Rapid City features a steppe climate , and is part of USDA Hardiness zone 5a. Its location makes its climate unlike both the higher elevations of the Black Hills to the west and the Great Plains to the east. It is characterized by long arid summers and long dry winters, with short but distinct spring and autumn seasons. Precipitation averages 16.3 inches or 414.0 millimetres annually, but has historically ranged from 9.12 inches or 231.6 millimetres in 1974 to 27.70 inches or 703.6 millimetres in 1946.
Winters are cold and dry, with December being the coldest month in recent years, with a daily average temperature of 24.9 °F or −3.9 °C. Chinook winds can warm temperatures above 50 °F or 10 °C, doing so on average about 21 times from December to February. Temperature inversions, however, occasionally produce warmer temperatures in the Black Hills. On average, highs do not climb above freezing on 42 days, while the low temperature reaches 0 °F or −17.8 °C on an average of seventeen nights. Snowfall is frequent but usually not heavy; March and April are typically the snowiest months. The seasonal total averages 41 inches or 1.04 metres, although historically ranging from 16.9 inches or 0.43 metres during 1980–81 to 80.9 inches or 2.05 metres during 1985–86. Extensive snow cover does not remain for long, with only nine days seasonally with 5 inches or 0.13 metres or more on the ground. Measurable snow has occurred in every month except July.
Compared to locations in the east, the area warms rather gradually early in the year, with the last measurable snow typically occurring in late April and precipitation totals beginning to increase; May snow occurs several times per decade. Toward the middle of the year, storms typically develop over the Black Hills during the afternoon and move onto the plains in the evening. Only in April through June have calendar-day precipitation amounts exceeding 3 inches or 76.2 millimetres been observed. June 15, 1963, with 3.78 inches or 96.0 millimetres, holds the single-day rainfall record; the record-wettest month is May 1996 with 8.18 inches or 207.8 millimetres. Rapid City has an average of twenty clear to partly cloudy days and 67 percent of its possible sunshine in June. This is the traditional “flood” season for Rapid and other creeks in the Eastern Hills. Temperatures warm rapidly as summer approaches.
Summer in Rapid City has relatively pleasant temperatures, and is relatively dry, and relatively sunny. July is the warmest month of the year, having a daily average temperature of 72.6 °F (22.6 °C). An average of 34 days reach 90 °F (32.2 °C)+ highs and 5.1 with 100 °F (37.8 °C)+ highs. Due to the elevation and aridity, lows rarely remain at or above 70 °F (21.1 °C) and during July and August fall to or below 50 °F or 10 °C on an average 7.6 days. Rapid City records an average of nine thunderstorm days in August, but only 1.56 inches or 39.6 millimetres of rain in that month.
Fall is a transition season: the average first freeze occurs in Rapid City on October 4 and in the Black Hills in late August through September. The Rapid City area’s first snowfall is usually in October, although higher elevations sometimes receive significant snow in September. Occasional cold fronts moving through the area bring blustery northwest winds.
Sunshine is abundant in the region in all months except December, averaging 2850 hours, 64% of the possible total, per year.
Rapid City holds a record for an extreme temperature drop of 47 °F or 26.1 °C in five minutes on January 10, 1911, from 60 °F or 16 °C to 13 °F or −11 °C. Official extreme temperatures range from −31 °F or −35 °C on February 2, 1996 up to 111 °F or 44 °C on July 15, 2006; the record low daily maximum is −18 °F or −28 °C on February 2, 1989, while the record high daily minimum is 75 °F or 24 °C on July 8, 1985 and July 28, 1960.