Climate

Because of its continental mid-latitude location, Indianapolis has a temperate climate with very warm, humid summers and cool winters. Imposed on the daily and seasonal temperature fluctuations are changes occurring every few days as surges of cold continental polar air move southward, or warm tropical air moves northward. These changes are more frequent and pronounced in winter than summer. A winter may be unusually cold, or a summer cool, if the influence of polar air is persistent. Alternatively, a summer may be unusually warm, or a winter mild, if air of tropical origin predominates. The interaction between these two broad classes of air masses, with contrasting temperature, humidity, and density, results in the development of low-pressure systems (cyclones) that generally move eastward over or close to the state, resulting in precipitation. These systems are least active in mid-summer, tending to move north of Indiana.

Climate data has been collected in Indianapolis since April 2, 1871. Even prior to this, some excellent descriptive data is available, as in the antebellum diaries of Calvin Fletcher. The first officially maintained climate instruments were installed at Blackford’s Block, at the southeast corner of Washington and Meridian streets. Measurements were made of wind speed and direction; maximum and minimum temperatures; humidity, using a psychrometer; and precipitation, with both a tipping bucket and 8″ rain gauge. Over the ensuing 70 years, the instruments were moved four times within a three-block area. In terms of interpreting the climate record, the most important move occurred on September 1, 1896, when the station was moved from its second site, the Saks Building at Wellington and Pennsylvania streets, to the Majestic Building, at the northeast corner of Maryland and Pennsylvania. At this time the instruments were taken from street level and placed on top of the building’s roof. On July 1, 1915, the station was moved to the Consolidated Building, 115 North Pennsylvania Street, and back to street level.

In June 1954, the city office was closed and consolidated with measurements at Weir Cook Airport (now Indianapolis International Airport), approximately 7 miles to the southwest of downtown Indianapolis. The site at Weir Cook had opened at the Administration Building on April 1, 1931, and the instruments were moved to their current location at the southwest corner of the Administration Building on September 28, 1977. The specific site location is 3944N, 8616W; station elevation is 792 (WBAN number 93819). Measurements were made of maximum, minimum, and hourly air temperature, humidity, sunshine hours, and precipitation. The station was maintained as a National Weather Service Class 1 forecasting station. Climate records were switched from the city office to the airport in 1942.

When describing the climate of a location it is usual to discuss the “climatic normal,” a 30-year average for the location of interest. In the United States this is computed and reported by the National Oceanic and Atmospheric Administration (NOAA). The climatic normal indicates the conditions to be expected at a given location and provides a reference against which to compare extremes. The extreme values reported include data from other sites in the locality of Indianapolis. None of the data has been corrected for changes in the location of the climate station, or for changes in site conditions at any one of the locations through time. Based on the instrumental record, there appear to be no systematic changes in the climate of Indianapolis over the last century or so.

All of the average values presented here refer to the climatic normals calculated from the Indianapolis International Airport data for the period 1950-1980.

Average annual precipitation for Indianapolis is 39.12″. Precipitation is fairly evenly distributed throughout the year. Each month normally receives at least 3 inches, and the wettest months, in spring-early summer, only an inch or so more (May 3.66″; June 3.99″; and July 4.32″). Therefore, there is no pronounced wet or dry season. Nearly all rainfall is associated with cyclonic disturbances. The greatest number of days of precipitation occurs in February (15 days expected), followed closely by May, June, and December (14 days). April and November are the months with the fewest days of rain. As one would expect from this, cloudiness is greatest in February with only three perfectly clear days expected, and it is sunniest in July with an average of 11 clear days. Autumn is distinctly sunnier than spring. The maximum monthly precipitation on record was 13.12″ reported in July, 1875, and the minimum monthly precipitation was 0.07″ in March, 1910. In terms of rainfall intensity, an event of approximately 2.5″ in a 24-hour period can be expected about once a year. The maximum recorded 24-hour precipitation was 6.80″ in September, 1895, and in the last 40 years the maximum daily total was 5.32″ on July 1, 1987. Mild droughts may occur in the summer when evaporation is highest.

Indianapolis lies beyond the influence of the lake effect snows generated by Lake Michigan that affect the northern part of Indiana. For the city the average snowfall is approximately 20″; however, it is very variable because winter temperatures are commonly near the freezing point. Snowfalls of 3″ or more occur on average two to three times in a winter. The greatest snowfall on record occurred in the winter season 1977-78 with 57.8″; the lowest in the winter of 1982-83. The maximum monthly amount was 30.6″ in January, 1978, and the maximum daily snowfall was 12.5″ on February 26, 1965.

Average daily temperatures are 26.0°F in January and 75.1°F in July. Average daily minimum temperatures are at their lowest in January (17.8°F), and highest in July (64.9°F). The average daily maximum is 34.2°F in January and 85.2°F in July. Annual extremes recorded within the area, not necessarily at the airport site, are: highest temperature 107°F on July 25, 1934; lowest temperature -27°F on January 19, 1994, superseding the earlier record low of -25°F on January 5, 1884. Residents of Indianapolis should expect to have 7 days in July and 4 in August with temperatures greater than 90°F; 21 days in December and 24 in January with minimum temperatures below freezing (32°F); and at least 2 days in December and 4 in January with minimum temperatures below 0°F. The average date of the first frost in autumn is October 20, and in the spring the average last occurrence April 22. Daytime relative humidity is highest on average in the winter (72% in December) because of the lower temperatures in this season, but high values in the summer (early afternoon average 56-61% May to August) exacerbate the high temperatures and result in high heat indexes.

Indianapolis is not a windy location. Surface winds are predominantly southwesterly in the summer, and on average less than 10 mph. Northwesterly winds are more frequent, although not predominant in winter. The windiest month is March with average speeds 11.7 mph; the least windy is August, with average speeds of 7.1 mph. Strong winds are uncommon and tend to be associated with thunderstorms and squalls. The greatest gust recorded in the Indianapolis area registered 111 mph from the northwest in June, 1929. However, it is important to note that during the early period of observations, wind speeds were measured using anemometers mounted on tall towers, well above the surface, and thus higher wind speeds are to be expected. Wind speed is now measured much closer to the ground (20 ft.). A more recent record gust was 90 mph from the west, recorded on January 25, 1950, and at the airport site the maximum gust reached 74 mph on April 6, 1988.

Severe storms that damage property and cause loss of life are most frequent in the spring. Statewide, Indiana has an annual average of 11 days with tornadoes. In Indianapolis thunderstorm frequency is greatest from May to August, on average occurring on 6 or 7 days in each month, approximately 43 days in total per year. There are no major topographic barriers or extensive water bodies within the city (other than small reservoirs, lakes, and rivers); therefore, the major causes of microscale (neighborhood scale) variations in climate are due to the built environment of the city and the associated human activities.

*Note: This entry is from the original print edition of the Encyclopedia of Indianapolis (1994). We are currently seeking an individual with knowledge of this topic to update this entry.