St. Louis, located near the confluence of the Mississippi and Missouri Rivers, has a geographical position that significantly shapes its climate and distinct seasonal patterns. This article explores St. Louis’s climate classification and the features that define its weather.
St. Louis’s Climate Classification
St. Louis is primarily categorized under the Köppen climate classification system as a hot-summer humid continental climate, denoted as Dfa. This classification indicates that the region experiences significant temperature variations across its seasons. The “D” signifies a continental climate (coldest month below 0°C/32°F). The “f” indicates a fully humid climate with precipitation distributed throughout the year. The “a” denotes a hot summer, characterized by at least one month above 22°C (71.6°F) and at least four months averaging above 10°C (50°F). While the urban core is sometimes classified as humid subtropical (Cfa) due to urban heat island effects, the broader metropolitan area consistently aligns with the Dfa designation.
Defining Features of St. Louis’s Climate
The Dfa classification translates into observable weather patterns throughout the year in St. Louis. Summers are notably hot and humid, with average high temperatures in July reaching around 88.5°F to 89°F. Humidity levels can peak around 67-68% during July, making the air feel muggy. The city typically experiences about 35 to 40 days annually where temperatures climb above 90°F. These warm, moist conditions frequently give rise to thunderstorms, particularly in the spring and summer months, which can sometimes be severe.
Winters are cold, often bringing significant snowfall. January is the coldest month, with average low temperatures ranging from 22.8°F to 25°F and average highs between 36.7°F and 41°F. The region typically receives between 13 to 18 inches of snow per winter. Temperatures can drop to 0°F or below on average two to three days each year. Spring and autumn are transitional periods, marked by fluctuating temperatures. Precipitation, averaging 41-43 inches annually, is well-distributed, though spring (April and May) tends to be the wettest.
Regional Influences and Gardening Zones
St. Louis’s continental location, far from ocean influence, contributes to significant temperature swings. The absence of major mountain ranges to the north or west allows unimpeded movement of air masses across the Great Plains. This enables cold arctic air from Canada to penetrate in winter, and warm, moist air from the Gulf of Mexico to stream northward in summer. The interaction of these contrasting air masses creates the wide variety of weather conditions observed throughout the year.
For gardeners, the climate directly impacts plant selection, guided by the USDA Plant Hardiness Zone system. This system categorizes regions based on their average annual extreme minimum winter temperature. The 2023 USDA update places St. Louis primarily within Zone 7a (0°F to 5°F), an update from the previous Zone 6b (-5°F to 0°F). This updated classification provides guidance for selecting appropriate trees, shrubs, and perennial plants that can thrive in local winter conditions.