Climate and monthly weather forecast
Decatur, IL

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The climate of Decatur

Located in the geographical coordinates of a Humid Continental zone as per the Köppen climate classification, Decatur, Illinois experiences a Dfa predominant climate. This climate classification refers to a humid continental climate with no dry season and hot summers.

The climate and weather in Decatur fluctuate across different months. Temperature highs range from 32.9°F (0.5°C) in January to a peak of 82°F (27.8°C) in July, while the lows oscillate between 21.2°F (-6°C) in January to 65.7°F (18.7°C) in July. Such a pattern suggests considerable temperature variations through the year.

Rainfall distribution extends from 0.87" (22mm) in February to 2.01" (51mm) in May. June and July also experience high rainfall, making the period from May to July the wettest season. In contrast, the least precipitation occurs in November and December. It's also interesting to note that despite a relatively consistent level of relative humidity, ranging from 70% in October to 84% in July, there are no specifically dry months.

Furthermore, Decatur receives sunlight for up to 15 hours a day in June, making it the longest day of the year. This compares to the shortest day in December with just 9.4 sunlight hours. The most sunshine hours occur from June to August, aligning with the summer season when UV index values peak at 6. The number of sunshine days also shows seasonal variation, with less than 15 days of sunshine in May and over 19 days in October and November.

Visibility remains consistent at 6 miles throughout the year, except in July and August where it decreases slightly. Wind speed varies minimally from 6.3mph (10.1km/h) in July and August to 10.7mph (17.2km/h) in January, with the maximum wind gusts observed in January and November at 16.7mph (26.9km/h) and 16.2mph (26.1km/h), respectively.

The best time to visit Decatur

Considering the average weather conditions, the best time to visit Decatur appears to be between late spring and early fall. May to September offers warm temperatures from 61.9°F (16.6°C) to 81.1°F (27.3°C), plentiful sunshine with 8.2 to 10.4 hours of sunlight a day, and moderate rainfall. Particularly, the months of June and July receive the most sunshine hours and maintain the highest UV index of 6, which is ideal for tourists seeking outdoor activities.

The worst time to visit Decatur

On the other hand, the harsh winter months from November through March should be avoided by visitors who are not accustomed to freezing conditions. The temperatures in these months drop to subzero levels with January experiencing the lowest temperature at 21.2°F (-6°C). Furthermore, these months have reduced daylight hours from 9.7 in December to 10.7 in February further dimishing the available daytime for outdoor pursuits.

Spring weather in Decatur

Spring in Decatur spans from March to May and welcomes an increase in average temperatures and rainfall. March starts with average high and low temperatures of 50.5°F (10.3°C) and 35.6°F (2°C), which increase to 72.9°F (22.7°C) and 56.7°F (13.7°C) by May respectively. Temperature variations along with a significant rise in rainfall from 1.3" (33mm) in March to 2.01" (51mm) in May mark the transition from winter to summer.

Summer weather in Decatur

Summer in Decatur extends from June to August and is characterized by the highest temperatures and consistent sunshine. The average temperatures vary from 72.9°F (22.7°C) to a peak of 82°F (27.8°C) in July. These summer months also receive the highest UV index, reaching the maximum of 6, providing ideal conditions for sunbathing and outdoor recreation.

Autumn weather in Decatur

Autumn weather in Decatur, from September to November, is marked by a cooling trend in temperatures and lower rainfall. With average highs dropping from 77°F (25°C) in September to 49.8°F (9.9°C) in November, accompanied by decreasing daylight hours, autumn signals the approach of winter.

Winter weather in Decatur

Winter in Decatur, spanning December through February, experiences the coldest temperatures and shorter days. Average high temperatures fall from 38.5°F (3.6°C) in December to 32.9°F (0.5°C) in January, while lows vary from 21.2°F (-6°C) in January to 27.9°F (-2.3°C) in December. This period also witnesses snowfall, with January observing the highest snowfall count of 1.85" (47mm) spread over 7.2 days.

Weather in January

The wintry weather of January in Decatur, Illinois is notorious for chilly temperatures and snowfall. With the coldest daily average low temperature plunging to 21.2°F (-6°C), and a significant snowfall of 1.85" (47mm), the month's climate demands bundling up and preparing for potential disruptions due to snow cover. Weather in January »

Weather in February

The winter in February, hovering a touch above January's chilling norms, retains Decatur's freezing aura but brings some respite with the temperature rising slightly. The slight dip in snowfall to 1.73" (44mm) makes way for marginally friendlier conditions outdoors. Weather in February »

Weather in March

March in Decatur, Illinois greets the advent of spring with a rewarding rise in temperature. With the highest daily average temperature blossoming to reach 50.5°F (10.3°C), the snowfall amount dwindens to less than 1 inch, signifying the retreat of the winter onslaught. Weather in March »

Weather in April

April in Decatur, Illinois unfurls the spring season in its entirety, with the city's inhabitants finally stowing away the shackles of winter. Infrequent rainfall of 1.93" (49mm) and a growing total of 13.3 daylight hours ensure a pleasant spring, enabling outdoor activities without the weight of winter. Weather in April »

Weather in May

In May, Decatur drops the spring chill entirely and takes the first confident strides into summer ambience, casting warmth in the city's climate. A peak in daylight to 14.4 hours couples well with a maximum temperature of 72.9°F (22.7°C), drawing smiling faces outdoors in casual clothes. Weather in May »

Weather in June

June's summer surge in Decatur, Illinois pushes the city towards sweltering metrics, with inhabitants bracing for the hottest days of the year. An escalation in average temperature to 80.6°F (27°C), combined with a matching UV index of 6, demands appropriate skin protection for those outdoors. Weather in June »

Weather in July

July steers Decatur into its summer apogee, featuring the calendar's maximum temperatures and humidity, with the mercury regularly striking 82°F (27.8°C). Manifesting the peak of summer, this month is synonymous with the buzz of air conditioners, and the glare of sunscreens in daylight. Weather in July »

Weather in August

August sustains Decatur's summer fervor, albeit with nuanced climatic alterations. A minor dip in temperature to 81.1°F (27.3°C) doesn't significantly impact the city's heatwave but signals a slowly encroaching autumn. Balmy winds, playful sunshine, and negligible snowfall maintain the summer's rhythm, swaying with the wildflowers blooming across the cityscape. Weather in August »

Weather in September

Autumnal winds whisper through Decatur in September, transforming summer's vanishing echo into a colourful foliage spectacle. Although the temperature witnessing a drop to 77°F (25°C), the city's weather remains comfortably warm and enjoys the highest visibility all year round. Weather in September »

Weather in October

October in Decatur acquaints the inhabitants with a more evident fall chill, balancing the warmth of afternoon sunshine with cold evenings. The weather is far from freezing temperatures, retaining that last bit of summer warmth. Weather in October »

Weather in November

Delving deeper into autumn, November in Decatur flaunts shorter, colder, and cloudier days, alongside a significant temperature tumble down to 49.8°F (9.9°C). Initial snowfall of 0.24" (6mm) specifies winter's arrival at the doorstep, causing the populace to delve deeper into their wardrobe for warmer clothing. Weather in November »

Weather in December

Quieting down for the year, December enwraps Decatur, Illinois, in winter grandeur, plummeting the temperature to 38.5°F (3.6°C) and snowfall enhancing by 0.83" (21mm). The harvest of pure white snow, biting winds, and frosted windows denote the city's deepest winter chill. Weather in December »
Updated: Feb 12, 2024
Published by: Weather U.S. | About Us
Data Sources | Weather Forecasting & Climate

Frequently asked questions

What is the coldest month in Decatur?

Decatur's coldest weather arrives in January, when temperatures average a high of 32.9°F (0.5°C) and a low of 21.2°F (-6°C).

How many days does it rain in Decatur?

In Decatur, during the entire year, the rain falls for 130.2 days and collects up to 16.34" (415mm) of precipitation.

When does it snow in Decatur?

Months with snowfall in Decatur are January through April, November and December.

When does it snow the most in Decatur?

January is the month with the most snowfall. Snow falls for 7.2 days and accumulates 1.85" (47mm) of snow.

How many days does it snow in Decatur?

Throughout the year, in Decatur, there are 23.1 snowfall days, and 5.67" (144mm) of snow is accumulated.

What is the month with the least sunshine in Decatur?

With an average of 4.1h of sunshine, January has the least sunshine of the year.

What is the month with the lowest UV index in Decatur?

The months with the lowest UV index are January, February, November and December, with an average maximum UV index of 2.

What is the driest month in Decatur?

February and December are months with the least rain in Decatur, Illinois, when the rain falls for 5.8 days and typically aggregates up to 0.87" (22mm) of rain.

When is Daylight Saving Time (DST) in Decatur?

On Sunday, March 10. 2024, at 2:00 am, Daylight Saving Time starts, and the time zone changes from CST to CDT. Daylight Saving Time ends Sunday, November 3. 2024, at 2:00 am, and the time zone reverts from CDT to CST.

What is the rainiest month in Decatur?

May is the month with the most rainfall in Decatur, Illinois. Rain falls for 17 days and accumulates 2.01" (51mm) of precipitation.

When it does not snow in Decatur?

In Decatur, Illinois, snow does not fall in May through October.

When are the longest days in Decatur?

June has the longest days of the year in Decatur, with an average of 15h and 0min of daylight.

What month has the most sunshine in Decatur?

The month with the most sunshine in Decatur is June, with an average of 10.4h of sunshine.

When is the highest UV index in Decatur?

June through August, with an average maximum UV index of 6, are months with the highest UV index.

What are the warmest months in Decatur?

July, noted for its average high-temperature of 82°F (27.8°C) and an average low-temperature of 65.7°F (18.7°C), is considered the warmest month.

What is the most humid month in Decatur?

July is the most humid month, with an average relative humidity of 84%.

What is the least humid month in Decatur?

With an average relative humidity of 70%, October is the least humid month in Decatur.

What is the month with the shortest days in Decatur?

December has the shortest days of the year in Decatur, Illinois, with an average of 9h and 24min of daylight.
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Long-term
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Average temperature
Decatur, IL

Average temperature - Decatur, IL

The warmest month (with the highest average high temperature) is July (82°F).
The month with the lowest average high temperature is January (32.9°F).

The month with the highest average low temperature is July (65.7°F).
The coldest month (with the lowest average low temperature) is January (21.2°F).

Average pressure
Decatur, IL

Average pressure - Decatur, IL

The month with the highest atmospheric pressure is January (30.15"Hg).
The month with the lowest atmospheric pressure is June (29.92"Hg).

Average wind speed
Decatur, IL

Average wind speed - Decatur, IL

The windiest month (with the highest average wind speed) is January (10.7mph).
The calmest months (with the lowest average wind speed) are July and August (6.3mph).

Average humidity
Decatur, IL

Average humidity - Decatur, IL

The month with the highest relative humidity is July (84%).
The month with the lowest relative humidity is October (70%).

Average rainfall
Decatur, IL

Average rainfall - Decatur, IL

The wettest month (with the highest rainfall) is May (2.01").
The driest months (with the least rainfall) are February and December (0.87").

Average rainfall days
Decatur, IL

Average rainfall days - Decatur, IL

The month with the highest number of rainy days is May (17 days).
The month with the least rainy days is February (5.8 days).

Average snowfall
Decatur, IL

Average snowfall - Decatur, IL

The month with the highest snowfall is January (1.85").
The months with the least snowfall are May, June, July, August, September and October (0").

Average snowfall days
Decatur, IL

Average snowfall days - Decatur, IL

The month with the highest number of snowfall days is January (7.2 days).
The months with the least snowfall days are May, June, July, August, September and October (0 days).

Average daylight / Average sunshine
Decatur, IL

Average daylight / Average sunshine - Decatur, IL

The month with the longest days is June (Average daylight: 15h and 0min).
The month with the shortest days is December (Average daylight: 9h and 24min).

The month with the most sunshine is June (Average sunshine: 10h and 24min).
The month with the least sunshine is January (Average sunshine: 4h and 6min).

Average sunshine days
Decatur, IL

Average sunshine days - Decatur, IL

The month with the most sunshine days is October (19.8 days).
The month with the least sunshine days is May (11.5 days).

Average UV index
Decatur, IL

Average UV index - Decatur, IL

The months with the highest UV index are June, July and August (UV index 6).
The months with the lowest UV index are January, February, November and December (UV index 2).

Average cloud cover
Decatur, IL

Average cloud cover - Decatur, IL

The month with the most cloud cover is January (Cloud cover 56).
The month with the least cloud cover is September (Cloud cover 29).

Average visibility
Decatur, IL

Average visibility - Decatur, IL

The months with the highest visibility are January, February, March, April, May, June, September, October, November and December (6mi).
The months with the lowest visibility are July and August (5mi).

Decatur, Illinois, USA

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