June Gloom is a seasonal meteorological phenomenon common to the coastal regions of Southern California, characterized by persistent low-hanging clouds and overcast skies during late spring and early summer. Locals often refer to this pattern using nicknames like “May Gray” or “No-Sky July” when it extends beyond June. The term describes the atmospheric conditions when the Pacific Ocean’s cool air mass, known as the marine layer, interacts with warmer air over the land, creating predictable annual cloud cover.
What June Gloom Looks and Feels Like
June Gloom is defined by a blanket of low, gray stratus clouds that cover the coastal plain. These clouds obscure the sun, creating a diffused light that makes the morning feel overcast and dim. The phenomenon is often accompanied by dampness, mist, drizzle, or fog, which can reduce visibility near the shoreline.
Temperatures are notably cooler under this cloud layer, with coastal highs remaining in the 60s or low 70s Fahrenheit, even when inland areas reach the 80s or 90s. The persistent cloudiness typically lasts through the morning hours, often “burning off” between mid-morning and early afternoon as solar heating strengthens, giving way to clear skies.
The Scientific Cause of Coastal Cloud Cover
June Gloom is caused by the interplay between the marine layer and a thermal inversion. The process begins over the ocean, where the cold California Current and coastal upwelling chill the air immediately above the surface. This creates a dense, cool, and moisture-rich air mass called the marine layer.
As this moist air moves inland, pushed by onshore winds, water vapor condenses into low-lying stratus clouds. A temperature inversion layer then acts as a stable, atmospheric lid that traps the marine layer below it. This inversion occurs because the air above the marine layer is warmer and drier, which is the reverse of the typical atmospheric condition.
The warm air aloft, often compressed by the North Pacific High-pressure system, prevents the cool, cloudy air from rising and dissipating. The strength of this temperature difference influences the depth of the marine layer and how long the clouds persist. This capping effect confines the cloud deck to a relatively narrow vertical band, sustaining the overcast conditions.
Where and When This Weather Pattern Occurs
June Gloom is a characteristic feature of the Southern California climate, extending along the coastline from Santa Barbara County through San Diego County. The marine layer is pushed inland by the sea breeze, but its penetration is limited by coastal mountain ranges, which block the flow of cool air.
Consequently, the most persistent effects are concentrated in the coastal plain and adjacent valleys, while areas just a few miles farther inland remain sunny and warmer. The pattern is most intense during late spring and early summer, peaking in May and June, which gives the phenomenon its name.
This timing is due to the strong presence of the North Pacific High-pressure system, which helps create the necessary thermal inversion. The combination of cold ocean water and the strong overlying warm air mass creates the most stable conditions for the stratus clouds to form and linger during this May-to-June window.