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Why can coastal southern California have cool and foggy summers despite its Mediterranean climate?

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Köppen climate classification map — Köppen climate classification The major climatic types are based on patterns of average precipitation, average temperature, and natural vegetation. This map depicts the world distribution of climate types based on the classification originally invented by Wladimir Köppen in 1900.

Coastal southern California experiences cool, foggy summers due to the interaction between atmospheric pressure systems and cold ocean currents, a phenomenon consistent with its Mediterranean climate.

The primary driver is the subtropical high, a region of semipermanent high atmospheric pressure. During the summer, this system shifts poleward, and the subsiding air on its eastern side creates a temperature inversion—a layer of warm air trapping cooler air near the surface. This inversion acts as a barrier to vertical convection, which promotes the formation of fog along the coast.1

This effect is reinforced by the California Current, a cold southward-flowing ocean current. As the marine air layer passes over these cold waters, it is cooled from below, further moderating summer temperatures and contributing to the persistent coastal fog. While the Mediterranean climate is generally characterized by hot, dry summers, this marine influence provides a significant cooling effect for coastal regions compared to inland areas.123