Allentown weather in September

In September, Allentown sees average daytime highs of 76°F and overnight lows near 58°F, with 4.4 in of rain across 8.8 wet days and about 12.3 hours of daylight. It is the 4th-warmest and 1st-wettest month of the year here.

Highs ease over the month, from about 81°F in the first days to 71°F by the end. The sky is clear or mostly clear about 35% 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 Allentown's 2nd-clearest and 9th-windiest month. For the whole year in context, see Allentown's year-round climate.

Daytime high
76°
4th warmest
Overnight low
58°
9th coldest
Rainfall
4.4 in
8.8 wet days
Daylight
12 h 18 min
5:48 AM – 6:04 PM
Cloud cover
56%
2nd clearest
Wind
6 mph
9th windiest
Solar energy
4.21 kWh/m²
Travel score
7.0
out of 10

Allentown is city in and seat of Lehigh County, Pennsylvania, United States.

Density
2,575/km²
people per km²
Founded
1762

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 81°F in the first days to 71°F by the end.

A typical September day in Allentown reaches about 76°F in the afternoon and slips back to 58°F overnight — a 19°F spread between the day's warmth and the night's chill. On most days the high lands between 67°F and 86°F. Set against its neighbours, September runs about 7°F cooler than August and about 12°F warmer than October.

40°60°80°151015202530
Avg high (°F)Avg low (°F)
18162
28162
38162
48062
58062
68061
77961
87960
97960
107860
117859
127859
137759
147758
157658
167657
177657
187557
197556
207556
217456
227455
237355
247355
257354
267254
277253
287153
297153
307152

How September really feels in Allentown

Once humidity and wind are folded in, September averages 0 days that feel above 90°F and 0 that feel at or below 32°F — the comfort signal a plain temperature reading hides.

Days feeling ≥ 90°F
0
heat index over the day's high
Days feeling ≤ 32°F
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.

Allentown weather by month

How September compares with the rest of the year at a glance. September is the 4th-warmest and 1st-wettest month here — the current month is highlighted below.

MonthHigh / Low (°F)Rainfall (inches)Wet daysSnow daysDaylight (hours)Travel score
Jan38° / 24°3.49.54.89.40.0
Feb41° / 25°3.08.9410.40.2
Mar49° / 32°4.0113.511.72.1
Apr62° / 43°3.910.60.4135.0
May71° / 52°3.711.5014.26.3
Jun81° / 62°4.111.4014.86.0
Jul85° / 67°3.811.2014.65.0
Aug83° / 65°3.710.1013.65.8
Sep76° / 58°4.48.8012.37.0
Oct64° / 46°3.98.90.110.95.8
Nov52° / 36°3.28.10.69.73.0
Dec42° / 29°4.09.42.99.10.4

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 35% of the time in September.

Beyond those clear spells, partly cloudy skies cover about 15% of September, with mostly cloudy or overcast conditions the remaining 50%.

0%25%50%75%100%151015202530
Clearer (%)Cloudier (%)
15050
25050
35050
45050
55050
64951
75050
85050
95050
105050
115050
125050
135050
145050
155149
165149
175050
185050
195050
205149
215050
225050
235149
245149
255149
265248
275149
285149
295149
305248

Chance of precipitation in September

Any precipitation

Allentown gets around 4.4 in of rain over 8.8 wet days in September, the wettest month. The line is the day-by-day chance of measurable precipitation.

Day to day, the chance of measurable rain averages around 28% and peaks near 31% on the wettest days.

0%10%20%30%40%151015202530
Rain chance (%)
126
226
326
426
526
626
726
827
927
1027
1127
1227
1327
1427
1527
1628
1728
1829
1929
2030
2130
2231
2331
2431
2531
2631
2731
2831
2931
3031

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 46% of days. Sticky, muggy-or-worse air turns up on roughly 16% of them.

0%25%50%75%100%151015202530
Dry (%)Comfortable (%)Humid (%)Muggy (%)Oppressive (%)
1292421216
2292421206
3302420206
4312420196
5322320186
6342319186
7352319176
8362319166
9372319166
10382319155
11402219145
12412219145
13422119134
14442119134
15452019123
16472019113
17481919113
18501919102
19501919102
2053181892
2153181892
2256171881
2357171781
2458161781
2560161671
2660161671
2763151570
2863151570
2965141460
3066141460

Wind in September

Winds average 6 mph in September.

Across September, daily wind ranges from about 5 mph on the calmest days to 6 mph at its breeziest. It stays fairly steady from day to day.

0246810151015202530
Wind speed (mph)Feels
15calm
25calm
35calm
45calm
55calm
65calm
75calm
85calm
96calm
106calm
116calm
126calm
136calm
146calm
156calm
166calm
176calm
186calm
196calm
206calm
216calm
226calm
236calm
246calm
256calm
266calm
276calm
286calm
296calm
306calm

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 5:33 AM to 6:03 AM, and sunset from 6:30 PM to 5:40 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202530
Daylight (h)Sunrise (AM)Sunset (PM)
112.95:336:30
212.95:346:28
312.95:356:27
412.85:366:25
512.85:376:23
612.75:386:22
712.75:396:20
812.65:406:18
912.65:416:16
1012.55:426:15
1112.55:436:13
1212.45:446:11
1312.45:456:09
1412.45:466:08
1512.35:476:06
1612.35:486:04
1712.25:496:02
1812.25:506:01
1912.15:515:59
2012.15:525:57
2112.05:535:55
2212.05:545:54
2311.95:555:52
2411.95:565:50
2511.95:575:48
2611.85:595:47
2711.86:005:45
2811.76:015:43
2911.76:025:42
3011.66:035:40

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 4.21 kWh/m².

For context, solar energy here swings from a low near 1.76 kWh/m² to a high of 6.02 kWh/m² in June across the year — September's 4.21 kWh/m² falls in the middle.

024151015202530
Solar energy (kWh/m²)Level
14.8Bright
24.7Bright
34.7Bright
44.7Bright
54.6Bright
64.6Bright
74.5Bright
84.5Bright
94.5Bright
104.4Bright
114.4Bright
124.4Bright
134.3Bright
144.3Bright
154.3Bright
164.2Bright
174.2Bright
184.1Bright
194.1Bright
204.1Bright
214.0Bright
224.0Moderate
233.9Moderate
243.9Moderate
253.9Moderate
263.8Moderate
273.8Moderate
283.7Moderate
293.7Moderate
303.7Moderate
Planning a trip in September

Is September a good time to visit Allentown?

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

7.0
Sightseeing
1st-best month of the year
6.0
Beach & pool
4th-best month of the year

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

What to pack for Allentown in September

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

In September, Allentown sees daytime highs around 76°F and nights near 58°F. Worth packing: light clothes and rain gear. About 9 days see rain.

light clothesrain gear

UV index in Allentown 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 Allentown peaks around July 10 at about 8 (very high) — sunscreen, a hat and midday shade are essential. It bottoms out near December 22 at about 1 (low).

The index reaches 3 or more — the level where the WHO recommends sun protection — in 8 months of the year.

025710JanFebMarAprMayJunJulAugSepOctNovDecLowModerateHighVery highHighModerateLowJul 10UV 8Dec 22UV 1
UV index (midday)Level
Jan2Low
Feb3Moderate
Mar5Moderate
Apr6High
May8Very high
Jun8Very high
Jul8Very high
Aug6High
Sep6High
Oct3Moderate
Nov2Moderate
Dec1Low

Air quality in Allentown 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 Allentown is worst in July — around 17 µg/m³ PM2.5 (moderate), about 2.1× the cleanest month (November, 8 µg/m³).

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

01020JanFebMarAprMayJunJulAugSepOctNovDecGoodModerateGoodJul 1417 µg/m³Nov 148 µg/m³
PM2.5 (µg/m³)Level
Jan10Good
Feb12Good
Mar13Moderate
Apr13Moderate
May16Moderate
Jun16Moderate
Jul17Moderate
Aug15Moderate
Sep12Good
Oct9Good
Nov8Good
Dec9Good
Location & terrain

Where is Allentown?

Allentown sits at 40.6°N, 75.5°W, 338 ft above sea level, in United States. The nearest listed city is Whitehall Township, 4.3 mi away.

Topography around Allentown

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

Within 2 miles, the terrain around Allentown has only modest variations in elevation — a maximum elevation change of 285 ft around an average of 344 ft above sea level; within 10 miles, significant variations in elevation — a maximum elevation change of 1,125 ft; within 50 miles, large variations in elevation — a maximum elevation change of 2,516 ft.

The land around Allentown is covered by trees (43%) and artificial surfaces (42%) within 2 miles, trees (47%) and grassland (22%) within 10 miles, and trees (71%) within 50 miles.

2 miles
285 ft
10 miles
1,125 ft
50 miles
2,516 ft
treesgrasslandcroplandartificial surfacesbare terrainwater
September weather nearby: Whitehall Township 4.3 mi · Bethlehem 6.2 mi · Easton 15 mi · Limerick 26 mi · Pottstown 27 mi · Lansdale 27 mi · Reading 30 mi · Phoenixville 33 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 Allentown (40.6084°, -75.4902°, elevation 338 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).