San Diego is often thought of as having a consistently dry, desert-like climate. While the area is geographically semi-arid, its direct proximity to the Pacific Ocean introduces specific atmospheric factors that create noticeable, persistent humidity. This high moisture level results from a continuous interaction between the cold ocean water and the air immediately above it. The feeling of mugginess is not due to tropical storms or rainfall but is a daily, localized meteorological phenomenon.
The Role of the Pacific Ocean and Cold Currents
The primary source of San Diego’s coastal moisture is the cold California Current, which flows southward along the West Coast. This cold water movement is enhanced by coastal upwelling, where northwesterly winds push surface water offshore, drawing colder, nutrient-rich water up from the deep ocean. When air passes over this cold ocean surface, it is cooled directly from below, causing the air to reach its saturation point quickly and maintaining a high amount of water vapor near the coast. Coastal San Diego’s dew points remain high because the ocean acts as an endless reservoir, constantly supplying moisture. This results in high relative humidity, often ranging between 65% and 80% along the immediate coastline, which is the foundational element for the unique cloud and fog formations.
The Trapping Effect of the Marine Layer
The humidity created by the cold ocean is concentrated and held in place by the temperature inversion layer. This layer reverses the typical temperature decrease with altitude, placing warmer, lighter air atop cooler, denser air near the surface. This warmer air acts like a stable, atmospheric lid, preventing the cool, moist air mass below it from rising and mixing with drier air above. The air trapped beneath this inversion is called the marine layer, where coastal humidity is held, and as it cools, water vapor condenses, forming low-lying stratus clouds or fog, responsible for overcast mornings like “May Gray” or “June Gloom.” The strength and depth of the inversion layer determines how persistent the marine layer is, ensuring high relative humidity is concentrated at low altitudes, directly impacting coastal areas.
Why Inland Areas Feel Less Humidity
The experience of high humidity changes rapidly when traveling east from the coastline into San Diego’s interior valleys and foothills. This dramatic shift occurs because the atmosphere’s trapping mechanism breaks down and the air mass is heated. The relatively low coastal mountain ranges and hills act as physical barriers, preventing the full penetration of the marine layer beyond a short distance inland. While the marine layer may push a few miles inland in the morning, it typically dissipates much earlier than at the coast, as the sun rises and the inland terrain absorbs solar radiation and heats the air above it, a process called sensible heating. This warming increases the air’s capacity to hold water vapor, which causes the relative humidity to drop significantly, resulting in a distinct microclimate where temperatures can be 10 to 20 degrees Fahrenheit warmer ten miles from the ocean, with humidity levels dropping from the 70% range down to 30% or 50%.