Isumi climate — normals and trends
Isumi (Japan) has a humid subtropical climate (Cfa) under the Köppen-Geiger classification. Its mean annual temperature is 16.7 °C and it receives 1,668 mm of precipitation per year.
These figures are normals computed over 86 complete calendar years, from 1940 to 2025, using ERA5 reanalysis data from the Copernicus programme.
The warmest month is August (26.1 °C on average) and the coldest is February (7.8 °C). Rainfall peaks in October.
Isumi averages 0 frost days, 1 hot days (≥ 30 °C) and 157 rainy days per year.
Isumi has warmed by +1.6 °C between 1940 and 2025, or +0.18 °C per decade. This trend is statistically significant.
The hottest day of the period reached 32.1 °C (on 6 August 2025) and the coldest -0.9 °C (on 5 February 1952).
Data from ERA5 reanalysis (Copernicus Climate Change Service), redistributed under a CC-BY 4.0 licence. Normals are recomputed whenever the archive is updated.
Climate normals · ERA5
IsumiChiba
- coords
- 35.23°, 140.40°
- period
- 1940–2025 · 86 yrs
Mean daily temperature
16.7°C
Average daily high
18.3°C
Average daily low
15.1°C
Annual rainfall
1,668mm
Sunshine / day
8.4h
Mean relative humidity
75%
Average peak wind
27.0km/h
Seasonal range
18.3°C
Frost days (< 0 °C)
0 d/yr
Hot days (≥ 30 °C)
1 d/yr
Rainy days (≥ 1 mm)
157 d/yr
Annual snowfall
2.4 cm
Average feels-like high
17.8 °C
Average feels-like low
13.1 °C
Average peak gust
44.5 km/h
Tropical nights (≥ 20 °C)
111 d/yr
Solar radiation
14.4 MJ/m²
Has the climate changed?
One stripe per year, 1940 to 2025
+1.6 °C
since 1940
+0.2 °C per decade · r² = 0.52 · p = < 0.001
Trend fitted by linear regression (ordinary least squares) on annual means. A trend is shown only when it is statistically significant (p < 0.05).
A climograph overlays rainfall (bars) and temperature (line): the standard reading in climatology, where you can see at a glance whether the warm season is also the dry one.
Records for the period
Hottest day
32.1 °C
6 August 2025
Coldest day
-0.9 °C
5 February 1952
Wettest day
217 mm
27 October 2007
Windiest day
91.5 km/h
17 September 1995
Strongest gust
164.9 km/h
1 October 2018
Hottest feels-like
41.0 °C
6 August 2025
Coldest feels-like
-8.9 °C
25 January 2023
Snowiest day
11.5 cm
15 February 1951
Sunniest day
13.9 h
17 June 1987
Normals computed over complete calendar years 1940–2025.
A specific year
Pick a year and see how it compares to normal — warmer, drier or more extreme than usual?
Across the whole year
Month by month
| High | vs. normal | Precipitation | vs. normal | |
|---|---|---|---|---|
| Jan | 11.0 °C | +1.0 °C | 61 mm | -30 mm |
| Feb | 10.0 °C | +0.1 °C | 29 mm | -73 mm |
| Mar | 14.0 °C | +1.7 °C | 169 mm | +25 mm |
| Apr | 17.1 °C | +0.9 °C | 167 mm | +41 mm |
| May | 20.2 °C | +0.4 °C | 200 mm | +58 mm |
| Jun | 24.9 °C | +2.4 °C | 103 mm | -74 mm |
| Jul | 28.3 °C | +2.5 °C | 103 mm | -25 mm |
| Aug | 29.2 °C | +1.8 °C | 106 mm | -5 mm |
| Sep | 27.5 °C | +2.3 °C | 111 mm | -68 mm |
| Oct | 22.1 °C | +1.1 °C | 245 mm | +14 mm |
| Nov | 17.1 °C | +0.1 °C | 95 mm | -51 mm |
| Dec | 14.1 °C | +1.4 °C | 115 mm | +22 mm |
The year's extremes
What was the weather like that day?
Your birthday, a wedding, a memorable trip: discover the actual weather for any date since 1940.
Frequently asked questions
What is the climate of Isumi?
Isumi has a humid subtropical climate (Köppen Cfa). Over 1940-2025, the average annual temperature is 16.7 °C and it receives 1,668 mm of precipitation per year.
What is the hottest day ever recorded in Isumi?
The hottest day measured in Isumi between 1940 and 2025 was 6 August 2025, reaching 32.1 °C.
What is the coldest day in Isumi?
The coldest day measured in Isumi between 1940 and 2025 was 5 February 1952, at -0.9 °C.
Does it snow in Isumi?
Yes. Isumi gets on average 2.4 cm of snow per year and has around 0 frost days.
Has the climate of Isumi warmed?
Yes: Isumi warmed by +1.6 °C between 1940 and 2025, a statistically significant trend.
What is the wettest day in Isumi?
The wettest day measured in Isumi between 1940 and 2025 was 27 October 2007, with 217 mm of rain in 24 hours.
What was the windiest day in Isumi?
The windiest day measured in Isumi between 1940 and 2025 was 17 September 1995, with winds reaching 91.5 km/h.
What is the warmest month in Isumi?
The warmest month in Isumi is August (26.1 °C on average); the coldest is February (7.8 °C).
How much does it rain in Isumi?
Isumi receives on average 1,668 mm of precipitation per year, spread over about 157 rainy days.
Is it sunny in Isumi?
Isumi gets on average 8.4 hours of sunshine per day, roughly 3,066 hours per year over 1940-2025.
What is the biggest single-day snowfall in Isumi?
The heaviest 24-hour snowfall measured in Isumi between 1940 and 2025 was 15 February 1951, with 11.5 cm.
Are there tropical nights in Isumi?
Yes: Isumi has on average 111 tropical nights per year (temperature staying above 20 °C), over 1940-2025.
What was the most extreme apparent temperature in Isumi?
In Isumi, between 1940 and 2025, the highest apparent temperature reached 41.0 °C on 6 August 2025, and the lowest was -8.9 °C on 25 January 2023.
Compare Isumi with another city
Pick a second city: its normals will be set against these, indicator by indicator.
Nearby cities
The climate around Isumi