The persistent low clouds and coastal mist that often greet San Diego mornings are part of a predictable, natural weather pattern unique to this region. Locals call this phenomenon “May Gray” or “June Gloom,” and it is a defining feature of the coastal climate, especially during late spring and early summer. The marine layer, the scientific term for this cool, moist air mass, frequently rolls in from the Pacific Ocean overnight. This weather system plays a significant role in moderating coastal temperatures.
The Atmospheric Engine Temperature Inversion
The primary mechanism responsible for San Diego’s frequent fog is a meteorological structure known as a temperature inversion. Normally, air near the Earth’s surface is warmest and gradually cools as altitude increases, allowing vertical mixing. In an inversion, this profile is flipped: a layer of warmer air sits above cooler air near the surface. This warm air aloft acts like an invisible lid, preventing the cool, moist air mass below from rising and mixing with the drier air above.
This trapped layer of cool, saturated air is called the marine layer. The moisture within this layer condenses into low-lying clouds, or stratus, when the air cools to its dew point. The resulting fog is essentially a cloud resting on the ground. Its density depends on the height and thickness of the marine layer. If the inversion base is very low, typically below 800 feet, the result is dense fog, but a deeper layer may only produce a high deck of stratus clouds.
The Role of the Pacific Ocean and Local Topography
The temperature inversion is particularly strong and persistent in San Diego due to the influence of the Pacific Ocean and the region’s geography. The cold California Current, which flows southward along the coast, is a major contributor to the cool marine layer. This current keeps the sea surface temperatures cool, chilling the air mass immediately above the water.
This cool, moist air mass moves inland, meeting warmer air sinking from the high-pressure system that often settles over the region. The stark contrast between the cold ocean air and the warmer air above intensifies the inversion layer, creating a robust atmospheric cap. Furthermore, the coastal mountains act as a natural barrier, helping to compress and contain the cool, moist air against the coast. This intersection of cold water, warm sinking air, and confining topography strengthens the system.
The Cycle of May Gray and June Gloom
The annual peak of this foggy weather, known as “May Gray” and “June Gloom,” is directly tied to the changing strength and position of the North Pacific High-pressure system. As spring transitions into summer, this high-pressure system strengthens and moves closer to the Southern California coast. The intensification results in increased sinking air, which strengthens the temperature inversion and traps the marine layer more effectively.
This seasonal pattern results in May and June being the cloudiest months for coastal Southern California. The fog is not permanent and typically follows a predictable diurnal process known as “burn off.” As the sun rises, solar radiation warms the land and the air within the marine layer. This heating eventually causes the stratus clouds to either evaporate or lift higher, revealing the clear, sunny skies. Clearing time varies, but it often occurs by late morning or early afternoon, depending on the thickness of the marine layer.