Isumi in Summer
In summer (June, July and August), Isumi sees average daytime highs around 26°C and overnight lows near 23°C, with 423 mm of rain over about 36 wet days across the season. It is the warmest season of the year and the 2nd-wettest season.
Over the season highs climb from about 22°C in early June to 27°C by late August. The sky is clear or mostly clear about 22% of the time across summer, and the air most often feels oppressive.
Summer is Isumi's best season of the year to visit on our travel score. For the whole year in context, see Isumi's year-round climate.
- Avg daytime high
- 26°C
- warmest season of the year
- Avg overnight low
- 23°C
- Rainfall
- 423 mm
- 36 wet days
- Avg wind
- 17 km/h
- Travel score
- 6.1
- best season of the year
- Months
- Jun · Jul · Aug
Temperature in Isumi in Summer
Over the season highs climb from about 22°C in early June to 27°C by late August.
A typical summer day in Isumi reaches about 26°C and drops to 23°C overnight. The warmest of the three months is August, the coolest June.
Isumi weather by month
How the three summer months (June, July and August) compare with the rest of the year — the season's months are highlighted.
| 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 Summer
The sky is clear or mostly clear about 22% of the time across summer.
Chance of precipitation in Summer
Isumi gets around 423 mm of rain over 36 wet days across summer. The line is the day-by-day chance of measurable precipitation.
Humidity in Summer
Most of summer feels oppressive — the stacked bands show each day's mix of comfort levels, read off the dew point.
Wind in Summer
Winds average 17 km/h across summer.
Sunrise & sunset in Summer
Daylight runs from about 14 h 12 min in early June to 12 h 48 min by late August. The lit band spans sunrise to sunset each day.
Solar energy in Summer
Average shortwave solar energy reaching the ground each day across summer, about 5.3 kWh/m².
Hazard calendar for Isumi
In summer, Isumi overlaps 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 summer a good time to visit Isumi?
Our two travel scores rate summer for general sightseeing and for beach or pool weather — 0 (poor) to 10 (ideal), averaged across the three months.
summer is a reasonable time to visit Isumi for sightseeing.
What to pack for Isumi in Summer
From summer's temperature range, rain chance, sun and wind — what's worth putting in the bag.
In summer, Isumi sees daytime highs around 26°C and nights near 23°C. Pack light clothes and rain gear. Worth having: sun protection. About 36 days see rain.
Sea temperature near Isumi in Summer
The nearest coast averages about 24°C across summer — 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 Summer
Clear-sky midday UV across summer averages around 10 (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 Summer
Average fine-particle pollution (PM2.5) through the year — so you can see where summer 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.