Karuizawa weather in September

In September, Karuizawa sees average daytime highs of 22°C and overnight lows near 16°C, with 241 mm of rain across 20.6 wet days and about 12.2 hours of daylight. It is the 3rd-warmest and 3rd-wettest month of the year here.

Highs ease over the month, from about 24°C in the first days to 20°C by the end. The sky is clear or mostly clear about 16% of the time in September, and the air most often feels humid.

Daylight stretches from about 12 h 48 min at the start of September to 11 h 42 min by month's end. Set against the rest of the year, September is Karuizawa's 10th-clearest and 9th-windiest month. For the whole year in context, see Karuizawa's year-round climate.

Daytime high
22°
3rd warmest
Overnight low
16°
10th coldest
Rainfall
241 mm
20.6 wet days
Daylight
12 h 12 min
5:33 AM – 5:47 PM
Cloud cover
72%
10th clearest
Wind
5 km/h
9th windiest
Solar energy
3.81 kWh/m²
Travel score
4.8
out of 10

Temperature in September

Average highAverage lowShaded bands: 10–90% of days

Highs ease over the month, from about 24°C in the first days to 20°C by the end.

A typical September day in Karuizawa reaches about 22°C in the afternoon and slips back to 16°C overnight — a 7°C spread between the day's warmth and the night's chill. On most days the high lands between 18°C and 26°C. Set against its neighbours, September runs about 4°C cooler than August and about 5°C warmer than October.

10°15°20°25°30°151015202530
Avg high (°C)Avg low (°C)
12518
22418
32418
42418
52418
62418
72417
82417
92417
102317
112317
122317
132316
142316
152216
162216
172215
182215
192215
202115
212115
222114
232114
242114
252014
262013
272013
282013
292013
302013

How September really feels in Karuizawa

Once humidity and wind are folded in, September averages 0 days that feel above 32°C and 0 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
0
wind chill below the day's low

September sits comfortably in between — no days that feel dangerously hot or drop to a hard freeze once humidity and wind are folded in.

Karuizawa weather by month

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

MonthHigh / Low (°C)Rainfall (mm)Wet daysSnow daysDaylight (hours)Travel score
Jan / -4°597.46.59.80.0
Feb / -4°608.66.710.60.0
Mar / -1°11013.17.111.71.3
Apr14° / 12213.82.112.93.7
May19° / 10°16516.80.113.95.1
Jun21° / 15°25422.7014.44.3
Jul25° / 19°28224014.24.5
Aug26° / 19°24024.1013.44.5
Sep22° / 16°24120.6012.24.8
Oct17° / 19515.5011.14.7
Nov11° / 728.80.610.13.1
Dec / -1°497.64.49.60.6

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 16% of the time in September.

Beyond those clear spells, partly cloudy skies cover about 15% of September, with mostly cloudy or overcast conditions the remaining 69%. Skies tend to open up as the month goes on.

0%25%50%75%100%151015202530
Clearer (%)Cloudier (%)
12872
22872
32971
42971
52971
62971
72971
82872
92971
102971
112971
122971
132971
143070
153070
163070
173169
183070
193070
203169
213169
223268
233268
243367
253367
263367
273466
283466
293466
303664

Chance of precipitation in September

Any precipitation

Karuizawa gets around 241 mm of rain over 20.6 wet days in September. The line is the day-by-day chance of measurable precipitation.

Day to day, the chance of measurable rain averages around 69% and peaks near 77% on the wettest days. The odds ease toward the end of the month.

0%20%40%60%80%151015202530
Rain chance (%)
177
277
377
476
576
675
775
874
974
1073
1172
1272
1371
1470
1569
1669
1768
1868
1967
2066
2166
2265
2364
2464
2563
2663
2762
2862
2961
3061

Humidity in September

DryComfortableHumidMuggyOppressiveMiserable

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

By dew point, September lands in the humid band about 32% of days. Sticky, muggy-or-worse air turns up on roughly 19% of them.

0%25%50%75%100%151015202530
Dry (%)Comfortable (%)Humid (%)Muggy (%)Oppressive (%)
111442422
221442401
321543391
431544371
541644361
641744341
751844321
861944301
972044290
1092243260
11102342250
12112441230
13132640210
14152739190
15172937170
16193036160
17213134140
18233332120
19253430110
2028352890
2130362680
2233362470
2335372260
2438372050
2540371840
2643371640
2745371530
2848371320
2950361220
3053351110

Wind in September

Winds average 5 km/h in September.

Across September, daily wind ranges from about 5 km/h on the calmest days to 5 km/h at its breeziest. It stays fairly steady from day to day.

0246810151015202530
Wind speed (km/h)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

Sunrise & sunset in September

DaylightNight

Daylight runs from about 12 h 48 min at the start of September to 11 h 42 min by month's end. The lit band spans sunrise to sunset each day.

Days shrink by about 66 minutes over September. Sunrise moves from 5:21 AM to 5:45 AM, and sunset from 6:10 PM to 5:25 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202530
Daylight (h)Sunrise (AM)Sunset (PM)
112.85:216:10
212.85:226:08
312.75:226:07
412.75:236:05
512.75:246:04
612.65:256:02
712.65:266:01
812.55:265:59
912.55:275:57
1012.55:285:56
1112.45:295:54
1212.45:305:53
1312.35:315:51
1412.35:315:50
1512.35:325:48
1612.25:335:47
1712.25:345:45
1812.15:355:43
1912.15:355:42
2012.15:365:40
2112.05:375:39
2212.05:385:37
2312.05:395:36
2411.95:405:34
2511.95:405:33
2611.85:415:31
2711.85:425:30
2811.85:435:28
2911.75:445:27
3011.75:455:25

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 3.81 kWh/m².

For context, solar energy here swings from a low near 2.51 kWh/m² to a high of 5.52 kWh/m² in May across the year — September's 3.81 kWh/m² falls in the middle.

012345151015202530
Solar energy (kWh/m²)Level
14.3Bright
24.2Bright
34.2Bright
44.2Bright
54.1Bright
64.1Bright
74.0Bright
84.0Bright
94.0Moderate
104.0Moderate
113.9Moderate
123.9Moderate
133.9Moderate
143.8Moderate
153.8Moderate
163.8Moderate
173.7Moderate
183.7Moderate
193.7Moderate
203.6Moderate
213.6Moderate
223.6Moderate
233.6Moderate
243.5Moderate
253.5Moderate
263.5Moderate
273.5Moderate
283.4Moderate
293.4Moderate
303.4Moderate
Planning a trip in September

Hazard calendar for Karuizawa

In seasonPeak

In September, Karuizawa sits inside its rainy season season. The full year is below, with each hazard's peak months called out.

Karuizawa's rainy season runs May–October, when most of the year's rain falls.

Rainy season
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec

Is September a good time to visit Karuizawa?

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

4.8
Sightseeing
2nd-best month of the year
3.4
Beach & pool
3rd-best month of the year

September is a reasonable time to visit Karuizawa for sightseeing.

What to pack for Karuizawa in September

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

In September, Karuizawa sees daytime highs around 22°C and nights near 16°C. Pack rain gear. Worth having: light clothes. About 21 days see rain.

rain gearlight clothes

UV index in Karuizawa in September

LowModerateHighVery highExtreme

Clear-sky midday UV in September peaks around 9 (very high — sun protection essential). The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in Karuizawa peaks around July 18 at about 10 (very high) — sunscreen, a hat and midday shade are essential. It bottoms out near December 23 at about 2 (moderate).

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

025710JanFebMarAprMayJunJulAugSepOctNovDecModerateHighVery highModerateJul 18UV 10Dec 23UV 2
UV index (midday)Level
Jan2Moderate
Feb4Moderate
Mar6High
Apr7High
May8Very high
Jun9Very high
Jul10Very high
Aug9Very high
Sep9Very high
Oct6High
Nov3Moderate
Dec2Moderate

Air quality in Karuizawa in September

GoodModeratePoorUnhealthyVery unhealthy

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

Air quality in Karuizawa is worst in April — around 21 µg/m³ PM2.5 (moderate), about 2.2× the cleanest month (December, 10 µg/m³).

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

0102030JanFebMarAprMayJunJulAugSepOctNovDecGoodModerateGoodApr 1821 µg/m³Dec 1810 µg/m³
PM2.5 (µg/m³)Level
Jan11Good
Feb14Moderate
Mar19Moderate
Apr21Moderate
May20Moderate
Jun18Moderate
Jul15Moderate
Aug14Moderate
Sep13Moderate
Oct12Moderate
Nov11Good
Dec10Good
Location & terrain

Where is Karuizawa?

Karuizawa sits at 36.4°N, 138.6°E, 952 m above sea level, in Japan. The nearest listed city is Miyota, 11 km away.

Topography around Karuizawa

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

Within 3 km, the terrain around Karuizawa has large variations in elevation — a maximum elevation change of 632 m around an average of 1,004 m above sea level; within 16 km, very significant variations in elevation — a maximum elevation change of 2,316 m; within 80 km, extreme variations in elevation — a maximum elevation change of 2,985 m.

The land around Karuizawa is covered by trees (86%) within 3 km, trees (86%) within 16 km, and trees (80%) within 80 km.

3 km
632 m
16 km
2,316 m
80 km
2,985 m
treesgrasslandcroplandartificial surfacesbare terrainwater
September weather nearby: Miyota 11 km · Komoro 18 km · Saku 20 km · Annaka 24 km · Tōmi 24 km · Tomioka 26 km · Ueda 32 km · Nakanojō 32 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 Karuizawa (36.35°, 138.6333°, elevation 952 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. 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).