Clearlake weather in September

In September, Clearlake sees average daytime highs of 82°F and overnight lows near 59°F, with 0.2 in of rain across 1 wet days and about 12.3 hours of daylight. It is the 3rd-warmest and 10th-wettest month of the year here.

Highs ease over the month, from about 85°F in the first days to 79°F by the end. The sky is clear or mostly clear about 84% of the time in September, and the air most often feels dry.

Daylight stretches from about 12 h 54 min at the start of September to 11 h 36 min by month's end. Set against the rest of the year, September is Clearlake's 3rd-clearest and 9th-windiest month. For the whole year in context, see Clearlake's year-round climate.

Daytime high
82°
3rd warmest
Overnight low
59°
10th coldest
Rainfall
0.2 in
1 wet days
Daylight
12 h 12 min
5:57 AM – 6:12 PM
Cloud cover
16%
3rd clearest
Wind
4 mph
9th windiest
Solar energy
6.03 kWh/m²
Travel score
8.2
out of 10

Clearlake is city in Lake County, California, United States.

Density
554/km²
people per km²
Founded
1980

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 85°F in the first days to 79°F by the end.

A typical September day in Clearlake reaches about 82°F in the afternoon and slips back to 59°F overnight — a 23°F spread between the day's warmth and the night's chill. On most days the high lands between 72°F and 90°F. Set against its neighbours, September runs about 5°F cooler than August and about 10°F warmer than October.

40°60°80°151015202530
Avg high (°F)Avg low (°F)
18562
28562
38561
48461
58461
68461
78461
88360
98360
108360
118360
128260
138260
148259
158259
168259
178159
188159
198158
208158
218158
228058
238058
248057
258057
267957
277957
287956
297856
307856

Clearlake weather by month

How September compares with the rest of the year at a glance. September is the 3rd-warmest and 10th-wettest month here — the current month is highlighted below.

MonthHigh / Low (°F)Rainfall (inches)Wet daysSnow daysDaylight (hours)Travel score
Jan53° / 38°6.110.91.49.62.8
Feb55° / 40°5.99.41.710.53.6
Mar59° / 42°4.710.41.411.74.6
Apr64° / 45°2.16.50.3136.2
May71° / 51°1.44.7014.17.8
Jun80° / 58°0.52.1014.78.2
Jul87° / 64°0.00.2014.56.6
Aug86° / 63°0.10.4013.56.7
Sep82° / 59°0.21012.38.2
Oct72° / 52°1.63.80118.2
Nov60° / 43°3.27.10.29.95.1
Dec52° / 38°6.210.50.89.32.8

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 84% of the time in September.

Beyond those clear spells, partly cloudy skies cover about 7% of September, with mostly cloudy or overcast conditions the remaining 9%. Skies tend to cloud over as the month goes on.

0%25%50%75%100%151015202530
Clearer (%)Cloudier (%)
1946
2946
3946
4946
5937
6937
7937
8937
9937
10928
11928
12928
13928
14928
15928
16928
17928
18919
199010
209010
219010
229010
239010
248911
258911
268911
278812
288812
298614
308614

Chance of precipitation in September

Any precipitation

Clearlake gets around 0.2 in of rain over 1 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 4% and peaks near 7% on the wettest days.

0%10%20%30%151015202530
Rain chance (%)
11
21
31
41
52
62
72
82
92
103
113
123
133
143
154
164
174
184
194
204
215
225
235
245
256
266
276
287
297
307

Humidity in September

DryComfortableHumidMuggyOppressiveMiserable

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

By dew point, September lands in the dry band about 95% of days. It rarely turns muggy — the humid and sticky bands barely register.

0%25%50%75%100%151015202530
Dry (%)Comfortable (%)Humid (%)
19181
29271
39271
49271
59361
69361
79361
89361
99361
109361
119451
129451
139451
149451
159451
169451
179550
189550
199550
209640
219640
229640
239640
249640
259730
269730
279730
289730
299820
309820

Wind in September

Winds average 4 mph in September.

Across September, daily wind ranges from about 4 mph on the calmest days to 4 mph at its breeziest. It stays fairly steady from day to day.

0246810151015202530
Wind speed (mph)Feels
14calm
24calm
34calm
44calm
54calm
64calm
74calm
84calm
94calm
104calm
114calm
124calm
134calm
144calm
154calm
164calm
174calm
184calm
194calm
204calm
214calm
224calm
234calm
244calm
254calm
264calm
274calm
284calm
294calm
304calm

Sunrise & sunset in September

DaylightNight

Daylight runs from about 12 h 54 min at the start of September to 11 h 36 min by month's end. The lit band spans sunrise to sunset each day.

Days shrink by about 78 minutes over September. Sunrise moves from 5:43 AM to 6:11 AM, and sunset from 6:37 PM to 5:49 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202530
Daylight (h)Sunrise (AM)Sunset (PM)
112.95:436:37
212.85:446:35
312.85:456:34
412.85:466:32
512.75:476:31
612.75:486:29
712.65:496:27
812.65:506:26
912.65:516:24
1012.55:526:22
1112.55:536:21
1212.45:546:19
1312.45:556:17
1412.35:556:16
1512.35:566:14
1612.25:576:12
1712.25:586:11
1812.25:596:09
1912.16:006:07
2012.16:016:06
2112.06:026:04
2212.06:036:02
2311.96:046:01
2411.96:055:59
2511.96:065:57
2611.86:075:56
2711.86:085:54
2811.76:095:52
2911.76:105:51
3011.66:115:49

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 6.03 kWh/m².

For context, solar energy here swings from a low near 2.01 kWh/m² to a high of 8.26 kWh/m² in July across the year — September's 6.03 kWh/m² falls in the middle.

0246151015202530
Solar energy (kWh/m²)Level
16.8Intense
26.7Intense
36.7Intense
46.6Intense
56.6Intense
66.5Intense
76.5Intense
86.4Intense
96.4Intense
106.3Intense
116.3Intense
126.2Intense
136.2Intense
146.1Intense
156.0Intense
166.0Bright
175.9Bright
185.9Bright
195.8Bright
205.8Bright
215.7Bright
225.7Bright
235.6Bright
245.5Bright
255.5Bright
265.4Bright
275.3Bright
285.3Bright
295.2Bright
305.2Bright
Planning a trip in September

Hazard calendar for Clearlake

In seasonPeak

None of Clearlake's seasonal hazards are active in September. The full-year calendar below shows when the rainy season, cyclone risk and dust haze do peak.

Clearlake's rainy season runs November–March, 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 Clearlake?

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

8.2
Sightseeing
1st-best month of the year
8.8
Beach & pool
3rd-best month of the year

September is one of the better times to visit Clearlake for general sightseeing. The water and warmth also suit a beach or pool day.

What to pack for Clearlake in September

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

In September, Clearlake sees daytime highs around 82°F and nights near 59°F. Pack light clothes and sun protection.

light clothessun protection

UV index in Clearlake in September

LowModerateHighVery highExtreme

Clear-sky midday UV in September peaks around 6 — enough to warrant sun protection. The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in Clearlake peaks around June 23 at about 10 (extreme) — sunscreen, a hat and midday shade are essential. It bottoms out near December 15 at about 2 (moderate).

The index reaches 3 or more — the level where the WHO recommends sun protection — in 9 months of the year.

025710JanFebMarAprMayJunJulAugSepOctNovDecModerateHighVery highExtremeVery highHighModerateJun 23UV 10Dec 15UV 2
UV index (midday)Level
Jan2Moderate
Feb3Moderate
Mar5Moderate
Apr6High
May9Very high
Jun10Extreme
Jul10Extreme
Aug9Very high
Sep6High
Oct5High
Nov2Moderate
Dec2Low

Air quality in Clearlake 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 Clearlake is worst in September — around 31 µg/m³ PM2.5 (moderate), about 5.8× the cleanest month (June, 5 µg/m³).

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

010203040JanFebMarAprMayJunJulAugSepOctNovDecGoodModerateGoodSep 1031 µg/m³Jun 106 µg/m³
PM2.5 (µg/m³)Level
Jan6Good
Feb8Good
Mar9Good
Apr8Good
May6Good
Jun5Good
Jul14Moderate
Aug29Moderate
Sep31Moderate
Oct14Moderate
Nov8Good
Dec6Good
Location & terrain

Where is Clearlake?

Clearlake sits at 39.0°N, 122.6°W, 1,417 ft above sea level, in United States. The nearest listed city is Windsor, 30 mi away.

Topography around Clearlake

How much the land rises and falls, and what covers it, within 2, 10 and 50 miles of Clearlake.

Within 2 miles, the terrain around Clearlake has significant variations in elevation — a maximum elevation change of 853 ft around an average of 1,493 ft above sea level; within 10 miles, large variations in elevation — a maximum elevation change of 3,409 ft; within 50 miles, very significant variations in elevation — a maximum elevation change of 6,982 ft.

The land around Clearlake is covered by trees (39%) and grassland (28%) within 2 miles, trees (52%) and grassland (38%) within 10 miles, and trees (52%) and grassland (30%) within 50 miles.

2 miles
853 ft
10 miles
3,409 ft
50 miles
6,982 ft
treesgrasslandcroplandartificial surfaceswater
September weather nearby: Windsor 30 mi · Ukiah 34 mi · Santa Rosa 36 mi · Rohnert Park 43 mi · Agua Caliente 45 mi · Napa 49 mi · Woodland 50 mi · Petaluma 50 mi

United States travel facts

Power plugs
Type A / B
120 V · 60 Hz
Prices
≈ 100% of US
price level, 2025
GDP per capita
$90,027
2025
Happiness
6.8 / 10
World Happiness, 2025
Safe drinking water
98%
of population, 2024
Homicide rate
5.8
per 100k, 2023
Road deaths
12.7
per 100k/yr, 2019
Income group
High
World Bank
Religion
Protestant 47%unaffiliated 23%Roman Catholic 21%Jewish 2%

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 Clearlake (38.9582°, -122.6264°, elevation 1,417 ft), 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).