Home › Cities › United States › Illinois › Alton

Alton, Illinois climate

Cfa Humid subtropical

Alton has a strongly seasonal mild-temperate climate (Humid subtropical, Köppen Cfa). The hottest month is July with an average daily high near 31°C, while the coldest is January averaging around −6°C at night. Around 1063 mm of precipitation falls each year, peaking in May (139 mm) and easing off in February (64 mm).

☀️ Summers

Hot

Highs near 31°C in July.

About 25 days a year above 32 °C

❄️ Winters

Cold

Lows near −6°C in January.

About 75 freezing nights a year

🌧️ Rain

Wet

About 1063 mm of rain a year.

Wettest in May

⛅ Sky & trend

Partly cloudy

A roughly even mix of sun and cloud.

+0.6 °C vs 50 years ago

Temperature & precipitation

Daily high–low temperature range with the monthly average, plus monthly precipitation · 1991–2020 normals · hover a month for the exact figures

Clear vs cloudy days

Share of days each month that are clear, partly cloudy or cloudy · 1991–2020 · hover a month for the day counts

Alton climate normals (1991–2020)

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Avg high (°C)	4	7	12	19	24	29	31	30	27	20	12	6
Avg temp (°C)	−1	1	7	13	19	24	26	25	21	14	7	2
Avg low (°C)	−6	−4	2	7	13	19	21	20	15	9	3	−3
Precipitation (mm)	64	64	88	125	139	86	100	84	86	74	87	66

Jan – Jun	Jan	Feb	Mar	Apr	May	Jun
Avg high (°C)	4	7	12	19	24	29
Avg temp (°C)	−1	1	7	13	19	24
Avg low (°C)	−6	−4	2	7	13	19
Precipitation (mm)	64	64	88	125	139	86

Jul – Dec	Jul	Aug	Sep	Oct	Nov	Dec
Avg high (°C)	31	30	27	20	12	6
Avg temp (°C)	26	25	21	14	7	2
Avg low (°C)	21	20	15	9	3	−3
Precipitation (mm)	100	84	86	74	87	66

How often — frequency of weather events

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Days with measurable rain (days)	6	7	9	9	10	8	7	6	6	6	8	7
Days with high ≥ 32°C (days)	—	—	—	0.0	0.3	5	10	7	3	0.1	—	0.0
Days with low ≤ 0°C (days)	21	18	10	1	—	—	—	—	—	0.1	7	18
Days with snowfall ≥ 2.5 cm (days)	0.4	0.5	0.1	0.0	—	—	—	—	—	—	0.1	0.2

Jan – Jun	Jan	Feb	Mar	Apr	May	Jun
Days with measurable rain (days)	6	7	9	9	10	8
Days with high ≥ 32°C (days)	—	—	—	0.0	0.3	5
Days with low ≤ 0°C (days)	21	18	10	1	—	—
Days with snowfall ≥ 2.5 cm (days)	0.4	0.5	0.1	0.0	—	—

Jul – Dec	Jul	Aug	Sep	Oct	Nov	Dec
Days with measurable rain (days)	7	6	6	6	8	7
Days with high ≥ 32°C (days)	10	7	3	0.1	—	0.0
Days with low ≤ 0°C (days)	—	—	—	0.1	7	18
Days with snowfall ≥ 2.5 cm (days)	—	—	—	—	0.1	0.2

Sky & atmosphere

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Cloud cover (%)	64	67	65	61	62	55	50	48	46	52	59	65
Clear days (≤ 30% cloud)	5	4	6	7	6	8	9	10	12	10	7	6
Partly cloudy days (30–70%)	10	9	9	9	10	11	13	12	9	9	9	9
Cloudy days (≥ 70%)	16	15	16	14	15	11	9	9	9	12	14	16
Relative humidity (%)	84	81	77	75	78	77	74	71	69	71	77	83
Wind speed (mph)	7.6	7.7	7.8	7.7	6.2	5.0	4.4	4.3	4.9	6.1	6.9	7.0

Jan – Jun	Jan	Feb	Mar	Apr	May	Jun
Cloud cover (%)	64	67	65	61	62	55
Clear days (≤ 30% cloud)	5	4	6	7	6	8
Partly cloudy days (30–70%)	10	9	9	9	10	11
Cloudy days (≥ 70%)	16	15	16	14	15	11
Relative humidity (%)	84	81	77	75	78	77
Wind speed (mph)	7.6	7.7	7.8	7.7	6.2	5.0

Jul – Dec	Jul	Aug	Sep	Oct	Nov	Dec
Cloud cover (%)	50	48	46	52	59	65
Clear days (≤ 30% cloud)	9	10	12	10	7	6
Partly cloudy days (30–70%)	13	12	9	9	9	9
Cloudy days (≥ 70%)	9	9	9	12	14	16
Relative humidity (%)	74	71	69	71	77	83
Wind speed (mph)	4.4	4.3	4.9	6.1	6.9	7.0

Methodology & sources

Temperature & precipitation — the official 1991–2020 climate normals from NOAA's U.S. Climate Normals, measured at Alton - Melvin Price L&d (NOAA GHCN station USC00110137), about 4 km from the city centre.

Cloud, humidity, wind & sunshine — modelled estimates from NASA POWER, NASA's satellite-and-reanalysis climatology. This is the standard global source for atmospheric variables, which are not measured at most weather stations.

How we build these numbers →