Letterkenny weather in September

In September, Letterkenny sees average daytime highs of 15°C and overnight lows near 10°C, with 88 mm of rain across 15.5 wet days and about 12.5 hours of daylight. It is the 4th-warmest and 9th-wettest month of the year here.

Highs ease over the month, from about 16°C in the first days to 14°C by the end. The sky is clear or mostly clear about 4% of the time in September, and the air most often feels dry.

Daylight stretches from about 13 h 36 min at the start of September to 11 h 24 min by month's end. Set against the rest of the year, September is Letterkenny's 9th-clearest and 8th-windiest month. For the whole year in context, see Letterkenny's year-round climate.

Daytime high
15°
4th warmest
Overnight low
10°
10th coldest
Rainfall
88 mm
15.5 wet days
Daylight
12 h 24 min
6:12 AM – 6:38 PM
Cloud cover
80%
9th clearest
Wind
16 km/h
8th windiest
Solar energy
2.55 kWh/m²
Travel score
3.9
out of 10

Letterkenny is town in County Donegal, Ireland.

Density
472/km²
people per km²

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 16°C in the first days to 14°C by the end.

A typical September day in Letterkenny reaches about 15°C in the afternoon and slips back to 10°C overnight — a 5°C spread between the day's warmth and the night's chill. On most days the high lands between 13°C and 18°C. Set against its neighbours, September runs about 2°C cooler than August and about 3°C warmer than October.

10°15°20°151015202530
Avg high (°C)Avg low (°C)
11611
21611
31611
41611
51611
61611
71611
81611
91611
101610
111610
121610
131610
141610
151610
161510
171510
181510
191510
201510
211510
221510
23159
24159
25159
26159
27159
28149
29149
30149

How September really feels in Letterkenny

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.

Letterkenny weather by month

How September compares with the rest of the year at a glance. September is the 4th-warmest and 9th-wettest month here — the current month is highlighted below.

MonthHigh / Low (°C)Rainfall (mm)Wet daysSnow daysDaylight (hours)Travel score
Jan / 10818.527.60.8
Feb / 9115.91.89.31.0
Mar / 88171.311.41.5
Apr11° / 8016.10.313.82.3
May14° / 8615.9015.83.3
Jun16° / 10°92160173.9
Jul17° / 12°10118016.54.3
Aug17° / 12°10517.8014.74.3
Sep15° / 10°8815.5012.53.9
Oct13° / 10517.1010.22.8
Nov10° / 11018.80.38.11.6
Dec / 11318.11.270.9

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 4% of the time in September.

Beyond those clear spells, partly cloudy skies cover about 12% of September, with mostly cloudy or overcast conditions the remaining 84%.

0%25%50%75%100%151015202530
Clearer (%)Cloudier (%)
11486
21486
31486
41486
51387
61387
71387
81387
91486
101486
111585
121585
131585
141585
151684
161684
171684
181684
191684
201783
211783
221783
231783
241783
251783
261882
271882
281882
291882
301882

Chance of precipitation in September

Any precipitation

Letterkenny gets around 88 mm of rain over 15.5 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 52% and peaks near 55% on the wettest days.

0%20%40%60%151015202530
Rain chance (%)
153
253
353
454
554
654
755
855
955
1055
1155
1254
1354
1454
1553
1652
1752
1851
1950
2049
2149
2249
2349
2449
2549
2650
2750
2851
2951
3052

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 88% of days. It rarely turns muggy — the humid and sticky bands barely register.

0%25%50%75%100%151015202530
Dry (%)Comfortable (%)Humid (%)
182171
282171
382171
482171
583161
683161
783161
884151
985141
1085141
1186131
1287121
1387121
1489110
1589110
1690100
1790100
1890100
1990100
209190
219190
229190
239190
249190
259190
269190
279190
289280
299280
309370

Wind in September

Winds average 16 km/h in September.

Across September, daily wind ranges from about 16 km/h on the calmest days to 17 km/h at its breeziest. It stays fairly steady from day to day.

05101520151015202530
Wind speed (km/h)Feels
116light breeze
216light breeze
316light breeze
416light breeze
516light breeze
616light breeze
716light breeze
816light breeze
916light breeze
1016light breeze
1116light breeze
1216light breeze
1316light breeze
1416light breeze
1516light breeze
1616light breeze
1716light breeze
1816light breeze
1916light breeze
2016light breeze
2116light breeze
2216light breeze
2316light breeze
2416light breeze
2516light breeze
2617light breeze
2717light breeze
2817light breeze
2917light breeze
3017light breeze

Sunrise & sunset in September

DaylightNight

Daylight runs from about 13 h 36 min at the start of September to 11 h 24 min by month's end. The lit band spans sunrise to sunset each day.

Days shrink by about 132 minutes over September. Sunrise moves from 5:43 AM to 6:39 AM, and sunset from 7:18 PM to 6:01 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202530
Daylight (h)Sunrise (AM)Sunset (PM)
113.65:437:18
213.55:457:15
313.45:477:13
413.45:497:10
513.35:517:08
613.25:537:05
713.15:557:02
813.15:577:00
913.05:596:57
1012.96:016:54
1112.86:026:52
1212.76:046:49
1312.76:066:46
1412.66:086:44
1512.56:106:41
1612.46:126:38
1712.46:146:36
1812.36:166:33
1912.26:186:30
2012.16:206:28
2112.16:226:25
2212.06:246:22
2311.96:266:20
2411.86:276:17
2511.86:296:14
2611.76:316:12
2711.66:336:09
2811.56:356:07
2911.46:376:04
3011.46:396:01

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 2.55 kWh/m².

For context, solar energy here swings from a low near 0.42 kWh/m² to a high of 4.85 kWh/m² in May across the year — September's 2.55 kWh/m² falls in the middle.

01234151015202530
Solar energy (kWh/m²)Level
13.1Moderate
23.0Moderate
33.0Moderate
43.0Moderate
52.9Moderate
62.9Moderate
72.9Moderate
82.8Moderate
92.8Moderate
102.7Moderate
112.7Moderate
122.7Moderate
132.6Moderate
142.6Moderate
152.6Moderate
162.5Moderate
172.5Moderate
182.5Moderate
192.5Moderate
202.4Moderate
212.4Moderate
222.3Moderate
232.3Moderate
242.3Moderate
252.2Moderate
262.2Moderate
272.1Moderate
282.1Moderate
292.0Moderate
302.0Dim
Planning a trip in September

Is September a good time to visit Letterkenny?

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

3.9
Sightseeing
3rd-best month of the year
Beach & pool
never really warm enough

September is a weaker month for outdoor sightseeing in Letterkenny.

What to pack for Letterkenny in September

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

In September, Letterkenny sees daytime highs around 15°C and nights near 10°C. Pack rain gear. Worth having: a warm layer. About 16 days see rain.

rain geara warm layer

Sea temperature near Letterkenny in September

The nearest coast averages about 15°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 45 km away) is warmest in August at about 15°C and coldest in February near 9°C.

The water stays below 20°C all year — cool enough that most swimmers want a wetsuit.

10°15°JanFebMarAprMayJunJulAugSepOctNovDecbracingcool
Sea temperature (°C)Comfort
Jan9Bracing
Feb9Bracing
Mar9Bracing
Apr9Bracing
May11Bracing
Jun13Bracing
Jul15Bracing
Aug15Cool
Sep15Bracing
Oct13Bracing
Nov12Bracing
Dec10Bracing

UV index in Letterkenny in September

LowModerateHighVery highExtreme

Clear-sky midday UV in September peaks around 3 — enough to warrant sun protection. The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in Letterkenny peaks around May 24 at about 5 (high) — a little sun protection helps at midday. It bottoms out near January 1 at about 0 (low).

The index reaches 3 or more — the level where the WHO recommends sun protection — in 6 months of the year.

025710JanFebMarAprMayJunJulAugSepOctNovDecLowModerateHighModerateLowMay 24UV 5Jan 1UV 0
UV index (midday)Level
Jan0Low
Feb1Low
Mar2Moderate
Apr3Moderate
May5High
Jun5High
Jul5High
Aug5Moderate
Sep3Moderate
Oct2Low
Nov1Low
Dec0Low
Location & terrain

Where is Letterkenny?

Letterkenny sits at 55.0°N, 7.7°W, 8 m above sea level, in Ireland. The nearest listed city is Londonderry County Borough, 28 km away.

Londonderry County BoroughDerryOmaghColeraineSligoBallymenaAntrimPortadown

Topography around Letterkenny

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

Within 3 km, the terrain around Letterkenny has only modest variations in elevation — a maximum elevation change of 199 m around an average of 73 m above sea level; within 16 km, significant variations in elevation — a maximum elevation change of 452 m; within 80 km, large variations in elevation — a maximum elevation change of 706 m.

The land around Letterkenny is covered by grassland (60%) and trees (25%) within 3 km, grassland (72%) and trees (20%) within 16 km, and grassland (53%) and water (38%) within 80 km.

3 km
199 m
16 km
452 m
80 km
706 m
treesgrasslandcroplandartificial surfaceswater
September weather nearby: Londonderry County Borough 28 km · Derry 28 km · Omagh 48 km · Coleraine 71 km · Sligo 89 km · Ballymena 94 km · Antrim 101 km · Portadown 101 km

Ireland travel facts

Travel advisory
Level 1
Exercise Normal Precautions
Power plugs
Type G
230 V · 50 Hz
Prices
≈ 85% of US
price level, 2025
GDP per capita
$131,592
2025
Happiness
6.9 / 10
World Happiness, 2025
Safe drinking water
96%
of population, 2024
Homicide rate
0.7
per 100k, 2023
Road deaths
3.1
per 100k/yr, 2019
Income group
High
World Bank
Religion
Roman Catholic 69%none 15%Protestant 4%Orthodox 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 Letterkenny (54.95°, -7.7333°, elevation 8 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. Topography is sampled from the Copernicus DEM GLO-90 elevation model (© ESA/Airbus) and ESA WorldCover 2021 land cover (CC BY 4.0).