Alajuela weather in September

In September, Alajuela sees average daytime highs of 24°C and overnight lows near 17°C, with 424 mm of rain across 28.3 wet days and about 12.1 hours of daylight. It is the 8th-warmest and 2nd-wettest month of the year here.

Day-to-day highs hold fairly steady near 24°C through the month. The sky is clear or mostly clear about 0% of the time in September, and the air most often feels humid.

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

Daytime high
24°
8th warmest
Overnight low
17°
7th coldest
Rainfall
424 mm
28.3 wet days
Daylight
12 h 06 min
5:30 AM – 5:33 PM
Cloud cover
92%
11th clearest
Wind
4 km/h
12th windiest
Solar energy
4.28 kWh/m²
Travel score
3.7
out of 10
Air quality
Good
PM10 18 µg/m³ · 2018

Air quality: WHO Ambient Air Quality Database. Density & description: Wikidata (CC0), GeoNames.

Temperature in September

Average highAverage lowShaded bands: 10–90% of days

Day-to-day highs hold fairly steady near 24°C through the month.

A typical September day in Alajuela reaches about 24°C in the afternoon and slips back to 17°C overnight — a 7°C spread between the day's warmth and the night's chill. On most days the high lands between 23°C and 26°C. Set against its neighbours, September runs on par with August and on par with October.

15°20°25°30°151015202530
Avg high (°C)Avg low (°C)
12517
22517
32517
42517
52517
62517
72517
82517
92517
102417
112417
122417
132417
142417
152417
162417
172417
182417
192417
202417
212417
222417
232417
242417
252417
262417
272417
282417
292417
302417

Alajuela weather by month

How September compares with the rest of the year at a glance. September is the 8th-warmest and 2nd-wettest month here — the current month is highlighted below.

MonthHigh / Low (°C)Rainfall (mm)Wet daysDaylight (hours)Travel score
Jan25° / 16°295.811.57.8
Feb25° / 16°204.111.78.0
Mar26° / 17°205.211.97.5
Apr27° / 17°6211.212.26.2
May25° / 18°33324.712.54.2
Jun25° / 18°39027.312.63.8
Jul24° / 18°2672512.54.3
Aug25° / 18°35226.412.34.0
Sep24° / 17°42428.312.13.7
Oct24° / 17°44428.811.83.8
Nov24° / 17°22421.511.54.8
Dec24° / 17°7210.811.46.9

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 0% of the time in September.

Beyond those clear spells, partly cloudy skies cover about 2% of September, with mostly cloudy or overcast conditions the remaining 98%.

0%25%50%75%100%151015202530
Clearer (%)Cloudier (%)
1298
2397
3397
4397
5397
6397
7397
8298
9298
10298
11298
12298
13298
14298
15298
16298
17298
18298
19298
20298
21298
22298
23298
24298
25298
26298
27298
28298
29298
30298

Chance of precipitation in September

Any precipitation

Alajuela gets around 424 mm of rain over 28.3 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 95% and peaks near 97% on the wettest days.

0%20%40%60%80%100%151015202530
Rain chance (%)
191
291
391
492
592
692
792
893
993
1093
1194
1294
1394
1495
1595
1695
1796
1896
1996
2096
2197
2297
2397
2497
2597
2697
2797
2896
2996
3096

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 57% of days. Sticky, muggy-or-worse air turns up on roughly 43% of them.

0%25%50%75%100%151015202530
Humid (%)Muggy (%)
16040
26040
36040
46040
56040
66040
75941
85941
95842
105743
115743
125644
135545
145545
155545
165446
175446
185446
195446
205446
215545
225545
235545
245644
255644
265644
275644
285644
295743
305743

Wind in September

Winds average 4 km/h in September, the calmest month.

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

0246810151015202530
Wind speed (km/h)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 12 min at the start of September to 11 h 54 min by month's end. The lit band spans sunrise to sunset each day.

Days shrink by about 18 minutes over September. Sunrise moves from 5:31 AM to 5:29 AM, and sunset from 5:42 PM to 5:24 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202530
Daylight (h)Sunrise (AM)Sunset (PM)
112.25:315:42
212.25:315:42
312.25:315:41
412.25:315:41
512.25:305:40
612.15:305:39
712.15:305:39
812.15:305:38
912.15:305:37
1012.15:305:37
1112.15:305:36
1212.15:305:35
1312.15:305:35
1412.15:305:34
1512.15:305:33
1612.15:305:33
1712.05:295:32
1812.05:295:31
1912.05:295:31
2012.05:295:30
2112.05:295:30
2212.05:295:29
2312.05:295:28
2412.05:295:28
2512.05:295:27
2612.05:295:26
2712.05:295:26
2811.95:295:25
2911.95:295:24
3011.95:295:24

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 4.28 kWh/m².

For context, solar energy here swings from a low near 3.65 kWh/m² to a high of 5.95 kWh/m² in March across the year — September's 4.28 kWh/m² falls in the middle.

012345151015202530
Solar energy (kWh/m²)Level
14.4Bright
24.4Bright
34.4Bright
44.4Bright
54.4Bright
64.4Bright
74.4Bright
84.3Bright
94.3Bright
104.3Bright
114.3Bright
124.3Bright
134.3Bright
144.3Bright
154.3Bright
164.3Bright
174.3Bright
184.2Bright
194.2Bright
204.2Bright
214.2Bright
224.2Bright
234.2Bright
244.2Bright
254.2Bright
264.2Bright
274.1Bright
284.1Bright
294.1Bright
304.1Bright
Planning a trip in September

Hazard calendar for Alajuela

In seasonPeak

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

Alajuela's rainy season runs May–November, 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 Alajuela?

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

3.7
Sightseeing
12th-best month of the year
3.2
Beach & pool
12th-best month of the year

September is a weaker month for outdoor sightseeing in Alajuela.

What to pack for Alajuela in September

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

In September, Alajuela sees daytime highs around 24°C and nights near 17°C. Pack rain gear. Worth having: light clothes. About 28 days see rain.

rain gearlight clothes

Sea temperature near Alajuela in September

The nearest coast averages about 29°C in September — the other half of the beach question. The full-year curve and swim-comfort zones are below.

The sea at the nearest coast (about 48 km away) is warmest in April at about 29°C and coldest in November near 28°C.

The water stays above 20°C all year — warm enough for a swim any month.

26°28°30°JanFebMarAprMayJunJulAugSepOctNovDecwarmtropicalcomfortable to swim
Sea temperature (°C)Comfort
Jan29Tropical
Feb29Tropical
Mar29Tropical
Apr29Tropical
May29Tropical
Jun29Tropical
Jul29Tropical
Aug29Tropical
Sep29Tropical
Oct28Tropical
Nov28Warm
Dec28Tropical

UV index in Alajuela in September

LowModerateHighVery highExtreme

Clear-sky midday UV in September peaks around 12 (very high — sun protection essential). The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in Alajuela peaks around February 22 at about 14 (extreme) — sunscreen, a hat and midday shade are essential. It bottoms out near December 17 at about 10 (very high).

The index reaches 3 or more — the level where the WHO recommends sun protection — every month of the year.

025710JanFebMarAprMayJunJulAugSepOctNovDecExtremeFeb 22UV 14Dec 17UV 10
UV index (midday)Level
Jan11Extreme
Feb14Extreme
Mar14Extreme
Apr14Extreme
May12Extreme
Jun13Extreme
Jul12Extreme
Aug12Extreme
Sep12Extreme
Oct12Extreme
Nov11Extreme
Dec10Very high

Air quality in Alajuela 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 Alajuela is worst in May — around 19 µg/m³ PM2.5 (moderate), about 2.1× the cleanest month (January, 9 µg/m³).

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

0102030JanFebMarAprMayJunJulAugSepOctNovDecGoodModerateGoodMay 1718 µg/m³Jan 149 µg/m³
PM2.5 (µg/m³)Level
Jan9Good
Feb10Good
Mar11Good
Apr17Moderate
May19Moderate
Jun17Moderate
Jul18Moderate
Aug17Moderate
Sep16Moderate
Oct14Moderate
Nov12Good
Dec10Good
Location & terrain

Where is Alajuela?

Alajuela sits at 10.0°N, 84.2°W, 955 m above sea level, in Costa Rica. The nearest listed city is Mercedes Norte, 9.0 km away.

Topography around Alajuela

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

Within 3 km, the terrain around Alajuela has only modest variations in elevation — a maximum elevation change of 200 m around an average of 945 m above sea level; within 16 km, very significant variations in elevation — a maximum elevation change of 2,297 m; within 80 km, extreme variations in elevation — a maximum elevation change of 3,470 m.

The land around Alajuela is covered by artificial surfaces (48%) and trees (32%) within 3 km, trees (56%), grassland (23%) and artificial surfaces (20%) within 16 km, and trees (68%) within 80 km.

3 km
200 m
16 km
2,297 m
80 km
3,470 m
treesgrasslandcroplandartificial surfaceswater
September weather nearby: Mercedes Norte 9.0 km · San Francisco 10 km · Heredia 11 km · San Pablo 13 km · San Rafael 13 km · Colima 16 km · San Juan 16 km · San Jose 17 km

Costa Rica travel facts

Travel advisory
Level 2
Exercise Increased Caution
Power plugs
Type A / B
120 V · 60 Hz
Prices
≈ 59% of US
price level, 2025
GDP per capita
$19,970
2025
Happiness
7.4 / 10
World Happiness, 2025
Safe drinking water
81%
of population, 2024
Homicide rate
17.7
per 100k, 2023
Road deaths
14.8
per 100k/yr, 2019
Income group
High
World Bank
English
Moderate
EF EPI 516
Religion
Roman Catholic 48%none 27%Evangelical and Pentecostal 20%other 3%

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 Alajuela (10.0172°, -84.2127°, elevation 955 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. 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).