If Southern California has felt unusually muggy lately, especially near San Diego and the coast, there is a good reason. Late summer can bring together several ingredients that raise the amount of moisture in the air: warm coastal ocean water, weaker coastal upwelling, humid marine air, and occasional monsoonal moisture moving in from the south and southeast.

Warm ocean water matters

The Pacific Ocean acts as a major control on coastal Southern California weather. When nearshore water is warmer, evaporation can add more water vapor to the air immediately above the ocean. Onshore winds can then carry that moist marine air inland, making coastal communities feel noticeably more humid.

Why does the ocean get warmer?

One important factor is upwelling. Along the California coast, winds can push surface water away from shore and allow colder water from below to rise toward the surface. When upwelling weakens, that supply of cold water can decrease and coastal surface temperatures can rise.

Ocean currents, changing winds, persistent summer sunshine, and the seasonal warming of the upper ocean also contribute. The result is not usually one single current or weather system; it is the combined state of the coastal ocean and atmosphere.

The marine layer can make it feel even muggier

Southern California’s familiar marine layer is made of cool, moisture-rich air near the ocean surface. When that air moves inland overnight and during the morning, relative humidity near the coast can become very high. Even when skies clear later in the day, the amount of moisture in the air can remain elevated.

Monsoon moisture is another source

During summer and early fall, atmospheric circulation can occasionally transport subtropical moisture into Southern California from Mexico and the surrounding region. This is commonly associated with the North American monsoon. When monsoonal moisture overlaps with already-humid coastal air, Southern California can feel much more tropical than its usual dry reputation suggests.

Humidity vs. dew point

Relative humidity is useful, but dew point is often a better number for understanding how muggy the air actually feels. Relative humidity changes substantially as air temperature changes. Dew point more directly reflects how much moisture is present in the air. A rising dew point near the beach is a good clue that the atmosphere itself has become more moisture-rich.

So where is Southern California’s humidity coming from?

There usually isn’t a single answer. In late summer, think of Southern California humidity as a combination of warm nearshore water + marine air + coastal wind patterns + occasional subtropical moisture. If coastal water temperatures rise while upwelling is weak, the ocean can help reinforce the muggy conditions people feel along the beaches.

That ocean-atmosphere connection is exactly why watching water temperature, wind, waves, and local weather together can tell you much more than looking at the air temperature alone.