Isumi weather in September

In September, Isumi sees average daytime highs of 25°C and overnight lows near 23°C, with 188 mm of rain across 14.7 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 27°C in the first days to 24°C by the end. The sky is clear or mostly clear about 26% of the time in September, and the air most often feels oppressive.

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 Isumi's 9th-clearest and 9th-windiest month. For the whole year in context, see Isumi's year-round climate.

Daytime high
25°
3rd warmest
Overnight low
23°
10th coldest
Rainfall
188 mm
14.7 wet days
Daylight
12 h 12 min
5:26 AM – 5:39 PM
Cloud cover
65%
9th clearest
Wind
18 km/h
9th windiest
Solar energy
4.2 kWh/m²
Travel score
5.9
out of 10

Isumi is city in Chiba Prefecture, Japan.

Density
226/km²
people per km²
Founded
2005

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

Temperature in September

Average highAverage lowShaded bands: 10–90% of days

Highs ease over the month, from about 27°C in the first days to 24°C by the end.

A typical September day in Isumi reaches about 25°C in the afternoon and slips back to 23°C overnight — a 3°C spread between the day's warmth and the night's chill. On most days the high lands between 23°C and 28°C. Set against its neighbours, September runs about 2°C cooler than August and about 4°C warmer than October.

15°20°25°30°151015202530
Avg high (°C)Avg low (°C)
12725
22724
32724
42724
52724
62724
72724
82624
92624
102624
112624
122624
132623
142623
152623
162523
172523
182523
192523
202522
212522
222522
232422
242422
252422
262421
272421
282421
292421
302321

Isumi 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 daysDaylight (hours)Travel score
Jan10° / 10810.79.92.5
Feb11° / 10110.610.72.4
Mar13° / 15914.611.73.2
Apr16° / 13°12612.612.94.8
May20° / 17°13811.813.85.9
Jun23° / 20°19013.514.35.8
Jul26° / 24°13311.814.16.2
Aug28° / 25°1001113.36.2
Sep25° / 23°18814.712.25.9
Oct21° / 19°23914.911.15.8
Nov17° / 14°14713.410.15.2
Dec13° / 10611.79.73.6

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 26% 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 59%. Skies tend to cloud over as the month goes on.

0%25%50%75%100%151015202530
Clearer (%)Cloudier (%)
14852
24852
34852
44753
54753
64753
74753
84654
94555
104555
114456
124357
134357
144357
154258
164159
174159
183961
193961
203862
213763
223664
233664
243763
253763
263664
273664
283664
293664
303664

Chance of precipitation in September

Any precipitation

Isumi gets around 188 mm of rain over 14.7 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 49% and peaks near 51% on the wettest days.

0%20%40%60%151015202530
Rain chance (%)
145
245
346
446
547
647
748
848
948
1049
1149
1249
1349
1450
1550
1650
1750
1850
1950
2051
2151
2251
2350
2450
2550
2650
2750
2850
2950
3050

Humidity in September

DryComfortableHumidMuggyOppressiveMiserable

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

By dew point, September lands in the oppressive band about 35% of days. Sticky, muggy-or-worse air turns up on roughly 78% of them.

0%25%50%75%100%151015202530
Dry (%)Comfortable (%)Humid (%)Muggy (%)Oppressive (%)Miserable (%)
1003184831
2004194830
3004204829
4004214727
5005224726
6015234725
7016234624
8017244622
9018254521
10029264519
11029264418
120310274317
130311284215
141412294114
151513304012
161514303811
171615313710
18171632359
19281733337
20291933316
212102034296
223112134275
233112235254
244122335233
254132436213
264132536192
275142636172
285152736161
296152836141
306162835131

Wind in September

Winds average 18 km/h in September.

Across September, daily wind ranges from about 17 km/h on the calmest days to 19 km/h at its breeziest. It stays fairly steady from day to day.

0510152025151015202530
Wind speed (km/h)Feels
117light breeze
217light breeze
317light breeze
417light breeze
517light breeze
617light breeze
717light breeze
817light breeze
917light breeze
1017light breeze
1118light breeze
1218light breeze
1318light breeze
1418light breeze
1518light breeze
1618light breeze
1718light breeze
1818light breeze
1919light breeze
2019light breeze
2119light breeze
2219light breeze
2319light breeze
2419light breeze
2519light breeze
2619light breeze
2719light breeze
2819light breeze
2919light breeze
3019light breeze

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 5:15 AM to 5:37 AM, and sunset from 6:01 PM to 5:18 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202530
Daylight (h)Sunrise (AM)Sunset (PM)
112.85:156:01
212.75:166:00
312.75:165:59
412.75:175:57
512.65:185:56
612.65:195:54
712.65:195:53
812.55:205:51
912.55:215:50
1012.45:225:48
1112.45:225:47
1212.45:235:45
1312.35:245:44
1412.35:255:42
1512.35:255:41
1612.25:265:39
1712.25:275:38
1812.15:285:36
1912.15:285:35
2012.15:295:33
2112.05:305:32
2212.05:315:30
2312.05:325:29
2411.95:325:27
2511.95:335:26
2611.85:345:24
2711.85:355:23
2811.85:355:21
2911.75:365:20
3011.75:375:18

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 4.2 kWh/m².

For context, solar energy here swings from a low near 2.25 kWh/m² to a high of 5.53 kWh/m² in August across the year — September's 4.2 kWh/m² falls in the middle.

024151015202530
Solar energy (kWh/m²)Level
14.9Bright
24.9Bright
34.8Bright
44.8Bright
54.7Bright
64.7Bright
74.6Bright
84.6Bright
94.5Bright
104.5Bright
114.4Bright
124.4Bright
134.3Bright
144.3Bright
154.2Bright
164.2Bright
174.1Bright
184.1Bright
194.0Bright
204.0Moderate
213.9Moderate
223.9Moderate
233.8Moderate
243.8Moderate
253.7Moderate
263.7Moderate
273.6Moderate
283.6Moderate
293.5Moderate
303.5Moderate
Planning a trip in September

Hazard calendar for Isumi

In seasonPeak

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

Isumi 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 September a good time to visit Isumi?

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

5.9
Sightseeing
3rd-best month of the year
5.5
Beach & pool
3rd-best month of the year

September is a reasonable time to visit Isumi for sightseeing.

What to pack for Isumi in September

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

In September, Isumi sees daytime highs around 25°C and nights near 23°C. Pack rain gear. Worth having: light clothes. About 15 days see rain.

rain gearlight clothes

Sea temperature near Isumi in September

The nearest coast averages about 26°C in September — the other half of the beach question. The full-year curve and swim-comfort zones are below.

The sea offshore is warmest in August at about 26°C and coldest in February near 16°C.

It's comfortable for a long swim from May 22 to Dec 9 — about 6.6 months when the water holds above 20°C.

15°20°25°JanFebMarAprMayJunJulAugSepOctNovDecbracingcoolrefreshingwarmtropicalcomfortable to swim
Sea temperature (°C)Comfort
Jan18Cool
Feb16Cool
Mar16Cool
Apr18Cool
May20Cool
Jun22Refreshing
Jul24Refreshing
Aug26Warm
Sep26Warm
Oct24Refreshing
Nov22Refreshing
Dec20Cool

UV index in Isumi in September

LowModerateHighVery highExtreme

Clear-sky midday UV in September peaks around 8 (very high — sun protection essential). The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in Isumi peaks around August 11 at about 10 (extreme) — 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 highExtremeVery highHighModerateAug 11UV 10Dec 23UV 2
UV index (midday)Level
Jan3Moderate
Feb4Moderate
Mar5Moderate
Apr7High
May9Very high
Jun9Very high
Jul10Extreme
Aug10Extreme
Sep8Very high
Oct6High
Nov4Moderate
Dec2Moderate

Air quality in Isumi 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 Isumi is worst in April — around 22 µg/m³ PM2.5 (moderate), about 1.9× the cleanest month (September, 11 µg/m³).

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

0102030JanFebMarAprMayJunJulAugSepOctNovDecModerateGoodModerateApr 1322 µg/m³Sep 911 µg/m³
PM2.5 (µg/m³)Level
Jan14Moderate
Feb17Moderate
Mar21Moderate
Apr22Moderate
May21Moderate
Jun16Moderate
Jul12Good
Aug11Good
Sep11Good
Oct14Moderate
Nov14Moderate
Dec13Moderate
Location & terrain

Where is Isumi?

Isumi sits at 35.2°N, 140.4°E, 1 m above sea level, in Japan. The nearest listed city is Ōhara, 2.0 km away.

Topography around Isumi

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

Within 3 km, the terrain around Isumi has only modest variations in elevation — a maximum elevation change of 86 m around an average of 12 m above sea level; within 16 km, only modest variations in elevation — a maximum elevation change of 229 m; within 80 km, significant variations in elevation — a maximum elevation change of 396 m.

The land around Isumi is covered by water (59%) and trees (27%) within 3 km, water (62%) and trees (27%) within 16 km, and water (55%) and trees (21%) within 80 km.

3 km
86 m
16 km
229 m
80 km
396 m
treesgrasslandcroplandartificial surfaceswater
September weather nearby: Ōhara 2.0 km · Katsuura 12 km · Mobara 24 km · Kamogawa 31 km · Ōamishirasato 32 km · Ōami 33 km · Tōgane 36 km · Sodegaura 40 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 Isumi (35.23°, 140.4049°, elevation 1 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).