For most people in the US, the month of June is associated with warm temperatures and abundant sunshine. For parts of coastal California, however, it is a month known for cloudy and relatively cool conditions. This regional phenomenon called “June Gloom” is the result of the interaction of several natural elements, including geography, ocean currents, and weather patterns.
With the California Current running south along the coast from the Gulf of Alaska, the water in the area is cold. Ocean temperatures in the region usually hover in the upper 50s to low 60s during the summer, cooling the air that flows over it.
Another significant factor is the temperature inversion aloft created by the North Pacific High, a semi-permanent area of high pressure. This is part of a larger planetary circulation of air known as a Hadley cell, a current of high altitude air traveling poleward from the tropics. As the air cools, it descends around 30N latitude. It compresses and warms as it sinks, making the air aloft warmer than the cold, moist air at the surface. Since air temperatures normally decrease with height, this situation acts like a cap on the cool air below and prevents it from rising any higher.
When the air under the inversion layer, known as the marine layer, is cooled to the point where the moisture condenses, an expansive sheet of low level stratus clouds form. The region’s prevailing westerly winds, as well as the sea-breeze circulation that often develops during the summer months, carries these clouds inland. While they create overcast conditions and some light drizzle, the clouds do not produce any significant rain. They also tend to dissipate by the afternoon as the land heats up.
The thickness and inland extent of the marine layer clouds depend on the strength of the high-pressure system. A stronger high will thin the clouds and keep them confined to the coast. A weaker high with allow the clouds to thicken and move further inland. Separated by only a few miles, the cloud-covered coast can be significantly cooler than sunny areas further east.
These conditions are most common in June, but are not necessarily limited to the month. They have been known to develop in May and last on and off through August. The monikers for these events include “May Gray”, “No Sky July”, and “Fogust”. However, high pressure usually builds over southern California in July, decreasing the impact of the marine layer or eliminating it altogether.
“June Gloom” clouds along west coast. Credit: NWS/UCSD