Koesan weather in September

In September, Koesan sees average daytime highs of 25°C and overnight lows near 16°C, with 139 mm of rain across 8.9 wet days and about 12.2 hours of daylight. It is the 4th-warmest and 3rd-wettest month of the year here.

Highs ease over the month, from about 27°C in the first days to 23°C by the end. The sky is clear or mostly clear about 30% 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 Koesan's 9th-clearest and 12th-windiest month. For the whole year in context, see Koesan's year-round climate.

Daytime high
25°
4th warmest
Overnight low
16°
9th coldest
Rainfall
139 mm
8.9 wet days
Daylight
12 h 12 min
6:16 AM – 6:30 PM
Cloud cover
59%
9th clearest
Wind
8 km/h
12th windiest
Solar energy
4.12 kWh/m²
Travel score
6.9
out of 10

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 23°C by the end.

A typical September day in Koesan reaches about 25°C in the afternoon and slips back to 16°C overnight — a 9°C spread between the day's warmth and the night's chill. On most days the high lands between 21°C and 28°C. Set against its neighbours, September runs about 4°C cooler than August and about 6°C warmer than October.

10°20°30°151015202530
Avg high (°C)Avg low (°C)
12719
22719
32719
42719
52618
62618
72618
82618
92618
102617
112617
122517
132517
142516
152516
162516
172516
182516
192415
202415
212415
222415
232414
242414
252314
262314
272313
282313
292313
302313

How September really feels in Koesan

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.

Koesan weather by month

How September compares with the rest of the year at a glance. September is the 4th-warmest and 3rd-wettest month here — the current month is highlighted below.

MonthHigh / Low (°C)Rainfall (mm)Wet daysSnow daysDaylight (hours)Travel score
Jan / -6°234.73.39.80.0
Feb / -4°334.62.110.60.4
Mar11° / 547.12.111.73.0
Apr18° / 837.70.212.96.1
May23° / 12°948013.97.1
Jun27° / 17°1258.8014.56.5
Jul28° / 21°25816.7014.34.8
Aug29° / 22°21915.5013.44.8
Sep25° / 16°1398.9012.26.9
Oct19° / 585.20117.0
Nov12° / 446.60.4103.4
Dec / -4°264.62.19.50.2

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 30% of the time in September.

Beyond those clear spells, partly cloudy skies cover about 17% of September, with mostly cloudy or overcast conditions the remaining 53%. Skies tend to open up as the month goes on.

0%25%50%75%100%151015202530
Clearer (%)Cloudier (%)
14159
24159
34159
44159
54258
64258
74159
84258
94258
104357
114357
124357
134456
144555
154555
164654
174753
184951
194951
205050
215149
225149
235149
245248
255248
265248
275347
285347
295446
305446

Chance of precipitation in September

Any precipitation

Koesan gets around 139 mm of rain over 8.9 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 31% and peaks near 43% on the wettest days. The odds ease toward the end of the month.

0%10%20%30%40%50%151015202530
Rain chance (%)
143
242
341
440
540
639
738
837
936
1035
1135
1234
1333
1432
1531
1630
1729
1828
1927
2026
2125
2224
2323
2423
2522
2622
2721
2821
2920
3020

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 28% of days. Sticky, muggy-or-worse air turns up on roughly 20% of them.

0%25%50%75%100%151015202530
Dry (%)Comfortable (%)Humid (%)Muggy (%)Oppressive (%)Miserable (%)
11153331191
22163430171
32183529161
43193627141
54203726130
65213725120
76223824100
8723382390
9824382280
101026372170
111227371960
121428361850
131628341740
141829331640
152130311530
162331301330
172632281220
182832261120
193133251020
20343323910
21363322810
22393320710
23423319610
24443218510
25473216410
26493115400
27533014300
28552913300
29562813300
30592712200

Wind in September

Winds average 8 km/h in September, the calmest month.

Across September, daily wind ranges from about 8 km/h on the calmest days to 8 km/h at its breeziest. It stays fairly steady from day to day.

0246810151015202530
Wind speed (km/h)Feels
18calm
28calm
38calm
48calm
58calm
68calm
78calm
88calm
98calm
108calm
118calm
128calm
138calm
148calm
158calm
168calm
178calm
188calm
198calm
208calm
218calm
228calm
238calm
248calm
258calm
268calm
278calm
288calm
298calm
308calm

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 6:04 AM to 6:28 AM, and sunset from 6:53 PM to 6:08 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202530
Daylight (h)Sunrise (AM)Sunset (PM)
112.86:046:53
212.86:056:52
312.76:056:50
412.76:066:49
512.76:076:47
612.66:086:46
712.66:096:44
812.66:106:43
912.56:106:41
1012.56:116:40
1112.46:126:38
1212.46:136:36
1312.46:146:35
1412.36:156:33
1512.36:156:32
1612.26:166:30
1712.26:176:29
1812.26:186:27
1912.16:196:25
2012.16:206:24
2112.06:206:22
2212.06:216:21
2311.96:226:19
2411.96:236:17
2511.96:246:16
2611.86:256:14
2711.86:256:13
2811.76:266:11
2911.76:276:10
3011.76:286:08

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 4.12 kWh/m².

For context, solar energy here swings from a low near 2.37 kWh/m² to a high of 5.83 kWh/m² in May across the year — September's 4.12 kWh/m² falls in the middle.

012345151015202530
Solar energy (kWh/m²)Level
14.1Bright
24.1Bright
34.1Bright
44.1Bright
54.1Bright
64.1Bright
74.1Bright
84.1Bright
94.1Bright
104.1Bright
114.1Bright
124.1Bright
134.1Bright
144.1Bright
154.1Bright
164.1Bright
174.1Bright
184.1Bright
194.1Bright
204.1Bright
214.1Bright
224.1Bright
234.1Bright
244.1Bright
254.0Bright
264.0Bright
274.0Bright
284.0Bright
294.0Bright
304.0Moderate
Planning a trip in September

Hazard calendar for Koesan

In seasonPeak

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

Koesan's rainy season runs June–September, 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 Koesan?

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

6.9
Sightseeing
3rd-best month of the year
6.0
Beach & pool
2nd-best month of the year

September is a reasonable time to visit Koesan for sightseeing. The water and warmth also suit a beach or pool day.

What to pack for Koesan in September

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

In September, Koesan sees daytime highs around 25°C and nights near 16°C. Worth packing: light clothes and rain gear. About 9 days see rain.

light clothesrain gear

UV index in Koesan in September

LowModerateHighVery highExtreme

Clear-sky midday UV in September peaks around 8 — enough to warrant sun protection. The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in Koesan peaks around August 14 at about 9 (very high) — sunscreen, a hat and midday shade are essential. It bottoms out near December 23 at about 2 (low).

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

025710JanFebMarAprMayJunJulAugSepOctNovDecModerateHighVery highHighModerateAug 14UV 9Dec 23UV 2
UV index (midday)Level
Jan2Low
Feb3Moderate
Mar5Moderate
Apr6High
May7High
Jun7Very high
Jul8Very high
Aug9Very high
Sep8Very high
Oct5High
Nov3Moderate
Dec2Low

Air quality in Koesan 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 Koesan is worst in March — around 80 µg/m³ PM2.5 (unhealthy), about 1.8× the cleanest month (July, 45 µg/m³).

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

0306090JanFebMarAprMayJunJulAugSepOctNovDecUnhealthyPoorUnhealthyMar 1479 µg/m³Jul 2345 µg/m³
PM2.5 (µg/m³)Level
Jan69Unhealthy
Feb72Unhealthy
Mar80Unhealthy
Apr65Unhealthy
May58Unhealthy
Jun54Poor
Jul45Poor
Aug45Poor
Sep56Unhealthy
Oct67Unhealthy
Nov72Unhealthy
Dec65Unhealthy
Location & terrain

Where is Koesan?

Koesan sits at 36.8°N, 127.8°E, 116 m above sea level, in South Korea. The nearest listed city is Jungpyong, 19 km away.

MungyeongSejongAnseongSangju

Topography around Koesan

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

Within 3 km, the terrain around Koesan has significant variations in elevation — a maximum elevation change of 298 m around an average of 173 m above sea level; within 16 km, large variations in elevation — a maximum elevation change of 866 m; within 80 km, very significant variations in elevation — a maximum elevation change of 1,408 m.

The land around Koesan is covered by trees (58%) within 3 km, trees (74%) within 16 km, and trees (72%) within 80 km.

3 km
298 m
16 km
866 m
80 km
1,408 m
treesgrasslandcroplandartificial surfacesbare terrainwater
September weather nearby: Jungpyong 19 km · Chungju 22 km · Chinch'ŏn 32 km · Cheongju-si 33 km · Mungyeong 43 km · Sejong 51 km · Anseong 52 km · Sangju 55 km

South Korea travel facts

Travel advisory
Level 1
Exercise Normal Precautions
Power plugs
Type C / F
220 V · 60 Hz
Prices
≈ 57% of US
price level, 2025
GDP per capita
$36,227
2025
Happiness
6.0 / 10
World Happiness, 2025
Safe drinking water
99%
of population, 2024
Homicide rate
0.5
per 100k, 2023
Road deaths
8.6
per 100k/yr, 2019
Income group
High
World Bank
English
Moderate
EF EPI 522
Religion
none 60%Protestant 17%Buddhist 16%Catholic 6%

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 Koesan (36.8108°, 127.7947°, elevation 116 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).