California is a geographically massive state, and its climate is characterized by an immense degree of regional variation. The answer to whether the state has humidity is not a simple yes or no, but rather depends entirely on a person’s location at any given time. This climatic diversity means that the experience of moisture in the air ranges from extremely damp and foggy to exceptionally arid and dry. Understanding this complex environment requires looking at how atmospheric moisture is measured and how the state’s distinct topography controls its distribution.
Understanding Relative Humidity and Dew Point
Relative humidity (RH) represents the amount of water vapor currently in the air as a percentage of the maximum amount it can hold at that specific temperature. Because warm air holds more moisture than cold air, a high RH percentage can be misleading when assessing comfort. For example, air at a cool 50°F might have 100% RH while containing less actual moisture than 50% RH air at 90°F.
Meteorologists prefer to use the dew point, which is the temperature at which the air must be cooled to become fully saturated (100% relative humidity). The dew point is an absolute measure of the actual moisture content, regardless of the current air temperature. Dew points above 65°F generally begin to feel sticky and humid to people, while those below 55°F are perceived as dry and comfortable.
Coastal California: The Marine Influence
The Pacific coastline is dominated by the marine layer, which creates a perpetually humid climate. This layer is a shallow mass of cool, moist air that forms over the cold ocean water and is capped by a temperature inversion. The frequent occurrence of fog and low-lying stratus clouds results from this layer, especially during the late spring and summer months.
The cool, moist air moderates coastal temperatures, creating a naturally air-conditioned climate. For instance, while inland temperatures may soar above 100°F in summer, the average daily high in San Francisco’s coastal areas during July and August often remains between 62 and 68°F. Relative humidity frequently remains above 80% through the night and morning, leading to coastal fog. Sea surface temperatures off the coast, hovering between 52°F and 58°F year-round, ensure the air mass remains consistently cool and near saturation.
Inland California: Valleys and Deserts
A sharp contrast exists in the state’s interior regions, particularly the Central Valley and the deserts of Southern California. In the Central Valley, summer temperatures routinely climb above 100°F, accompanied by very low relative humidity, sometimes dropping below 10%. This environment is the source of the phenomenon often called “dry heat,” where the lack of moisture allows perspiration to evaporate rapidly, providing a cooling effect.
The Mojave and Colorado Deserts experience extreme aridity, with low dew points indicating minimal moisture content. Death Valley, for example, is one of the hottest places on Earth, with summer temperatures exceeding 120°F. The low humidity in these desert regions also contributes to extreme diurnal temperature swings, where the lack of atmospheric moisture allows temperatures to drop significantly after the sun sets.
Geographical Factors Driving Variation
The difference in humidity across the state is controlled by three major geographical and meteorological factors. The cold California Current flows southward along the coast, bringing frigid water. This cold water facilitates upwelling, where deep, cold water rises to the surface, chilling the air above it and creating the persistent marine layer.
A semi-permanent high-pressure system sits over the Pacific Ocean, especially during summer, causing air to sink and warm over the land. This sinking air compresses the moist marine layer near the coast, but inland, it contributes to atmospheric stability and dry conditions. Two major mountain chains, the Coast Ranges and the Sierra Nevada, further divide the state’s climates.
These mountain ranges create a rain shadow effect, blocking moist air movement from the Pacific into the interior. As the air rises over the mountains, it cools and releases its moisture as precipitation on the western, windward slopes. By the time the air descends on the eastern side into the Central Valley and the deserts, it is compressed, warms, and becomes drier, resulting in inland aridity.