Climate Insights for Alton

Sully County, Illinois, United States

Hottest
July
31°C peak
Coldest
January
-4°C low
Wettest
May
128mm rain
Best to Visit
June, September
Mild temps & low rain

Climate Overview

The climate in Alton is temperate. The average annual temperature is 14°C and approximately 1094mm of precipitation falls per year. Alton receives a notable amount of rainfall throughout the year, including during its driest month. Alton is in the Northern Hemisphere. Summer begins at the end of June and lasts through September. The summer months are: July, June, August. The coldest months are January, February, December, when temperatures can drop to -4°C. Light snow can occur between January and December.

Temperature

-10° 10° 20° 30° 40° Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Max: 4° Average: 0° Min: -4° Feb Max: 8° Average: 2° Min: -2° Mar Max: 14° Average: 8° Min: 4° Apr Max: 20° Average: 13° Min: 8° May Max: 25° Average: 19° Min: 15° Jun Max: 30° Average: 25° Min: 20° Jul Max: 31° Average: 26° Min: 22° Aug Max: 29° Average: 24° Min: 20° Sep Max: 28° Average: 22° Min: 16° Oct Max: 21° Average: 15° Min: 10° Nov Max: 13° Average: 7° Min: 3° Dec Max: 8° Average: 3° Min: -1°
Max
Average
Min

Temperatures in Alton vary considerably across the year, with a 35°C difference between the coldest and warmest months. The warmest month is July, when average highs reach 31°C and the mean temperature sits around 26°C. The coldest month is January, when overnight lows fall to -4°C and the daily average is 0°C. On average, daily temperatures fluctuate by about 10°C between overnight lows and afternoon highs. The sharpest temperature change occurs between October and November, with a 7°C shift in average temperatures.

Precipitation

0mm 25mm 50mm 75mm 100mm 125mm 150mm Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Precipitation: 70mm Snow: 17mm Feb Precipitation: 66mm Snow: 15mm Mar Precipitation: 114mm Snow: 3mm Apr Precipitation: 122mm Snow: 1mm May Precipitation: 128mm Snow: 0mm Jun Precipitation: 94mm Snow: 0mm Jul Precipitation: 114mm Snow: 0mm Aug Precipitation: 106mm Snow: 0mm Sep Precipitation: 73mm Snow: 0mm Oct Precipitation: 81mm Snow: 0mm Nov Precipitation: 71mm Snow: 5mm Dec Precipitation: 55mm Snow: 5mm
Precipitation
Snow

Alton is a moderately rainy area, receiving approximately 1094mm of precipitation annually, which averages out to around 91mm per month. The wettest month is May, averaging 128mm of rainfall. April and March also tend to see above-average precipitation. The driest month is December, with just 55mm. That's 73mm less than May, the wettest month. Rainfall is relatively evenly distributed throughout the year, so there is no pronounced dry or wet season to plan around. Light snowfall can occur between January and December, totalling around 46cm annually. January is the snowiest month with approximately 17cm.

Wind

0km/h 5km/h 10km/h 15km/h Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Wind Speed: 13km/h Feb Wind Speed: 13km/h Mar Wind Speed: 14km/h Apr Wind Speed: 14km/h May Wind Speed: 11km/h Jun Wind Speed: 10km/h Jul Wind Speed: 8km/h Aug Wind Speed: 9km/h Sep Wind Speed: 9km/h Oct Wind Speed: 12km/h Nov Wind Speed: 12km/h Dec Wind Speed: 12km/h
Wind Speed

Alton is relatively calm, with an average annual wind speed of 11km/h. For context, sustained winds below 12km/h are generally light, while anything above 25km/h is considered strong. The windiest month is March, averaging 14km/h, though still comfortable for most outdoor activities. The calmest conditions occur in July, with average winds dropping to 8km/h, making it an ideal period for wind-sensitive activities. There is moderate seasonal variation in wind speed, with a 6km/h gap between the calmest and windiest months. Wind tends to be stronger during winter months, driven by seasonal pressure systems. The seasonal averages break down to 13km/h in winter, 13km/h in spring, 9km/h in summer, 11km/h in autumn.

Seasons

Winter
Dec 21
Spring
Mar 20
Summer
Jun 21
Autumn
Sep 23

7 days until Autumn

It is currently summer in Alton, which began on June 21, 2026. Alton is in the Northern Hemisphere, where spring begins in March, summer in June, autumn in September, and winter in December. The warmest period peaks in July at around 31°C, while the coldest stretch bottoms out in January near -4°C. Autumn is next, beginning on September 23, 2026 (7 days from now).

Spring

Temperatures range from 4 to 25°C. Rainfall is heavy, totalling around 365mm across the season with a monthly average of 122mm. Winds are generally light, averaging 13km/h. An umbrella or waterproof jacket is advisable as spring showers are frequent.

Summer

Temperatures range from 20 to 31°C. Rainfall is heavy, totalling around 314mm across the season with a monthly average of 105mm. Winds are generally light, averaging 9km/h. Light, breathable clothing is ideal. Perfect conditions for outdoor activities.

Autumn

Temperatures range from 3 to 28°C. Precipitation is moderate at around 225mm for the season, averaging 75mm per month. Expect around 5cm of snow accumulation during this period. Winds are generally light, averaging 11km/h. A waterproof jacket and sturdy footwear are recommended as rain becomes more frequent.

Winter

Temperatures range from -4 to 8°C. Precipitation is moderate at around 191mm for the season, averaging 64mm per month. Expect around 37cm of snow accumulation during this period. Winds are generally light, averaging 13km/h. Warm winter clothing and waterproof boots are a must, and travel may be affected by snow.

Best Time to Visit

Jan
Feb
Mar
Apr 13°
May 19°
Jun 25°
Jul 26°
Aug 24°
Sep 22°
Oct 15°
Nov
Dec

The best time to visit Alton is during June and September, when the weather is most comfortable for travellers. During these months, average temperatures hover around 23°C with modest rainfall averaging 83mm per month. By contrast, July can be uncomfortably hot with highs reaching 31°C, while January is the coldest period with lows dropping to -4°C. Despite being the best months overall, some rain is still likely. Pack a waterproof layer and plan a mix of indoor and outdoor activities.

Monthly Climate Data

The table below shows monthly climate averages for Alton based on historical weather data. Min and max temperatures represent typical overnight lows and afternoon highs for each month, shown in degrees Celsius (°C). Precipitation is measured in millimetres (mm) and indicates the average total rainfall per month. Snow is measured in centimetres (cm) and reflects average monthly snowfall accumulation. Wind speed is the average sustained speed in kilometres per hour (km/h). All values are based on averages from over 10 years of historical weather data. The current month is highlighted for quick reference.

Month
Min Temperature
Max Temperature
Wind Speed
Precipitation
Snow
Jan
-4 °C
4 °C
13 km/h
70 mm
17 cm
Feb
-2 °C
8 °C
13 km/h
66 mm
15 cm
Mar
4 °C
14 °C
14 km/h
114 mm
3 cm
Apr
8 °C
20 °C
14 km/h
122 mm
1 cm
May
15 °C
25 °C
11 km/h
128 mm
0 cm
Jun
20 °C
30 °C
10 km/h
94 mm
0 cm
Jul
22 °C
31 °C
8 km/h
114 mm
0 cm
Aug
20 °C
29 °C
9 km/h
106 mm
0 cm
Sep Current
16 °C
28 °C
9 km/h
73 mm
0 cm
Oct
10 °C
21 °C
12 km/h
81 mm
0 cm
Nov
3 °C
13 °C
12 km/h
71 mm
5 cm
Dec
-1 °C
8 °C
12 km/h
55 mm
5 cm
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