Climate zones categorize large regions based on characteristic weather patterns, including temperature, precipitation, and resulting vegetation. For a geographically complex place like Los Angeles, a single classification is often insufficient. Multiple systems are used, tailored to different applications, such as scientific analysis or horticultural planning, highlighting the significant environmental diversity within the metropolitan area.
Los Angeles and the Mediterranean Climate
The primary scientific classification for Los Angeles is the Köppen system’s “Dry-Summer Subtropical” climate, commonly known as a Mediterranean climate. This designation is characterized by mild, wet winters and hot, dry summers. Nearly all precipitation occurs seasonally, between November and April.
Within the Köppen system, Los Angeles is subdivided into two main types: Csa and Csb. Inland valleys, such as the San Fernando Valley, are designated as Csa (Hot-Summer Mediterranean), where the warmest month averages above 71.6°F (22°C). Coastal areas, influenced by the Pacific Ocean, generally fall under Csb (Warm-Summer Mediterranean), where the warmest month averages below 71.6°F (22°C). This division highlights the rapid temperature gradient that occurs within a short distance of the coast.
Practical Gardening Zones (Sunset System)
For California residents and horticulturists, the Sunset Climate Zone system is the most practical guide for gardening. Unlike simpler systems, the Sunset map incorporates a comprehensive set of factors that affect plant growth, going beyond just minimum winter temperatures. This system accounts for:
- Coastal influence
- Intensity of summer heat
- Length of the growing season
- Humidity levels
- Local rainfall patterns
Los Angeles County encompasses a wide range of these zones, typically including Zones 18, 19, 20, 21, and 24. Zone 24 is the mildest, representing the immediate marine influence along the coast, while Zones 18 and 19 are found in the interior valleys, which experience greater temperature extremes and hotter summers. Zone 21, a thermal belt, is known as excellent citrus-growing country because temperatures rarely drop far below freezing. Selecting the correct plant requires considering the complete, year-round fluctuation of temperatures.
Defining Cold Tolerance (USDA System)
The USDA Plant Hardiness Zone Map is a standardized system based solely on the average annual minimum winter temperature. It determines a plant’s ability to survive the coldest period of the year. Populated areas of Los Angeles County typically range from Zone 9b to 11a.
Zone 9b indicates an average annual minimum temperature between 25°F and 30°F, while Zone 11a represents an average minimum between 35°F and 40°F. While this information is useful for selecting perennial plants that can withstand the winter cold, it provides an incomplete picture for Los Angeles gardeners. The USDA system does not factor in the significant summer heat, humidity, or the total duration of the growing season, which are all crucial elements in Southern California’s climate.
The Drivers of Local Climate Variation
The diverse climate classifications in Los Angeles County result directly from its unique topography and proximity to the Pacific Ocean. This geography creates distinct local climate pockets, referred to as microclimates, where conditions change drastically over short distances. The Pacific Ocean exerts a powerful moderating effect through the marine layer, keeping coastal temperatures cooler and more stable year-round.
Moving inland, the Transverse Ranges, an east-west oriented mountain chain, act as a barrier to this cool, moist air. This barrier blocks the marine influence and prevents cold air masses from the continent’s interior from reaching the coastal basin, creating a rain shadow effect. Inland valleys, such as the San Fernando and San Gabriel Valleys, are subject to greater temperature extremes, with hotter summers and colder winter nights compared to the immediate coast. Rapid changes in elevation also contribute to variation, as higher altitudes experience quicker cooling and occasionally receive snow.