Alton, Illinois climate: Normal weather by month, frost dates and records
Alton's normal year runs from 22°F January nights to 87°F July afternoons, with 41.9 inches of precipitation a year. (NOAA 1991-2020 normals)
In metric: a yearly mean of 13.2°C, January lows of −6°C, July highs of 31°C, 1,063 mm of precipitation a year.
- NOAA stationAlton - Melvin Price Lock and Dam
- Station IDUSC00110137
- Location38.87°N 90.15°W
- Elevation405 ft (123 m)
- Population24,967
On October 14, the normal high in Alton is 69°F and the normal low 49°F, and the chance of measurable rain or snow is 24%.
Normals describe a typical year at the station, not a forecast. The National Weather Service forecast for Alton.
- Jan 39° 22°
- Feb 44° 25°
- Mar 54° 35°
- Apr 66° 45°
- May 75° 56°
- Jun 84° 66°
- Jul 87° 69°
- Aug 86° 67°
- Sep 80° 60°
- Oct 68° 48°
- Nov 54° 37°
- Dec 44° 27°
Each month: normal high (large) and low, °F. Fill: the month's normal mean, colder blue to hotter red.
What is the weather like in Alton each month?
July is the warmest month, with normal highs of 87°F; January the coldest, with normal lows of 22°F. The most precipitation falls in May (5.46 in), the least in February (2.52 in).
| Month | High | Low | Mean | Precip. | Wet days | 90°F days | Freezing nights | Daylight |
|---|---|---|---|---|---|---|---|---|
| January | 39°F | 22°F | 30.3°F | 2.53 | 7 | <1 | 24 | 9 h 40 min |
| February | 44°F | 25°F | 34.5°F | 2.52 | 8 | <1 | 20 | 10 h 40 min |
| March | 54°F | 35°F | 44.1°F | 3.46 | 10 | <1 | 11 | 11 h 50 min |
| April | 66°F | 45°F | 55.6°F | 4.92 | 10 | <1 | 2 | 13 h 10 min |
| May | 75°F | 56°F | 65.6°F | 5.46 | 11 | <1 | <1 | 14 h 20 min |
| June | 84°F | 66°F | 74.6°F | 3.40 | 8 | 6 | <1 | 14 h 50 min |
| July | 87°F | 69°F | 78.3°F | 3.95 | 7 | 11 | <1 | 14 h 40 min |
| August | 86°F | 67°F | 76.6°F | 3.30 | 7 | 8 | <1 | 13 h 40 min |
| September | 80°F | 60°F | 70.0°F | 3.38 | 7 | 3 | <1 | 12 h 30 min |
| October | 68°F | 48°F | 57.9°F | 2.93 | 8 | <1 | <1 | 11 h 20 min |
| November | 54°F | 37°F | 45.4°F | 3.43 | 8 | <1 | 8 | 10 h 10 min |
| December | 44°F | 27°F | 35.3°F | 2.59 | 8 | <1 | 20 | 9 h 30 min |
| Year (annual) | 65°F | 46°F | 55.7°F | 41.87 | 99 | 29 | 85 |
The same normals in °C and millimetres
| Month | High | Low | Mean | Precip. |
|---|---|---|---|---|
| January | 4°C | −6°C | −0.9°C | 64 mm |
| February | 7°C | −4°C | 1.4°C | 64 mm |
| March | 12°C | 2°C | 6.7°C | 88 mm |
| April | 19°C | 7°C | 13.1°C | 125 mm |
| May | 24°C | 13°C | 18.7°C | 139 mm |
| June | 29°C | 19°C | 23.7°C | 86 mm |
| July | 31°C | 21°C | 25.7°C | 100 mm |
| August | 30°C | 20°C | 24.8°C | 84 mm |
| September | 27°C | 15°C | 21.1°C | 86 mm |
| October | 20°C | 9°C | 14.4°C | 74 mm |
| November | 12°C | 3°C | 7.4°C | 87 mm |
| December | 6°C | −3°C | 1.8°C | 66 mm |
NOAA publishes no 1991-2020 snowfall normal for this station. The nearest record that keeps one, St. Louis (14 miles away), gets 17 inches of snow in a normal year; elevation and distance can change that.
Source: NOAA NCEI 1991-2020 monthly normals, station USC00110137; annual normals. Wet days: 0.01 inch or more; 90°F days and freezing nights: NOAA's day counts. Daylight: sunrise to sunset on the 15th, computed from the latitude.
When is the best weather to visit Alton?
Normal highs sit between 65°F and 85°F in April, May, June, September and October. Of those, September has the fewest days with rain (7).
That is a measurement, not a recommendation: it counts the months whose normal afternoon high sits between 65°F and 85°F, where a light layer is enough by day.
Source: NOAA NCEI 1991-2020 monthly normals, station USC00110137.
When is the last frost in Alton?
In Alton, the last spring frost (32°F) comes after April 4 in half of years and after April 19 in one year in ten; the first autumn frost comes before November 5 in half of years and before October 25 in one year in ten.
Frost-free in half of yearsFrost in 1 to 9 years out of 10Frost most years
- Last spring frost, half of years after
- April 41 year in 10 after April 19; 9 in 10 after March 18
- First autumn frost, half of years before
- November 51 year in 10 before October 25; 9 in 10 before November 21
NOAA puts the median frost-free season at 214 days, and the season without a hard freeze (28°F) at 237 days.
A hard freeze (28°F) comes after March 25 in half of springs and before November 16 in half of autumns.
The USDA's 2023 Plant Hardiness Zone Map puts Alton in zone 7a across its 2 ZIP code areas (7a in 62002, the largest).
Frost dates for every NOAA station in Illinois · Frost dates by ZIP code
Source: NOAA NCEI 1991-2020 annual normals (frost probabilities), station USC00110137; USDA 2023 Plant Hardiness Zone Map, ZIP listings: PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed October 8, 2026. A 10% date means one year in ten has a frost after it in spring or before it in autumn.
What hardiness zone is each Alton ZIP code in?
The USDA's 2023 map gives each ZIP code area one zone. Alton's 2 ZIP code areas are all in zone 7a. Each ZIP's frost dates are in the frost tool.
Source: USDA 2023 Plant Hardiness Zone Map, ZIP listings: PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed October 8, 2026; ZIP areas in the place: Census Bureau 2020 ZCTA-to-place relationship file. A ZIP area counts when at least a tenth of its land (and 0.5 km²) lies in the place.
How hot and how cold does it get in Alton?
In a normal year in Alton, 29 days a year reach 90°F (1 of them 100°F or more), 85 nights fall to 32°F or colder and 99 days bring measurable precipitation.
- Days at 90°F or more
- 29a normal year
- Days at 100°F or more
- 1a normal year
- Nights at 32°F or colder
- 85a normal year
- Days that stay below freezing
- 21high of 32°F or less
- Days with 0.01 inch of precipitation
- 99a normal year
The highest temperature in this station's record (since 1943) is 116°F, on July 21, 1946; the lowest is −16°F, on December 26, 1983.
The wettest calendar year on that record is 2004, with 59.03 in; the snowiest season is 1961–62, with 38.4 inches.
Source: NOAA NCEI 1991-2020 annual normals, station USC00110137; records: ACIS (NOAA Regional Climate Centers), station USC00110137. Records use complete months only; a month missing more than three days is left out.
How sunny is Alton?
Over a year 75% of the sunlight that would reach the ground under clear skies gets through the clouds in the area around Alton, 4.2 kWh per square metre a day. July is the sunniest month (82%) and December the cloudiest (66%).
| Month | Share of clear-sky sunshine | Sunshine, kWh/m² a day |
|---|---|---|
| January | 67% | 2.1 |
| February | 66% | 2.8 |
| March | 67% | 3.8 |
| April | 73% | 5.0 |
| May | 74% | 5.6 |
| June | 81% | 6.4 |
| July | 82% | 6.3 |
| August | 81% | 5.6 |
| September | 81% | 4.8 |
| October | 76% | 3.4 |
| November | 72% | 2.4 |
| December | 66% | 1.8 |
Source: NASA POWER solar climatology, 2001-2020 (CERES SYN1deg). Satellite-based, for the 1-degree cell (about 100 km across) around the station, so it describes the area rather than the airport.
How much heating and cooling does Alton need?
A normal year in Alton brings 4,780 heating degree days, 11% fewer than the median U.S. place (5,373), and 1,424 cooling degree days, 57% more than the median (908). Six months need 300 or more heating degree days, a monthly mean of about 55°F or colder. For gardens and crops, NOAA counts 4,168 growing degree days a year above 50°F.
| Month | Heating | Cooling |
|---|---|---|
| January | 1,074 | 0 |
| February | 855 | 0 |
| March | 651 | 3 |
| April | 303 | 22 |
| May | 91 | 108 |
| June | 8 | 297 |
| July | 0 | 412 |
| August | 1 | 362 |
| September | 35 | 184 |
| October | 252 | 34 |
| November | 590 | 2 |
| December | 921 | 0 |
| Year (annual) | 4,780 | 1,424 |
Source: NOAA NCEI 1991-2020 monthly normals, station USC00110137. A degree day is one degree of a day's mean temperature below 65°F (heating) or above it (cooling); utilities use them to estimate energy use.
How does Alton's climate compare?
Alton is warmer than 127 of the other 143 places with their own NOAA record in Illinois, and wetter than 100. Across the 7,247 U.S. stations with normals, its annual mean of 55.7°F is warmer than 63%.
In the Köppen classification computed from these normals, Alton is Dfa: continental, no dry season, hot summer. What the classes mean.
Places with the most similar year elsewhere
Measured month by month on the normal high, low and precipitation, among places of 20,000 or more people in other states:
Source: NOAA NCEI 1991-2020 normals for every station PlainClimate covers. Similarity: the root-mean-square difference of the 36 monthly figures, precipitation weighted so an inch counts like 6°F.
Which places near Alton share this climate record?
28 places have no NOAA station with 1991-2020 normals of their own, and this station is the nearest full record. A place's own climate can differ from these figures, by several degrees where it sits much higher or lower than the station, nearer the coast, or inside a large city's heat; for the larger places the next nearest records are named with their yearly mean, so the spread around them is visible, and places of 50,000 or more have their own page with NOAA's gridded normals.
- Florissant, MOalso near: St. Louis (4 mi, 57.4°F a year); St. Charles (10 mi, 56.1°F a year)51,290 people · 11 mi SW
- Old Jamestown, MO21,121 people · 8 mi W
- Spanish Lake, MO18,183 people · 6 mi SW
- Ferguson, MO18,043 people · 11 mi SW
- Bellefontaine Neighbors, MO10,401 people · 9 mi SW
- Wood River, IL10,251 people · 4 mi E
- Bethalto, IL9,183 people · 6 mi NE
- Black Jack, MO6,575 people · 8 mi SW
- East Alton, IL5,621 people · 2 mi NE
- Glasgow Village, MO5,491 people · 8 mi S
- Dellwood, MO4,833 people · 10 mi SW
- Rosewood Heights, IL4,047 people · 4 mi E
- Holiday Shores, IL3,162 people · 12 mi E
- Castle Point, MO2,792 people · 9 mi SW
- Riverview, MO2,334 people · 9 mi S
- Moline Acres, MO2,102 people · 10 mi SW
- South Roxana, IL1,849 people · 6 mi SE
- Bunker Hill, IL1,591 people · 16 mi NE
- Roxana, IL1,408 people · 6 mi SE
- Hartford, IL1,152 people · 5 mi SE
- Calverton Park, MO1,134 people · 11 mi SW
- Worden, IL1,070 people · 17 mi E
- Hamel, IL926 people · 17 mi E
- Alhambra, IL602 people · 22 mi E
- West Alton, MO353 people · 3 mi W
- Moro, IL337 people · 8 mi NE
- Portage Des Sioux, MO323 people · 11 mi NW
- Prairietown, IL189 people · 14 mi NE
Source: Census Bureau 2024 Gazetteer (place centers) and Vintage 2024 population estimates. Distance and direction from the station to each place's center.
Questions about Alton's climate
What is the coldest month in Alton?
January, with a normal high of 39°F and a normal low of 22°F. The warmest is July (87°F by day, 69°F at night).
Does it snow in Alton?
Yes. NOAA publishes no 1991-2020 snowfall normal for Alton's NOAA station; the nearest record that keeps one, St. Louis (14 miles away), records 17 inches of snow in a normal year.
What is the rainiest month in Alton?
May, with 5.46 in of precipitation in a normal year; the driest is February with 2.52 in.
What plant hardiness zone is Alton in?
The USDA's 2023 map puts Alton in zone 7a across its 2 ZIP code areas (7a in 62002, the largest).
How late can frost come in Alton?
In half of years the last 32°F night comes after April 4; in one year in ten it comes after April 19. Waiting until April 19 leaves about a one-in-ten chance of a later frost.
Nearby and next
- Illinois climateThe statewide normals and every place in the state
- St. Louis, MO14 miles from this station, 57.4°F a year
- St. Charles, MO16 miles from this station, 56.1°F a year
- St Louis Science Center, MO17 miles from this station, 58.0°F a year
- Jerseyville, IL19 miles from this station, 53.4°F a year
- Compare two placesMonth by month, side by side
Download these normals (CSV) · JSON · How these figures are made