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
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.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg high (°C) | 27 | 27 | 27 | 27 | 27 | 27 | 27 | 26 | 26 | 26 | 26 | 26 | 26 | 26 | 26 | 25 | 25 | 25 | 25 | 25 | 25 | 25 | 24 | 24 | 24 | 24 | 24 | 24 | 24 | 23 |
| Avg low (°C) | 25 | 24 | 24 | 24 | 24 | 24 | 24 | 24 | 24 | 24 | 24 | 24 | 23 | 23 | 23 | 23 | 23 | 23 | 23 | 22 | 22 | 22 | 22 | 22 | 22 | 21 | 21 | 21 | 21 | 21 |
| Avg high (°C) | Avg low (°C) | |
|---|---|---|
| 1 | 27 | 25 |
| 2 | 27 | 24 |
| 3 | 27 | 24 |
| 4 | 27 | 24 |
| 5 | 27 | 24 |
| 6 | 27 | 24 |
| 7 | 27 | 24 |
| 8 | 26 | 24 |
| 9 | 26 | 24 |
| 10 | 26 | 24 |
| 11 | 26 | 24 |
| 12 | 26 | 24 |
| 13 | 26 | 23 |
| 14 | 26 | 23 |
| 15 | 26 | 23 |
| 16 | 25 | 23 |
| 17 | 25 | 23 |
| 18 | 25 | 23 |
| 19 | 25 | 23 |
| 20 | 25 | 22 |
| 21 | 25 | 22 |
| 22 | 25 | 22 |
| 23 | 24 | 22 |
| 24 | 24 | 22 |
| 25 | 24 | 22 |
| 26 | 24 | 21 |
| 27 | 24 | 21 |
| 28 | 24 | 21 |
| 29 | 24 | 21 |
| 30 | 23 | 21 |
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.
| Month | High / Low (°C) | Rainfall (mm) | Wet days | Daylight (hours) | Travel score |
|---|---|---|---|---|---|
| Jan | 10° / 7° | 108 | 10.7 | 9.9 | 2.5 |
| Feb | 11° / 6° | 101 | 10.6 | 10.7 | 2.4 |
| Mar | 13° / 9° | 159 | 14.6 | 11.7 | 3.2 |
| Apr | 16° / 13° | 126 | 12.6 | 12.9 | 4.8 |
| May | 20° / 17° | 138 | 11.8 | 13.8 | 5.9 |
| Jun | 23° / 20° | 190 | 13.5 | 14.3 | 5.8 |
| Jul | 26° / 24° | 133 | 11.8 | 14.1 | 6.2 |
| Aug | 28° / 25° | 100 | 11 | 13.3 | 6.2 |
| Sep | 25° / 23° | 188 | 14.7 | 12.2 | 5.9 |
| Oct | 21° / 19° | 239 | 14.9 | 11.1 | 5.8 |
| Nov | 17° / 14° | 147 | 13.4 | 10.1 | 5.2 |
| Dec | 13° / 9° | 106 | 11.7 | 9.7 | 3.6 |
Cloud cover in September
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.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Clearer (%) | 48 | 48 | 48 | 47 | 47 | 47 | 47 | 46 | 45 | 45 | 44 | 43 | 43 | 43 | 42 | 41 | 41 | 39 | 39 | 38 | 37 | 36 | 36 | 37 | 37 | 36 | 36 | 36 | 36 | 36 |
| Cloudier (%) | 52 | 52 | 52 | 53 | 53 | 53 | 53 | 54 | 55 | 55 | 56 | 57 | 57 | 57 | 58 | 59 | 59 | 61 | 61 | 62 | 63 | 64 | 64 | 63 | 63 | 64 | 64 | 64 | 64 | 64 |
| Clearer (%) | Cloudier (%) | |
|---|---|---|
| 1 | 48 | 52 |
| 2 | 48 | 52 |
| 3 | 48 | 52 |
| 4 | 47 | 53 |
| 5 | 47 | 53 |
| 6 | 47 | 53 |
| 7 | 47 | 53 |
| 8 | 46 | 54 |
| 9 | 45 | 55 |
| 10 | 45 | 55 |
| 11 | 44 | 56 |
| 12 | 43 | 57 |
| 13 | 43 | 57 |
| 14 | 43 | 57 |
| 15 | 42 | 58 |
| 16 | 41 | 59 |
| 17 | 41 | 59 |
| 18 | 39 | 61 |
| 19 | 39 | 61 |
| 20 | 38 | 62 |
| 21 | 37 | 63 |
| 22 | 36 | 64 |
| 23 | 36 | 64 |
| 24 | 37 | 63 |
| 25 | 37 | 63 |
| 26 | 36 | 64 |
| 27 | 36 | 64 |
| 28 | 36 | 64 |
| 29 | 36 | 64 |
| 30 | 36 | 64 |
Chance of precipitation in September
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.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Rain chance (%) | 45 | 45 | 46 | 46 | 47 | 47 | 48 | 48 | 48 | 49 | 49 | 49 | 49 | 50 | 50 | 50 | 50 | 50 | 50 | 51 | 51 | 51 | 50 | 50 | 50 | 50 | 50 | 50 | 50 | 50 |
| Rain chance (%) | |
|---|---|
| 1 | 45 |
| 2 | 45 |
| 3 | 46 |
| 4 | 46 |
| 5 | 47 |
| 6 | 47 |
| 7 | 48 |
| 8 | 48 |
| 9 | 48 |
| 10 | 49 |
| 11 | 49 |
| 12 | 49 |
| 13 | 49 |
| 14 | 50 |
| 15 | 50 |
| 16 | 50 |
| 17 | 50 |
| 18 | 50 |
| 19 | 50 |
| 20 | 51 |
| 21 | 51 |
| 22 | 51 |
| 23 | 50 |
| 24 | 50 |
| 25 | 50 |
| 26 | 50 |
| 27 | 50 |
| 28 | 50 |
| 29 | 50 |
| 30 | 50 |
Humidity in September
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.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Dry (%) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 2 | 2 | 2 | 3 | 3 | 4 | 4 | 4 | 5 | 5 | 6 | 6 |
| Comfortable (%) | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 2 | 2 | 3 | 3 | 4 | 5 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 11 | 12 | 13 | 13 | 14 | 15 | 15 | 16 |
| Humid (%) | 3 | 4 | 4 | 4 | 5 | 5 | 6 | 7 | 8 | 9 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 28 |
| Muggy (%) | 18 | 19 | 20 | 21 | 22 | 23 | 23 | 24 | 25 | 26 | 26 | 27 | 28 | 29 | 30 | 30 | 31 | 32 | 33 | 33 | 34 | 34 | 35 | 35 | 36 | 36 | 36 | 36 | 36 | 35 |
| Oppressive (%) | 48 | 48 | 48 | 47 | 47 | 47 | 46 | 46 | 45 | 45 | 44 | 43 | 42 | 41 | 40 | 38 | 37 | 35 | 33 | 31 | 29 | 27 | 25 | 23 | 21 | 19 | 17 | 16 | 14 | 13 |
| Miserable (%) | 31 | 30 | 29 | 27 | 26 | 25 | 24 | 22 | 21 | 19 | 18 | 17 | 15 | 14 | 12 | 11 | 10 | 9 | 7 | 6 | 6 | 5 | 4 | 3 | 3 | 2 | 2 | 1 | 1 | 1 |
| Dry (%) | Comfortable (%) | Humid (%) | Muggy (%) | Oppressive (%) | Miserable (%) | |
|---|---|---|---|---|---|---|
| 1 | 0 | 0 | 3 | 18 | 48 | 31 |
| 2 | 0 | 0 | 4 | 19 | 48 | 30 |
| 3 | 0 | 0 | 4 | 20 | 48 | 29 |
| 4 | 0 | 0 | 4 | 21 | 47 | 27 |
| 5 | 0 | 0 | 5 | 22 | 47 | 26 |
| 6 | 0 | 1 | 5 | 23 | 47 | 25 |
| 7 | 0 | 1 | 6 | 23 | 46 | 24 |
| 8 | 0 | 1 | 7 | 24 | 46 | 22 |
| 9 | 0 | 1 | 8 | 25 | 45 | 21 |
| 10 | 0 | 2 | 9 | 26 | 45 | 19 |
| 11 | 0 | 2 | 9 | 26 | 44 | 18 |
| 12 | 0 | 3 | 10 | 27 | 43 | 17 |
| 13 | 0 | 3 | 11 | 28 | 42 | 15 |
| 14 | 1 | 4 | 12 | 29 | 41 | 14 |
| 15 | 1 | 5 | 13 | 30 | 40 | 12 |
| 16 | 1 | 5 | 14 | 30 | 38 | 11 |
| 17 | 1 | 6 | 15 | 31 | 37 | 10 |
| 18 | 1 | 7 | 16 | 32 | 35 | 9 |
| 19 | 2 | 8 | 17 | 33 | 33 | 7 |
| 20 | 2 | 9 | 19 | 33 | 31 | 6 |
| 21 | 2 | 10 | 20 | 34 | 29 | 6 |
| 22 | 3 | 11 | 21 | 34 | 27 | 5 |
| 23 | 3 | 11 | 22 | 35 | 25 | 4 |
| 24 | 4 | 12 | 23 | 35 | 23 | 3 |
| 25 | 4 | 13 | 24 | 36 | 21 | 3 |
| 26 | 4 | 13 | 25 | 36 | 19 | 2 |
| 27 | 5 | 14 | 26 | 36 | 17 | 2 |
| 28 | 5 | 15 | 27 | 36 | 16 | 1 |
| 29 | 6 | 15 | 28 | 36 | 14 | 1 |
| 30 | 6 | 16 | 28 | 35 | 13 | 1 |
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.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Wind speed (km/h) | 17 | 17 | 17 | 17 | 17 | 17 | 17 | 17 | 17 | 17 | 18 | 18 | 18 | 18 | 18 | 18 | 18 | 18 | 19 | 19 | 19 | 19 | 19 | 19 | 19 | 19 | 19 | 19 | 19 | 19 |
| Feels | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze | light breeze |
| Wind speed (km/h) | Feels | |
|---|---|---|
| 1 | 17 | light breeze |
| 2 | 17 | light breeze |
| 3 | 17 | light breeze |
| 4 | 17 | light breeze |
| 5 | 17 | light breeze |
| 6 | 17 | light breeze |
| 7 | 17 | light breeze |
| 8 | 17 | light breeze |
| 9 | 17 | light breeze |
| 10 | 17 | light breeze |
| 11 | 18 | light breeze |
| 12 | 18 | light breeze |
| 13 | 18 | light breeze |
| 14 | 18 | light breeze |
| 15 | 18 | light breeze |
| 16 | 18 | light breeze |
| 17 | 18 | light breeze |
| 18 | 18 | light breeze |
| 19 | 19 | light breeze |
| 20 | 19 | light breeze |
| 21 | 19 | light breeze |
| 22 | 19 | light breeze |
| 23 | 19 | light breeze |
| 24 | 19 | light breeze |
| 25 | 19 | light breeze |
| 26 | 19 | light breeze |
| 27 | 19 | light breeze |
| 28 | 19 | light breeze |
| 29 | 19 | light breeze |
| 30 | 19 | light breeze |
Sunrise & sunset in September
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.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Daylight (h) | 12.8 | 12.7 | 12.7 | 12.7 | 12.6 | 12.6 | 12.6 | 12.5 | 12.5 | 12.4 | 12.4 | 12.4 | 12.3 | 12.3 | 12.3 | 12.2 | 12.2 | 12.1 | 12.1 | 12.1 | 12.0 | 12.0 | 12.0 | 11.9 | 11.9 | 11.8 | 11.8 | 11.8 | 11.7 | 11.7 |
| Sunrise (AM) | 5:15 | 5:16 | 5:16 | 5:17 | 5:18 | 5:19 | 5:19 | 5:20 | 5:21 | 5:22 | 5:22 | 5:23 | 5:24 | 5:25 | 5:25 | 5:26 | 5:27 | 5:28 | 5:28 | 5:29 | 5:30 | 5:31 | 5:32 | 5:32 | 5:33 | 5:34 | 5:35 | 5:35 | 5:36 | 5:37 |
| Sunset (PM) | 6:01 | 6:00 | 5:59 | 5:57 | 5:56 | 5:54 | 5:53 | 5:51 | 5:50 | 5:48 | 5:47 | 5:45 | 5:44 | 5:42 | 5:41 | 5:39 | 5:38 | 5:36 | 5:35 | 5:33 | 5:32 | 5:30 | 5:29 | 5:27 | 5:26 | 5:24 | 5:23 | 5:21 | 5:20 | 5:18 |
| Daylight (h) | Sunrise (AM) | Sunset (PM) | |
|---|---|---|---|
| 1 | 12.8 | 5:15 | 6:01 |
| 2 | 12.7 | 5:16 | 6:00 |
| 3 | 12.7 | 5:16 | 5:59 |
| 4 | 12.7 | 5:17 | 5:57 |
| 5 | 12.6 | 5:18 | 5:56 |
| 6 | 12.6 | 5:19 | 5:54 |
| 7 | 12.6 | 5:19 | 5:53 |
| 8 | 12.5 | 5:20 | 5:51 |
| 9 | 12.5 | 5:21 | 5:50 |
| 10 | 12.4 | 5:22 | 5:48 |
| 11 | 12.4 | 5:22 | 5:47 |
| 12 | 12.4 | 5:23 | 5:45 |
| 13 | 12.3 | 5:24 | 5:44 |
| 14 | 12.3 | 5:25 | 5:42 |
| 15 | 12.3 | 5:25 | 5:41 |
| 16 | 12.2 | 5:26 | 5:39 |
| 17 | 12.2 | 5:27 | 5:38 |
| 18 | 12.1 | 5:28 | 5:36 |
| 19 | 12.1 | 5:28 | 5:35 |
| 20 | 12.1 | 5:29 | 5:33 |
| 21 | 12.0 | 5:30 | 5:32 |
| 22 | 12.0 | 5:31 | 5:30 |
| 23 | 12.0 | 5:32 | 5:29 |
| 24 | 11.9 | 5:32 | 5:27 |
| 25 | 11.9 | 5:33 | 5:26 |
| 26 | 11.8 | 5:34 | 5:24 |
| 27 | 11.8 | 5:35 | 5:23 |
| 28 | 11.8 | 5:35 | 5:21 |
| 29 | 11.7 | 5:36 | 5:20 |
| 30 | 11.7 | 5:37 | 5:18 |
Solar energy in September
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.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Solar energy (kWh/m²) | 4.9 | 4.9 | 4.8 | 4.8 | 4.7 | 4.7 | 4.6 | 4.6 | 4.5 | 4.5 | 4.4 | 4.4 | 4.3 | 4.3 | 4.2 | 4.2 | 4.1 | 4.1 | 4.0 | 4.0 | 3.9 | 3.9 | 3.8 | 3.8 | 3.7 | 3.7 | 3.6 | 3.6 | 3.5 | 3.5 |
| Level | Bright | Bright | Bright | Bright | Bright | Bright | Bright | Bright | Bright | Bright | Bright | Bright | Bright | Bright | Bright | Bright | Bright | Bright | Bright | Moderate | Moderate | Moderate | Moderate | Moderate | Moderate | Moderate | Moderate | Moderate | Moderate | Moderate |
| Solar energy (kWh/m²) | Level | |
|---|---|---|
| 1 | 4.9 | Bright |
| 2 | 4.9 | Bright |
| 3 | 4.8 | Bright |
| 4 | 4.8 | Bright |
| 5 | 4.7 | Bright |
| 6 | 4.7 | Bright |
| 7 | 4.6 | Bright |
| 8 | 4.6 | Bright |
| 9 | 4.5 | Bright |
| 10 | 4.5 | Bright |
| 11 | 4.4 | Bright |
| 12 | 4.4 | Bright |
| 13 | 4.3 | Bright |
| 14 | 4.3 | Bright |
| 15 | 4.2 | Bright |
| 16 | 4.2 | Bright |
| 17 | 4.1 | Bright |
| 18 | 4.1 | Bright |
| 19 | 4.0 | Bright |
| 20 | 4.0 | Moderate |
| 21 | 3.9 | Moderate |
| 22 | 3.9 | Moderate |
| 23 | 3.8 | Moderate |
| 24 | 3.8 | Moderate |
| 25 | 3.7 | Moderate |
| 26 | 3.7 | Moderate |
| 27 | 3.6 | Moderate |
| 28 | 3.6 | Moderate |
| 29 | 3.5 | Moderate |
| 30 | 3.5 | Moderate |
Hazard calendar for Isumi
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.
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).
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.
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.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sea temperature (°C) | 18 | 16 | 16 | 18 | 20 | 22 | 24 | 26 | 26 | 24 | 22 | 20 |
| Comfort | Cool | Cool | Cool | Cool | Cool | Refreshing | Refreshing | Warm | Warm | Refreshing | Refreshing | Cool |
| Sea temperature (°C) | Comfort | |
|---|---|---|
| Jan | 18 | Cool |
| Feb | 16 | Cool |
| Mar | 16 | Cool |
| Apr | 18 | Cool |
| May | 20 | Cool |
| Jun | 22 | Refreshing |
| Jul | 24 | Refreshing |
| Aug | 26 | Warm |
| Sep | 26 | Warm |
| Oct | 24 | Refreshing |
| Nov | 22 | Refreshing |
| Dec | 20 | Cool |
UV index in Isumi in September
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.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| UV index (midday) | 3 | 4 | 5 | 7 | 9 | 9 | 10 | 10 | 8 | 6 | 4 | 2 |
| Level | Moderate | Moderate | Moderate | High | Very high | Very high | Extreme | Extreme | Very high | High | Moderate | Moderate |
| UV index (midday) | Level | |
|---|---|---|
| Jan | 3 | Moderate |
| Feb | 4 | Moderate |
| Mar | 5 | Moderate |
| Apr | 7 | High |
| May | 9 | Very high |
| Jun | 9 | Very high |
| Jul | 10 | Extreme |
| Aug | 10 | Extreme |
| Sep | 8 | Very high |
| Oct | 6 | High |
| Nov | 4 | Moderate |
| Dec | 2 | Moderate |
Air quality in Isumi in September
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).
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| PM2.5 (µg/m³) | 14 | 17 | 21 | 22 | 21 | 16 | 12 | 11 | 11 | 14 | 14 | 13 |
| Level | Moderate | Moderate | Moderate | Moderate | Moderate | Moderate | Good | Good | Good | Moderate | Moderate | Moderate |
| PM2.5 (µg/m³) | Level | |
|---|---|---|
| Jan | 14 | Moderate |
| Feb | 17 | Moderate |
| Mar | 21 | Moderate |
| Apr | 22 | Moderate |
| May | 21 | Moderate |
| Jun | 16 | Moderate |
| Jul | 12 | Good |
| Aug | 11 | Good |
| Sep | 11 | Good |
| Oct | 14 | Moderate |
| Nov | 14 | Moderate |
| Dec | 13 | Moderate |
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.
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
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).