Kosai weather in December

In December, Kosai sees average daytime highs of 12°C and overnight lows near 5°C, with 67 mm of rain across 6.3 wet days and about 9.7 hours of daylight. It is the 10th-warmest and 11th-wettest month of the year here.

Highs ease over the month, from about 14°C in the first days to 11°C by the end. The sky is clear or mostly clear about 62% of the time in December, and the air most often feels dry.

Daylight stretches from about 9 h 48 min at the start of December to 9 h 42 min by month's end. Set against the rest of the year, December is Kosai's 1st-clearest and 3rd-windiest month. For the whole year in context, see Kosai's year-round climate.

Daytime high
12°
10th warmest
Overnight low
3rd coldest
Rainfall
67 mm
6.3 wet days
Daylight
9 h 42 min
6:56 AM – 4:37 PM
Cloud cover
37%
1st clearest
Wind
16 km/h
3rd windiest
Solar energy
2.62 kWh/m²
Travel score
3.9
out of 10

Kosai is city in Shizuoka Prefecture, Japan.

Density
691/km²
people per km²
Founded
1972

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 14°C in the first days to 11°C by the end.

A typical December day in Kosai reaches about 12°C in the afternoon and slips back to 5°C overnight — a 7°C spread between the day's warmth and the night's chill. On most days the high lands between 9°C and 16°C. Set against its neighbours, December runs about 5°C cooler than November and about 2°C warmer than January.

10°15°20°151015202531
Avg high (°C)Avg low (°C)
1157
2157
3147
4147
5147
6146
7146
8136
9136
10136
11136
12135
13135
14125
15125
16125
17125
18125
19125
20124
21124
22114
23114
24114
25114
26114
27114
28114
29114
30114
31114

How December really feels in Kosai

Once humidity and wind are folded in, December averages 0 days that feel above 32°C and 4 that feel at or below 0°C — the comfort signal a plain temperature reading hides.

Days feeling ≥ 32°C
0
heat index over the day's high
Days feeling ≤ C
4
wind chill below the day's low

The story in December is cold — wind chill drags the overnight "feels like" below the raw low.

Kosai weather by month

How December compares with the rest of the year at a glance. December is the 10th-warmest and 11th-wettest month here — the current month is highlighted below.

MonthHigh / Low (°C)Rainfall (mm)Wet daysDaylight (hours)Travel score
Jan10° / 655.89.92.6
Feb11° / 816.810.73.0
Mar14° / 13510.511.74.3
Apr19° / 11°15110.412.86.0
May23° / 15°17011.313.86.5
Jun26° / 20°24215.614.35.4
Jul30° / 24°25015.814.14.2
Aug31° / 25°14415.313.33.8
Sep28° / 22°23614.612.25.2
Oct23° / 16°21111.811.16.6
Nov18° / 10°1138.910.25.9
Dec12° / 676.39.73.9

Cloud cover in December

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 62% of the time in December, the clearest month of the year.

Beyond those clear spells, partly cloudy skies cover about 11% of December, with mostly cloudy or overcast conditions the remaining 27%. Skies tend to open up as the month goes on.

0%25%50%75%100%151015202531
Clearer (%)Cloudier (%)
16634
26634
36634
46733
56832
66832
76931
87030
97030
107129
117129
127228
137228
147228
157228
167327
177327
187525
197525
207525
217525
227624
237723
247723
257822
267822
277921
287921
297921
307921
317921

Chance of precipitation in December

Any precipitation

Kosai gets around 67 mm of rain over 6.3 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 20% and peaks near 26% on the wettest days. The odds ease toward the end of the month.

0%10%20%30%40%151015202531
Rain chance (%)
126
226
325
425
524
624
723
822
922
1021
1121
1220
1320
1419
1519
1619
1719
1818
1918
2018
2118
2218
2318
2418
2517
2617
2717
2817
2916
3016
3116

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 99% 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
5982
6991
7991
8991
9991
10991
11991
12991
13991
14991
151000
161000
171000
181000
191000
201000
211000
221000
231000
241000
251000
261000
271000
281000
291000
301000
311000

Wind in December

Winds average 16 km/h in December.

Across December, daily wind ranges from about 14 km/h on the calmest days to 17 km/h at its breeziest. It stays fairly steady from day to day.

05101520151015202531
Wind speed (km/h)Feels
114light breeze
214light breeze
315light breeze
415light breeze
515light breeze
615light breeze
715light breeze
815light breeze
915light breeze
1015light breeze
1115light breeze
1215light breeze
1316light breeze
1416light breeze
1516light breeze
1616light breeze
1716light breeze
1816light breeze
1916light breeze
2016light breeze
2116light breeze
2216light breeze
2316light breeze
2416light breeze
2516light breeze
2616light breeze
2716light breeze
2817light breeze
2917light breeze
3017light breeze
3117light breeze

Sunrise & sunset in December

DaylightNight

Daylight runs from about 9 h 48 min at the start of December to 9 h 42 min by month's end. The lit band spans sunrise to sunset each day.

Days shrink by about 6 minutes over December. Sunrise moves from 6:44 AM to 7:02 AM, and sunset from 4:35 PM to 4:44 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202531
Daylight (h)Sunrise (AM)Sunset (PM)
19.86:444:35
29.86:454:35
39.86:464:35
49.86:474:35
59.86:484:35
69.86:494:35
79.86:494:35
89.76:504:35
99.76:514:35
109.76:524:35
119.76:524:35
129.76:534:36
139.76:544:36
149.76:544:36
159.76:554:36
169.76:564:37
179.76:564:37
189.76:574:37
199.76:574:38
209.76:584:38
219.76:584:39
229.76:594:39
239.76:594:39
249.77:004:40
259.77:004:41
269.77:004:41
279.77:014:42
289.77:014:42
299.77:014:43
309.77:024:43
319.77:024:44

Solar energy in December

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in December, about 2.62 kWh/m² — the year's lowest.

For context, solar energy here swings from a low near 2.62 kWh/m² to a high of 5.4 kWh/m² in May across the year — December's 2.62 kWh/m² sits near the bottom.

0123151015202531
Solar energy (kWh/m²)Level
12.6Moderate
22.6Moderate
32.6Moderate
42.6Moderate
52.6Moderate
62.6Moderate
72.6Moderate
82.6Moderate
92.6Moderate
102.6Moderate
112.6Moderate
122.6Moderate
132.6Moderate
142.6Moderate
152.6Moderate
162.6Moderate
172.6Moderate
182.6Moderate
192.7Moderate
202.7Moderate
212.7Moderate
222.7Moderate
232.7Moderate
242.7Moderate
252.7Moderate
262.8Moderate
272.8Moderate
282.8Moderate
292.8Moderate
302.8Moderate
312.8Moderate
Planning a trip in December

Hazard calendar for Kosai

In seasonPeak

In December, Kosai sits inside its pacific typhoon season. The full year is below, with each hazard's peak months called out.

Kosai sits in the Pacific typhoon belt; the season peaks July–October.

Pacific typhoon
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec

Is December a good time to visit Kosai?

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

3.9
Sightseeing
9th-best month of the year
0.0
Beach & pool
10th-best month of the year

December is a weaker month for outdoor sightseeing in Kosai.

What to pack for Kosai in December

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

In December, Kosai sees daytime highs around 12°C and nights near 5°C. Worth packing: a warm layer. About 6 days see rain.

a warm layer

Sea temperature near Kosai in December

The nearest coast averages about 19°C 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 13 km away) is warmest in August at about 27°C and coldest in February near 16°C.

It's comfortable for a long swim from May 25 to Nov 28 — about 6.2 months when the water holds above 20°C.

15°20°25°30°JanFebMarAprMayJunJulAugSepOctNovDecbracingcoolrefreshingwarmtropicalcomfortable to swim
Sea temperature (°C)Comfort
Jan16Cool
Feb16Cool
Mar16Cool
Apr17Cool
May19Cool
Jun22Refreshing
Jul25Warm
Aug27Warm
Sep26Warm
Oct23Refreshing
Nov21Refreshing
Dec19Cool

UV index in Kosai in December

LowModerateHighVery highExtreme

Clear-sky midday UV in December peaks around 3. The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in Kosai peaks around July 15 at about 10 (very high) — sunscreen, a hat and midday shade are essential. It bottoms out near December 22 at about 3 (moderate).

The index reaches 3 or more — the level where the WHO recommends sun protection — in 10 months of the year.

025710JanFebMarAprMayJunJulAugSepOctNovDecModerateHighVery highModerateJul 15UV 10Dec 22UV 3
UV index (midday)Level
Jan3Moderate
Feb4Moderate
Mar5Moderate
Apr7High
May9Very high
Jun10Very high
Jul10Very high
Aug9Very high
Sep9Very high
Oct6High
Nov4Moderate
Dec3Moderate

Air quality in Kosai 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 Kosai is worst in April — around 25 µg/m³ PM2.5 (moderate), about 1.6× the cleanest month (July, 15 µg/m³).

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

0102030JanFebMarAprMayJunJulAugSepOctNovDecModerateApr 1325 µg/m³Jul 2415 µg/m³
PM2.5 (µg/m³)Level
Jan17Moderate
Feb19Moderate
Mar25Moderate
Apr25Moderate
May24Moderate
Jun20Moderate
Jul15Moderate
Aug15Moderate
Sep16Moderate
Oct18Moderate
Nov18Moderate
Dec16Moderate
Location & terrain

Where is Kosai?

Kosai sits at 34.7°N, 137.5°E, 30 m above sea level, in Japan. The nearest listed city is Arai, 4.0 km away.

Topography around Kosai

How much the land rises and falls, and what covers it, within 3, 16 and 80 km of Kosai.

Within 3 km, the terrain around Kosai has only modest variations in elevation — a maximum elevation change of 83 m around an average of 33 m above sea level; within 16 km, significant variations in elevation — a maximum elevation change of 463 m; within 80 km, very significant variations in elevation — a maximum elevation change of 2,300 m.

The land around Kosai is covered by trees (41%) and artificial surfaces (31%) within 3 km, water (47%) and trees (25%) within 16 km, and water (43%) and trees (42%) within 80 km.

3 km
83 m
16 km
463 m
80 km
2,300 m
treesgrasslandcroplandartificial surfacesbare terrainwater
December weather nearby: Arai 4.0 km · Toyohashi 15 km · Toyokawa 17 km · Kozakai-chō 19 km · Hamamatsu 19 km · Shinshiro 22 km · Tahara 24 km · Hamakita 26 km

Japan travel facts

Travel advisory
Level 1
Exercise Normal Precautions
Power plugs
Type A / B
100 V · 50 / 60 Hz
Prices
≈ 65% of US
price level, 2025
GDP per capita
$35,951
2025
Happiness
6.1 / 10
World Happiness, 2025
Safe drinking water
99%
of population, 2024
Homicide rate
0.2
per 100k, 2023
Road deaths
3.6
per 100k/yr, 2019
Income group
High
World Bank
English
Very low
EF EPI 446
Religion
Shintoism 49%Buddhism 46%other 4%Christianity 1%

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 Kosai (34.7005°, 137.5225°, elevation 30 m), 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. Tropical-cyclone seasons are the published climatological basin seasons (NOAA National Hurricane Center / regional warning centres). 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).