San Marcos weather in September

In September, San Marcos sees average daytime highs of 88°F and overnight lows near 69°F, with 3.0 in of rain across 8.2 wet days and about 12.2 hours of daylight. It is the 4th-warmest and 6th-wettest month of the year here.

Highs ease over the month, from about 91°F in the first days to 86°F by the end. The sky is clear or mostly clear about 50% of the time in September, and the air most often feels oppressive.

Daylight stretches from about 12 h 36 min at the start of September to 11 h 42 min by month's end. Set against the rest of the year, September is San Marcos's 3rd-clearest and 12th-windiest month. For the whole year in context, see San Marcos's year-round climate.

Daytime high
88°
4th warmest
Overnight low
69°
9th coldest
Rainfall
3.0 in
8.2 wet days
Daylight
12 h 12 min
6:21 AM – 6:31 PM
Cloud cover
46%
3rd clearest
Wind
6 mph
12th windiest
Solar energy
5.15 kWh/m²
Travel score
4.7
out of 10

San Marcos is city in and county seat of Hays County, Texas, United States, that extends into Caldwell and Guadalupe counties.

Density
684/km²
people per km²
Founded
1808

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 91°F in the first days to 86°F by the end.

A typical September day in San Marcos reaches about 88°F in the afternoon and slips back to 69°F overnight — a 19°F spread between the day's warmth and the night's chill. On most days the high lands between 81°F and 95°F. Set against its neighbours, September runs about 6°F cooler than August and about 8°F warmer than October.

60°80°100°151015202530
Avg high (°F)Avg low (°F)
19273
29273
39172
49172
59172
69172
79071
89071
99071
108971
118971
128970
138970
148870
158870
168869
178869
188869
198768
208768
218768
228767
238767
248767
258666
268666
278666
288665
298565
308565

How September really feels in San Marcos

Once humidity and wind are folded in, September averages 18 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
18
heat index over the day's high
Days feeling ≤ 32°F
0
wind chill below the day's low

The story in September is heat — humidity pushes the afternoon "feels like" well above the raw temperature.

San Marcos weather by month

How September compares with the rest of the year at a glance. September is the 4th-warmest and 6th-wettest month here — the current month is highlighted below.

MonthHigh / Low (°F)Rainfall (inches)Wet daysDaylight (hours)Travel score
Jan61° / 42°2.57.410.35.4
Feb65° / 46°2.47.610.96.2
Mar71° / 52°3.09.411.86.8
Apr78° / 59°3.3912.76.9
May85° / 66°4.511.413.55.3
Jun90° / 72°3.29.813.93.9
Jul94° / 74°1.86.413.73.2
Aug94° / 74°2.46.413.13.0
Sep88° / 69°3.08.212.24.7
Oct80° / 60°3.27.511.37.1
Nov69° / 50°2.8710.57.0
Dec62° / 43°2.87.910.15.7

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 50% of the time in September.

Beyond those clear spells, partly cloudy skies cover about 18% of September, with mostly cloudy or overcast conditions the remaining 33%. Skies tend to open up as the month goes on.

0%25%50%75%100%151015202530
Clearer (%)Cloudier (%)
16832
26733
36733
46634
56634
66535
76436
86436
96436
106337
116337
126337
136436
146436
156436
166436
176535
186535
196634
206733
216832
226931
237129
247228
257327
267426
277426
287525
297525
307525

Chance of precipitation in September

Any precipitation

San Marcos gets around 3.0 in of rain over 8.2 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 27% and peaks near 31% on the wettest days.

0%10%20%30%40%151015202530
Rain chance (%)
126
227
327
428
528
629
729
830
930
1030
1131
1231
1331
1431
1530
1630
1730
1829
1928
2028
2127
2226
2325
2424
2523
2623
2722
2821
2921
3021

Humidity in September

DryComfortableHumidMuggyOppressiveMiserable

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

By dew point, September lands in the oppressive band about 33% of days. Sticky, muggy-or-worse air turns up on roughly 65% of them.

0%25%50%75%100%151015202530
Dry (%)Comfortable (%)Humid (%)Muggy (%)Oppressive (%)Miserable (%)
1261439381
2361439381
3361438381
4361438371
5471438371
6471438371
7471338371
8571337371
9571337371
10571337381
11671236381
12681235381
13781235380
14791234380
15791234370
168101333370
179101332360
1810111431350
1911111530340
2012121529320
2113121627310
2215121626300
2316131725280
2418131825270
2519131824250
2621141923240
2722141923220
2823141923210
2924142023200
3025142022190

Wind in September

Winds average 6 mph in September, the calmest month.

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

0246810151015202530
Wind speed (mph)Feels
16calm
26calm
36calm
46calm
56calm
66calm
76calm
86calm
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 36 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 54 minutes over September. Sunrise moves from 6:12 AM to 6:29 AM, and sunset from 6:50 PM to 6:13 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202530
Daylight (h)Sunrise (AM)Sunset (PM)
112.66:126:50
212.66:136:49
312.66:146:48
412.56:146:47
512.56:156:46
612.56:156:44
712.56:166:43
812.46:166:42
912.46:176:40
1012.46:176:39
1112.36:186:38
1212.36:196:37
1312.36:196:35
1412.26:206:34
1512.26:206:33
1612.26:216:31
1712.16:216:30
1812.16:226:29
1912.16:226:28
2012.16:236:26
2112.06:246:25
2212.06:246:24
2312.06:256:22
2411.96:256:21
2511.96:266:20
2611.96:266:18
2711.86:276:17
2811.86:286:16
2911.86:286:15
3011.76:296:13

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 5.15 kWh/m².

For context, solar energy here swings from a low near 2.69 kWh/m² to a high of 6.69 kWh/m² in July across the year — September's 5.15 kWh/m² falls in the middle.

0246151015202530
Solar energy (kWh/m²)Level
15.6Bright
25.6Bright
35.5Bright
45.5Bright
55.4Bright
65.4Bright
75.3Bright
85.3Bright
95.3Bright
105.2Bright
115.2Bright
125.1Bright
135.1Bright
145.1Bright
155.1Bright
165.0Bright
175.0Bright
185.0Bright
195.0Bright
205.0Bright
215.0Bright
225.0Bright
235.0Bright
245.0Bright
255.0Bright
265.0Bright
275.0Bright
284.9Bright
294.9Bright
304.9Bright
Planning a trip in September

Is September a good time to visit San Marcos?

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

4.7
Sightseeing
9th-best month of the year
7.0
Beach & pool
2nd-best month of the year

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

What to pack for San Marcos in September

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

In September, San Marcos sees daytime highs around 88°F and nights near 69°F. Pack light clothes. Worth having: rain gear and sun protection. About 8 days see rain.

light clothesrain gearsun protection

UV index in San Marcos in September

LowModerateHighVery highExtreme

Clear-sky midday UV in September peaks around 9 (very high — sun protection essential). The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in San Marcos peaks around August 11 at about 11 (extreme) — sunscreen, a hat and midday shade are essential. It bottoms out near December 20 at about 4 (moderate).

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

025710JanFebMarAprMayJunJulAugSepOctNovDecModerateHighVery highExtremeVery highHighModerateAug 11UV 11Dec 20UV 4
UV index (midday)Level
Jan4Moderate
Feb6High
Mar7High
Apr10Very high
May10Extreme
Jun10Extreme
Jul10Extreme
Aug11Extreme
Sep9Very high
Oct6High
Nov5Moderate
Dec4Moderate

Air quality in San Marcos 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 San Marcos is worst in July — around 17 µg/m³ PM2.5 (moderate), about 2.9× the cleanest month (December, 6 µg/m³).

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

01020JanFebMarAprMayJunJulAugSepOctNovDecGoodModerateGoodJul 1416 µg/m³Dec 136 µg/m³
PM2.5 (µg/m³)Level
Jan6Good
Feb7Good
Mar8Good
Apr9Good
May14Moderate
Jun14Moderate
Jul17Moderate
Aug12Good
Sep10Good
Oct6Good
Nov6Good
Dec6Good
Location & terrain

Where is San Marcos?

San Marcos sits at 29.9°N, 97.9°W, 617 ft above sea level, in United States. The nearest listed city is Kyle, 8.1 mi away.

Topography around San Marcos

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

Within 2 miles, the terrain around San Marcos has only modest variations in elevation — a maximum elevation change of 289 ft around an average of 643 ft above sea level; within 10 miles, only modest variations in elevation — a maximum elevation change of 751 ft; within 50 miles, significant variations in elevation — a maximum elevation change of 1,814 ft.

The land around San Marcos is covered by trees (42%), artificial surfaces (31%) and grassland (23%) within 2 miles, trees (41%) and grassland (39%) within 10 miles, and trees (48%) and grassland (42%) within 50 miles.

2 miles
289 ft
10 miles
751 ft
50 miles
1,814 ft
treesgrasslandcroplandartificial surfacesbare terrainwater
September weather nearby: Kyle 8.1 mi · New Braunfels 17 mi · Canyon Lake 19 mi · Seguin 22 mi · Cibolo 28 mi · Austin 29 mi · Schertz 30 mi · University of Texas 30 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 San Marcos (29.8833°, -97.9414°, elevation 617 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).