An inversion separating a deep potentially cool moist, nearly saturated boundary layer that has the ocean as a source region (called the marine layer) from potentially warmer air above, which extends to the tropopause. The lapse rate in the marine layer tends to be unstable with respect to the wet adiabatic rate and in the air above the inversion is very stable.
The marine layer is generally 50 to 100 mb or more deep and tends to be modified from beneath by diurnal heating. The diurnal heating creates a well mixed region characterized by the dry adiabatic lapse rate that works its way upward from the surface and is evidenced by evaporated cloud material.
The marine inversion is formed by differential advection. Usually the surface winds along the West Coast during the summer are westerly, bringing air in the boundary layer across the California current and the area of upwelling in the ocean adjacent to the coastline. This air is conductionally cooled by the surface of the ocean to the dew point, often, creating coastal stratus. The relatively strong winds (due to surface pressure gradients) in the boundary layer mixes these effects (both temperatures and mixing ratios) through a deeper and deeper layer as the air approaches the coast. When diurnal heating occurs as the air moves over the coastline, as explained in the previous paragraph, the cloud base is not at the surface and so is not technically fog, but stratocumulus. Whether surface based or not, this deep cloudy layer is often referred to as “advection fog.”
The wind direction above the inversion can be southerly, southwesterly, westerly or northwesterly depending upon the larger scale synoptic situation. In all of these cases, air in the middle and upper troposphere does not have a source region near the surface of the ocean, and will be potentially warm and dry (with large dew point depressions).
An inversion that forms nocturnally when pressure gradients are weak, surface winds are calm or very light, and when there are sufficient number of hours without direct solar insolation so that profound radiational cooling occurs. This radiational cooling is encouraged by clear skies, and low mixing ratios, and, hence, conditions for which the moderating influences of water vapor are minimized.
Since winds are light, deep mixing does not occur, so that the radiational cooling of the earth’s surface is conductionally passed only to the bottom of the boundary layer. Thus, the radiation inversion is most often ground-based, with no mixed layer analogous to the marine layer.
When sufficient radiational cooling occurs, air near the surface can be cooled to the dew point resulting in the formation of a shallow layer of ground-based stratus, often referred to as “radiation fog.”