marine layer clouds that persist longer into the day. Upwelling of colder-than-normal ocean water, associated with a La Niña, can strengthen this effect even more. Once this marine layer has formed, the prevailing westerly winds advect the clouds over the coastal lands. The extent of inland advection is limited by southern California's coastal mountain ranges. The winds will continue to push the cloud layer onshore until they encounter mountains at or above the altitude of the clouds themselves, with the mountains then preventing any further inland progress of the marine layer. The foothill regions of these mountains experience some of the thickest fog and drizzle, as they are essentially in the clouds at this point. The marine layer clouds of a June Gloom day usually are at their maximum at dawn, when the surface air is at a minimum temperature and the temperature difference in the inversion layer is at its maximum. The air beneath the inversion base being at its coolest also makes it likely that it will be saturated with a relative humidity of 100%. A sea breeze, which is caused by the temperature and pressure difference between warm areas inland and the cool air over the ocean, often develops on warm summer days as well, increasing the on-shore flow pattern and maintaining a constant flow of marine stratus clouds onto the coastal areas. A strong low pressure system passing over southern California will produce the deepest and most extensive June Gloom marine layer. The marine layer effect is weakened when a weak high pressure system is in place over the region, and the marine layer will be very weak or nonexistent when there is a strong high-pressure system affecting southern California. The National Weather Service graphic on the right explains the effects of atmospheric conditions upon the marine layer and local weather conditions in more detail. Similar weather elsewhere in the world While many parts of the world commonly have an offshore marine layer of stratus or stratocumulus clouds, other locations matching the daily and seasonal effects of Southern California's June Gloom are relatively rare. These include the western coast of Peru, the Macaronesian Islands, the western coasts of Morocco and Portugal, and Namibia in southern Africa. Actinoform clouds and drizzle prediction Researchers have discovered that the cloud fields forming June Gloom and related phenomena from other west-coast marine-influenced climates are excellent places to find and study