44° | 2.4" | 0 | 8 | 0 | 47° – 59° |
| December | 56° | 40° | 4.8" | 0 | 12 | 0 | 44° – 51° |
| Year | 71° | 48° | 23.9" | 0.0" | 68 | 0 |
“Wet days” have at least 0.01" of rain or melted snow. “Snow on ground” counts days with 1" or more. The 30-year range is the coldest and warmest that month’s average temperature got between 1991 and 2020.
Rain and snow in Castle Hill
Castle Hill gets 24 inches of rain and melted snow a year. December is the wettest month and July the driest. Rain or snow falls on about 68 days a year.
About 282 frost-free days a year
In half of years the last 32° night comes by March 8 and the first one by December 22. That leaves about 282 frost-free days for gardens, longer than 94% of U.S. towns.
Heating and cooling a home in Castle Hill
Engineers use “degree days” to estimate how hard a furnace or air conditioner works. In Castle Hill, 78% of that work is heating and 22% is cooling.
Degree days measure how far each day’s average temperature falls below or rises above 65°, added up over a year. Source: NOAA station normals.
Castle Hill and its neighbors
Castle Hill borders 2 towns with climate data. January nights across them vary by 1.0°. Walnut Creek has both the coldest January nights (39°) and the hottest July afternoons (87°). Castle Hill’s January low is 40° and its July high 84°.
Temperatures and rain come from NOAA’s 5 km grid, so each town gets its own value.
How Castle Hill compares
Compared with every U.S. town and city on TownTapestry, each counted once.
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°). The shift is larger than in 78% of U.S. towns.
Change by month1991–2020 average minus 1901–2000 average, at Castle Hill’s spot on NOAA’s grid
How much a month can swing
A normal is an average, and real years wander around it. May swings the most: between 1991 and 2020 its average temperature ranged from 55° to 69°. December is the most reliable, staying within 7°.
Common questions about Castle Hill’s climate
What is the coldest month in Castle Hill?
December, with a typical high of 56° and a low of 40°.
What is the hottest month in Castle Hill?
July, with a typical high of 84° and a low of 57°. About 27 days a year reach 90°.
How much snow does Castle Hill get?
Almost none. A normal year brings 0.0 inches.
When is the first frost in Castle Hill?
In half of years the first 32° night comes by December 22, and the last spring frost by March 8.
What is the rainiest month in Castle Hill?
December, with 4.8 inches of rain and melted snow. The year totals 24 inches.
Temperatures and rainfall come from NOAA’s gridded climate normals for Castle Hill itself (about a 3-mile square). Snow, frost dates and day counts come from NOAA’s weather station at Upper San Leandro Filters, CA (9 mi, snow) and Mt Diablo Junction, CA (7 mi, frost, heat and degree days). Normals are 30-year averages, not a forecast. Source: NOAA NCEI U.S. Climate Normals, 1991–2020 and 1901–2000.