Climate and Average Weather Year Round in Alton United States

In Alton, the summers are hot and muggy; the winters are very cold, snowy, and windy; and it is partly cloudy year round. Over the course of the year, the temperature typically varies from 24°F to 88°F and is rarely below 8°F or above 96°F.

Based on the tourism score, the best times of year to visit Alton for warm-weather activities are from late May to mid July and from mid July to late September.

The hot season lasts for 3.8 months, from May 24 to September 19, with an average daily high temperature above 79°F. The hottest month of the year in Alton is July, with an average high of 88°F and low of 70°F.

The cold season lasts for 3.0 months, from November 28 to February 27, with an average daily high temperature below 49°F. The coldest month of the year in Alton is January, with an average low of 25°F and high of 40°F.

In Alton, the average percentage of the sky covered by clouds experiences significant seasonal variation over the course of the year.

The clearer part of the year in Alton begins around June 13 and lasts for 4.4 months, ending around October 26.

The clearest month of the year in Alton is August, during which on average the sky is clear, mostly clear, or partly cloudy 69% of the time.

The cloudier part of the year begins around October 26 and lasts for 7.6 months, ending around June 13.

The cloudiest month of the year in Alton is February, during which on average the sky is overcast or mostly cloudy 53% of the time.

A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. The chance of wet days in Alton varies throughout the year.

The wetter season lasts 5.6 months, from March 22 to September 10, with a greater than 28% chance of a given day being a wet day. The month with the most wet days in Alton is May, with an average of 12.1 days with at least 0.04 inches of precipitation.

The drier season lasts 6.4 months, from September 10 to March 22. The month with the fewest wet days in Alton is January, with an average of 5.2 days with at least 0.04 inches of precipitation.

Among wet days, we distinguish between those that experience rain alone, snow alone, or a mixture of the two. The month with the most days of rain alone in Alton is May, with an average of 12.1 days. Based on this categorization, the most common form of precipitation throughout the year is rain alone, with a peak probability of 40% on May 23.

Rainfall

To show variation within the months and not just the monthly totals, we show the rainfall accumulated over a sliding 31-day period centered around each day of the year. Alton experiences significant seasonal variation in monthly rainfall.

Rain falls throughout the year in Alton. The month with the most rain in Alton is May, with an average rainfall of 4.0 inches.

The month with the least rain in Alton is January, with an average rainfall of 1.5 inches.

Snowfall

As with rainfall, we consider the snowfall accumulated over a sliding 31-day period centered around each day of the year. Alton experiences some seasonal variation in monthly snowfall.

The snowy period of the year lasts for 3.6 months, from November 27 to March 14, with a sliding 31-day snowfall of at least 1.0 inches. The month with the most snow in Alton is January, with an average snowfall of 3.3 inches.

The snowless period of the year lasts for 8.4 months, from March 14 to November 27. The least snow falls around July 23, with an average total accumulation of 0.0 inches.

The length of the day in Alton varies significantly over the course of the year. In 2024, the shortest day is December 21, with 9 hours, 26 minutes of daylight; the longest day is June 20, with 14 hours, 54 minutes of daylight.

The earliest sunrise is at 5:34 AM on June 13, and the latest sunrise is 1 hour, 55 minutes later at 7:29 AM on November 2. The earliest sunset is at 4:38 PM on December 6, and the latest sunset is 3 hours, 52 minutes later at 8:30 PM on June 27.

Daylight saving time (DST) is observed in Alton during 2024, starting in the spring on March 10, lasting 7.8 months, and ending in the fall on November 3.

The figure below presents a compact representation of key lunar data for 2024. The horizontal axis is the day, the vertical axis is the hour of the day, and the colored areas indicate when the moon is above the horizon. The vertical gray bars (new Moons) and blue bars (full Moons) indicate key Moon phases.

We base the humidity comfort level on the dew point, as it determines whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid. Unlike temperature, which typically varies significantly between night and day, dew point tends to change more slowly, so while the temperature may drop at night, a muggy day is typically followed by a muggy night.

Alton experiences extreme seasonal variation in the perceived humidity.

The muggier period of the year lasts for 4.2 months, from May 18 to September 23, during which time the comfort level is muggy, oppressive, or miserable at least 18% of the time. The month with the most muggy days in Alton is July, with 21.2 days that are muggy or worse.

The least muggy day of the year is January 31, when muggy conditions are essentially unheard of.

This section discusses the wide-area hourly average wind vector (speed and direction) at 10 meters above the ground. The wind experienced at any given location is highly dependent on local topography and other factors, and instantaneous wind speed and direction vary more widely than hourly averages.

The average hourly wind speed in Alton experiences significant seasonal variation over the course of the year.

The windier part of the year lasts for 7.2 months, from October 11 to May 19, with average wind speeds of more than 8.5 miles per hour. The windiest month of the year in Alton is March, with an average hourly wind speed of 10.7 miles per hour.

The calmer time of year lasts for 4.8 months, from May 19 to October 11. The calmest month of the year in Alton is August, with an average hourly wind speed of 6.4 miles per hour.

The predominant average hourly wind direction in Alton varies throughout the year.

The wind is most often from the south for 1.7 weeks, from February 21 to March 4 and for 9.2 months, from March 11 to December 17, with a peak percentage of 42% on June 20. The wind is most often from the north for 1.0 weeks, from March 4 to March 11, with a peak percentage of 29% on March 7. The wind is most often from the west for 2.1 months, from December 17 to February 21, with a peak percentage of 34% on January 1.

To characterize how pleasant the weather is in Alton throughout the year, we compute two travel scores.

The tourism score favors clear, rainless days with perceived temperatures between 65°F and 80°F. Based on this score, the best times of year to visit Alton for general outdoor tourist activities are from late May to mid July and from mid July to late September, with a peak score in the first week of September.

The beach/pool score favors clear, rainless days with perceived temperatures between 75°F and 90°F. Based on this score, the best time of year to visit Alton for hot-weather activities is from mid June to late August, with a peak score in the last week of July.

Methodology

For each hour between 8:00 AM and 9:00 PM of each day in the analysis period (1980 to 2016), independent scores are computed for perceived temperature, cloud cover, and total precipitation. Those scores are combined into a single hourly composite score, which is then aggregated into days, averaged over all the years in the analysis period, and smoothed.

Our cloud cover score is 10 for fully clear skies, falling linearly to 9 for mostly clear skies, and to 1 for fully overcast skies.

Our precipitation score, which is based on the three-hour precipitation centered on the hour in question, is 10 for no precipitation, falling linearly to 9 for trace precipitation, and to 0 for 0.04 inches of precipitation or more.

Our tourism temperature score is 0 for perceived temperatures below 50°F, rising linearly to 9 for 65°F, to 10 for 75°F, falling linearly to 9 for 80°F, and to 1 for 90°F or hotter.

Our beach/pool temperature score is 0 for perceived temperatures below 65°F, rising linearly to 9 for 75°F, to 10 for 82°F, falling linearly to 9 for 90°F, and to 1 for 100°F or hotter.

Definitions of the growing season vary throughout the world, but for the purposes of this report, we define it as the longest continuous period of non-freezing temperatures (≥ 32°F) in the year (the calendar year in the Northern Hemisphere, or from July 1 until June 30 in the Southern Hemisphere).

The growing season in Alton typically lasts for 7.1 months (215 days), from around March 31 to around November 1, rarely starting before March 13 or after April 18, and rarely ending before October 15 or after November 18.

Growing degree days are a measure of yearly heat accumulation used to predict plant and animal development, and defined as the integral of warmth above a base temperature, discarding any excess above a maximum temperature. In this report, we use a base of 50°F and a cap of 86°F.

Based on growing degree days alone, the first spring blooms in Alton should appear around March 18, only rarely appearing before March 5 or after April 5.

This section discusses the total daily incident shortwave solar energy reaching the surface of the ground over a wide area, taking full account of seasonal variations in the length of the day, the elevation of the Sun above the horizon, and absorption by clouds and other atmospheric constituents. Shortwave radiation includes visible light and ultraviolet radiation.

The average daily incident shortwave solar energy experiences significant seasonal variation over the course of the year.

The brighter period of the year lasts for 4.1 months, from April 25 to August 28, with an average daily incident shortwave energy per square meter above 5.9 kWh. The brightest month of the year in Alton is July, with an average of 6.7 kWh.

The darker period of the year lasts for 3.1 months, from November 5 to February 9, with an average daily incident shortwave energy per square meter below 3.0 kWh. The darkest month of the year in Alton is December, with an average of 2.0 kWh.

For the purposes of this report, the geographical coordinates of Alton are 38.891 deg latitude, -90.184 deg longitude, and 505 ft elevation.

The topography within 2 miles of Alton contains only modest variations in elevation, with a maximum elevation change of 243 feet and an average elevation above sea level of 474 feet. Within 10 miles also contains only modest variations in elevation (486 feet). Within 50 miles contains only modest variations in elevation (728 feet).

The area within 2 miles of Alton is covered by artificial surfaces (56%) and water (35%), within 10 miles by cropland (42%) and artificial surfaces (26%), and within 50 miles by cropland (61%) and trees (24%).