Climate zones are areas that share similar long-term weather patterns, including temperature, humidity, and precipitation. The total number of climate zones in the United States depends entirely on the criteria used to define them. Different classification systems focus on specific climatic factors to serve various purposes, resulting in vastly different numbers of zones. For instance, a practical classification provides detailed zones for agriculture, while a scientific approach groups areas into a smaller set of broad climate types.
The Most Common Classification System
The most familiar classification is the United States Department of Agriculture (USDA) Plant Hardiness Zone Map. This system is primarily a tool for gardeners and growers, informing them which plants are likely to survive the winter. The map’s classification is based on one specific metric: the average annual extreme minimum winter temperature at a given location.
The current USDA map divides the country and its territories into 13 major zones, each representing a 10-degree Fahrenheit range of the coldest winter temperature. Each zone is further subdivided into a half-zone designated with an “a” or a “b,” narrowing the range to a 5-degree interval. This detailed classification aids in predicting plant survival, as the minimum temperature often determines whether a plant can live through a specific winter.
Broader Scientific Climate Types
A different approach is used by climatologists through the Köppen classification system. This method focuses on both temperature and precipitation patterns throughout the year to categorize climates based on the type of vegetation they can naturally support. The Köppen system groups the world’s climates into five major categories: Tropical (A), Dry (B), Temperate (C), Continental (D), and Polar (E).
Due to the size and geographic diversity of the United States, all five primary climate groups are represented within its borders. For example, Tropical climates are found in Hawaii, Puerto Rico, and parts of South Florida. The Great Plains and the arid Southwest fall under the Dry climate group, while the vast interior is dominated by Continental climates, which feature significant seasonal temperature extremes.
The system uses a combination of two or three letters to create highly specific sub-classifications, reflecting seasonal precipitation and temperature variations. For instance, a Temperate climate (C) might be further classified as a Mediterranean climate (Cs) or a Humid Subtropical climate (Cfa). Considering these numerous sub-classifications, the United States contains dozens of distinct climate types, even though they all fall under the five major groups.
How Zone Boundaries Are Determined
Regardless of the classification system used, the physical boundaries of climate zones are shaped by geographical and atmospheric factors. The most fundamental factor is latitude, which dictates the angle of the sun’s rays and the amount of solar energy received, leading to colder zones further from the equator. This simple latitudinal pattern is heavily modified by other local and regional influences.
Elevation plays a significant role, as temperatures decrease with increasing altitude, creating cooler zones on mountain peaks. Large bodies of water, such as the Pacific and Atlantic Oceans, moderate temperature swings, leading to milder maritime climates along the coasts. Inland areas, far from the ocean’s influence, experience continental climates with wider seasonal temperature variations. Major mountain ranges, like the Cascades and the Rockies, create distinct climate zones by blocking prevailing winds and moisture, producing arid rain shadows on their leeward sides.