Indiana is located in the Midwest, situated between the Great Lakes and the Ohio River Valley, resulting in a humid continental climate that receives substantial precipitation throughout the year. The state’s average annual precipitation, which includes rain and melted snow, is a significant factor shaping its extensive agricultural landscape and water resources. The long-term statewide average hovers around 40.5 inches of precipitation annually, though the exact figure varies by location.
Determining the Statewide Annual Average
The standard figure used to define a region’s climate is calculated using 30-year climate normals, established by the National Oceanic and Atmospheric Administration (NOAA). For Indiana, the statewide average annual precipitation is approximately 40.5 inches, derived from weather records spanning from 1895 to 2020. This measurement represents the mean amount of moisture the state receives over a climatologically significant period. Recent updates to these normals, such as the 1991–2020 period, show a slight upward trend in total precipitation for many areas. For example, the Indianapolis area saw its total annual precipitation increase by over one inch compared to the previous 1981–2010 period. This measurable increase suggests that the state’s climate is becoming marginally wetter over time.
How Rainfall Differs by Region
While the statewide average is a useful starting point, Indiana’s precipitation is not uniformly distributed, showing a distinct gradient from north to south. The average annual precipitation in Northern Indiana, particularly in the northeastern counties, can be as low as 37 inches per year. Moving southward, the annual totals increase considerably, with Southern Indiana receiving the highest amounts of moisture. Areas near the Ohio River Valley can record averages up to 47 inches of precipitation annually, representing a difference of up to 10 inches compared to the northern counties. This significant difference is mainly attributed to the influence of the Gulf of Mexico, which readily transports warm, moist air northward. The southern portion of the state is also more frequently in the path of larger storm systems and low-pressure tracks that sweep across the Ohio Valley. A localized exception to the north-to-south trend occurs in the counties bordering Lake Michigan, where lake-effect snow events can locally enhance winter precipitation totals.
Seasonal Precipitation Patterns
Precipitation in Indiana follows a clear seasonal pattern, with late spring and early summer typically being the wettest period. May is frequently cited as the month with the highest average rainfall statewide, often registering between 4 and 5 inches of precipitation. In central Indiana, June has also emerged as a contender for the wettest month, averaging 4.9 inches based on recent climate normals. This spring peak is crucial for agriculture as it coincides with the primary growing season, but it also increases the risk of flooding. The precipitation during this period is often delivered through intense thunderstorm activity, resulting from the collision of warm, humid air from the Gulf with cooler continental air masses. Rainfall tends to decrease slightly during the latter part of the summer and into the autumn months. The driest period of the year is usually observed during the winter months, with February often ranking as the month with the lowest average precipitation statewide. Totals during this time commonly fall below 3 inches per month.
Notable Historical Variations
Indiana’s precipitation history is characterized by considerable deviations from the long-term average, highlighted by periods of both severe drought and exceptional wetness. A significant dry period occurred during the summer of 2012, which brought devastating drought conditions to the region. During this event, Indianapolis recorded only 0.09 inches of precipitation in June, making it the driest June on record. Conversely, the state experienced periods of extreme moisture, such as the five-year interval from 2015 to 2019, which averaged 47.2 inches of annual precipitation statewide. This figure significantly exceeded the long-term average of 40.5 inches. Analysis of long-term climate trends indicates a rise in the intensity and frequency of heavy rainfall events. This means a greater percentage of annual moisture is delivered in fewer, more powerful storms, which increases the risk of flash flooding and soil erosion across the state.