Castro Valley weather in December

In December, Castro Valley sees average daytime highs of 57°F and overnight lows near 42°F, with 3.2 in of rain across 8.7 wet days and about 9.4 hours of daylight. It is the 12th-warmest and 3rd-wettest month of the year here.

Highs ease over the month, from about 59°F in the first days to 56°F by the end. The sky is clear or mostly clear about 37% of the time in December, and the air most often feels dry.

Daylight stretches from about 9 h 36 min at the start of December to 9 h 24 min by month's end. Set against the rest of the year, December is Castro Valley's 10th-clearest and 9th-windiest month. For the whole year in context, see Castro Valley's year-round climate.

Daytime high
57°
12th warmest
Overnight low
42°
2nd coldest
Rainfall
3.2 in
8.7 wet days
Daylight
9 h 24 min
7:22 AM – 4:47 PM
Cloud cover
56%
10th clearest
Wind
5 mph
9th windiest
Solar energy
2.23 kWh/m²
Travel score
4.2
out of 10

Castro Valley is census-designated place in Alameda County, California, United States.

Density
1,401/km²
people per km²

Air quality: WHO Ambient Air Quality Database. Density & description: Wikidata (CC0), GeoNames.

Temperature in December

Average highAverage lowShaded bands: 10–90% of days

Highs ease over the month, from about 59°F in the first days to 56°F by the end.

A typical December day in Castro Valley reaches about 57°F in the afternoon and slips back to 42°F overnight — a 15°F spread between the day's warmth and the night's chill. On most days the high lands between 52°F and 62°F. Set against its neighbours, December runs about 7°F cooler than November and on par with January.

30°40°50°60°70°151015202531
Avg high (°F)Avg low (°F)
15943
25943
35943
45843
55843
65843
75842
85842
95842
105742
115742
125742
135742
145742
155742
165641
175641
185641
195641
205641
215641
225641
235641
245640
255640
265640
275640
285640
295640
305640
315640

Castro Valley weather by month

How December compares with the rest of the year at a glance. December is the 12th-warmest and 3rd-wettest month here — the current month is highlighted below.

MonthHigh / Low (°F)Rainfall (inches)Wet daysDaylight (hours)Travel score
Jan57° / 42°3.599.74.2
Feb60° / 43°3.78.910.54.9
Mar63° / 45°2.88.411.76.0
Apr67° / 47°1.34.812.97.1
May72° / 51°0.73148.2
Jun77° / 54°0.10.914.69.1
Jul80° / 57°0.00.214.39.0
Aug80° / 57°0.10.213.59.0
Sep79° / 55°0.10.412.39.0
Oct74° / 52°0.82.9118.6
Nov64° / 46°1.65.5106.4
Dec57° / 42°3.28.79.44.2

Cloud cover in December

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 37% of the time in December.

Beyond those clear spells, partly cloudy skies cover about 12% of December, with mostly cloudy or overcast conditions the remaining 52%. Skies tend to cloud over as the month goes on.

0%25%50%75%100%151015202531
Clearer (%)Cloudier (%)
15050
24951
34951
44852
54753
64753
74753
84654
94753
104753
114753
124753
134852
144852
154951
165050
174951
185050
195149
205149
215149
225050
235050
245050
255050
264951
274951
284852
294654
304654
314654

Chance of precipitation in December

Any precipitation

Castro Valley gets around 3.2 in of rain over 8.7 wet days in December. The line is the day-by-day chance of measurable precipitation.

Day to day, the chance of measurable rain averages around 28% and peaks near 31% on the wettest days.

0%10%20%30%40%151015202531
Rain chance (%)
127
228
329
429
530
630
731
831
931
1031
1131
1231
1330
1430
1530
1629
1728
1828
1927
2027
2126
2226
2326
2425
2525
2625
2725
2825
2926
3026
3126

Humidity in December

DryComfortableHumidMuggyOppressiveMiserable

Most of December feels dry — the stacked bands show each day's mix of comfort levels, read off the dew point.

By dew point, December lands in the dry band about 98% of days. It rarely turns muggy — the humid and sticky bands barely register.

0%25%50%75%100%151015202531
Dry (%)Comfortable (%)
1982
2982
3982
4982
5973
6973
7973
8982
9982
10982
11982
12982
13982
14982
15982
16982
17982
18982
19991
20991
21991
22991
23991
24991
25991
26991
27991
28991
29991
30991
31991

Wind in December

Winds average 5 mph in December.

Across December, daily wind ranges from about 5 mph on the calmest days to 5 mph at its breeziest. It stays fairly steady from day to day.

0246810151015202531
Wind speed (mph)Feels
15calm
25calm
35calm
45calm
55calm
65calm
75calm
85calm
95calm
105calm
115calm
125calm
135calm
145calm
155calm
165calm
175calm
185calm
195calm
205calm
215calm
225calm
235calm
245calm
255calm
265calm
275calm
285calm
295calm
305calm
315calm

Sunrise & sunset in December

DaylightNight

Daylight runs from about 9 h 36 min at the start of December to 9 h 24 min by month's end. The lit band spans sunrise to sunset each day.

Days shrink by about 12 minutes over December. Sunrise moves from 7:11 AM to 7:29 AM, and sunset from 4:46 PM to 4:54 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202531
Daylight (h)Sunrise (AM)Sunset (PM)
19.67:114:46
29.67:114:45
39.57:124:45
49.57:134:45
59.57:144:45
69.57:154:45
79.57:164:45
89.57:174:45
99.57:174:45
109.57:184:45
119.47:194:46
129.47:204:46
139.47:204:46
149.47:214:46
159.47:224:46
169.47:224:47
179.47:234:47
189.47:244:47
199.47:244:48
209.47:254:48
219.47:254:49
229.47:264:49
239.47:264:50
249.47:274:50
259.47:274:51
269.47:274:51
279.47:284:52
289.47:284:52
299.47:284:53
309.47:284:54
319.47:294:54

Solar energy in December

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in December, about 2.23 kWh/m² — the year's lowest.

For context, solar energy here swings from a low near 2.23 kWh/m² to a high of 8.04 kWh/m² in June across the year — December's 2.23 kWh/m² sits near the bottom.

0123151015202531
Solar energy (kWh/m²)Level
12.4Moderate
22.3Moderate
32.3Moderate
42.3Moderate
52.3Moderate
62.2Moderate
72.2Moderate
82.2Moderate
92.2Moderate
102.2Moderate
112.2Moderate
122.2Moderate
132.2Moderate
142.2Moderate
152.2Moderate
162.2Moderate
172.2Moderate
182.2Moderate
192.2Moderate
202.2Moderate
212.2Moderate
222.2Moderate
232.2Moderate
242.2Moderate
252.2Moderate
262.2Moderate
272.2Moderate
282.3Moderate
292.3Moderate
302.3Moderate
312.3Moderate
Planning a trip in December

Is December a good time to visit Castro Valley?

Our two travel scores rate December for general sightseeing and for beach or pool weather — 0 (poor) to 10 (ideal).

4.2
Sightseeing
11th-best month of the year
0.0
Beach & pool
11th-best month of the year

December is a reasonable time to visit Castro Valley for sightseeing.

What to pack for Castro Valley in December

From December's temperature range, rain chance, sun and wind — what's worth putting in the bag.

In December, Castro Valley sees daytime highs around 57°F and nights near 42°F. Worth packing: a warm layer and rain gear. About 9 days see rain.

a warm layerrain gear

Sea temperature near Castro Valley in December

The nearest coast averages about 55°F in December — the other half of the beach question. The full-year curve and swim-comfort zones are below.

The sea at the nearest coast (about 27 mi away) is warmest in August at about 59°F and coldest in April near 54°F.

The water stays below 68°F all year — cool enough that most swimmers want a wetsuit.

52°54°56°58°60°JanFebMarAprMayJunJulAugSepOctNovDecbracingcool
Sea temperature (°F)Comfort
Jan54Bracing
Feb54Bracing
Mar54Bracing
Apr54Bracing
May54Bracing
Jun55Bracing
Jul57Bracing
Aug59Bracing
Sep59Bracing
Oct58Bracing
Nov56Bracing
Dec55Bracing

UV index in Castro Valley in December

LowModerateHighVery highExtreme

Clear-sky midday UV in December peaks around 2. The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in Castro Valley peaks around June 23 at about 10 (extreme) — sunscreen, a hat and midday shade are essential. It bottoms out near December 15 at about 2 (moderate).

The index reaches 3 or more — the level where the WHO recommends sun protection — in 9 months of the year.

025710JanFebMarAprMayJunJulAugSepOctNovDecModerateHighVery highExtremeVery highHighModerateJun 23UV 10Dec 15UV 2
UV index (midday)Level
Jan2Moderate
Feb4Moderate
Mar5Moderate
Apr7High
May8Very high
Jun10Extreme
Jul10Extreme
Aug10Very high
Sep6High
Oct5High
Nov2Moderate
Dec2Moderate

Air quality in Castro Valley in December

GoodModeratePoorUnhealthyVery unhealthy

Average fine-particle pollution (PM2.5) through the year — so you can see where December falls, not just the annual average.

Air quality in Castro Valley is worst in August — around 25 µg/m³ PM2.5 (moderate), about 2.5× the cleanest month (June, 10 µg/m³).

Modeled monthly averages (CAMS reanalysis, 2015–2024).

0102030JanFebMarAprMayJunJulAugSepOctNovDecModerateGoodModerateAug 1824 µg/m³Jun 1110 µg/m³
PM2.5 (µg/m³)Level
Jan15Moderate
Feb18Moderate
Mar16Moderate
Apr12Moderate
May10Good
Jun10Good
Jul11Good
Aug25Moderate
Sep17Moderate
Oct20Moderate
Nov18Moderate
Dec13Moderate
Location & terrain

Where is Castro Valley?

Castro Valley sits at 37.7°N, 122.1°W, 164 ft above sea level, in United States. The nearest listed city is Ashland, 1.2 mi away.

Topography around Castro Valley

How much the land rises and falls, and what covers it, within 2, 10 and 50 miles of Castro Valley.

Within 2 miles, the terrain around Castro Valley has only modest variations in elevation — a maximum elevation change of 568 ft around an average of 217 ft above sea level; within 10 miles, large variations in elevation — a maximum elevation change of 2,064 ft; within 50 miles, very significant variations in elevation — a maximum elevation change of 4,452 ft.

The land around Castro Valley is covered by artificial surfaces (74%) within 2 miles, artificial surfaces (29%), trees (28%) and grassland (21%) within 10 miles, and water (27%), grassland (26%) and trees (25%) within 50 miles.

2 miles
568 ft
10 miles
2,064 ft
50 miles
4,452 ft
treesgrasslandcroplandartificial surfaceswaterwetland
December weather nearby: Ashland 1.2 mi · Hayward 1.9 mi · San Lorenzo 3.1 mi · San Leandro 4.3 mi · Union City 7.5 mi · Dublin 8.1 mi · San Ramon 8.1 mi · Danville 9.9 mi

United States travel facts

Power plugs
Type A / B
120 V · 60 Hz
Prices
≈ 100% of US
price level, 2025
GDP per capita
$90,027
2025
Happiness
6.8 / 10
World Happiness, 2025
Safe drinking water
98%
of population, 2024
Homicide rate
5.8
per 100k, 2023
Road deaths
12.7
per 100k/yr, 2019
Income group
High
World Bank
Religion
Protestant 47%unaffiliated 23%Roman Catholic 21%Jewish 2%

Sources: World Bank, US State Department, World Happiness Report, WHO/UNICEF, EF EPI, CIA World Factbook.

Data sources

Temperature, precipitation, cloud, humidity, wind and solar figures are a 1991–2020 climatology for Castro Valley (37.6941°, -122.0863°, elevation 164 ft), computed from hourly ERA5 reanalysis — contains modified Copernicus Climate Change Service information. Sun figures are astronomical calculations, not observations. City data © GeoNames, CC BY 4.0. Map outlines from Natural Earth (public domain); interactive map © Google. Sea temperatures are the NOAA OISST v2.1 1991–2020 monthly climatology (public domain), sampled at the nearest ocean point. UV index is derived from CAMS (Copernicus Atmosphere Monitoring Service) clear-sky UV data — contains modified Copernicus Atmosphere Monitoring Service information. Monthly air-quality (PM2.5) seasonality is from the CAMS EAC4 reanalysis (Copernicus Atmosphere Monitoring Service), scaled to the WHO annual mean where available. Topography is sampled from the Copernicus DEM GLO-90 elevation model (© ESA/Airbus) and ESA WorldCover 2021 land cover (CC BY 4.0).