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A marine layer is an air mass that develops over the surface of a large body of water, such as an ocean or large lake, in the presence of a temperature inversion. The inversion itself is usually initiated by the cooling effect caused when cold water on the surface of the ocean interacts with a comparatively warm air mass. [1]
A marine layer can come in a number of different forms depending on the atmospheric conditions. It may manifest itself as merely a cool, humid air mass without any cloud cover, or it may be accompanied by clouds. In many cases, marine layers can consist of dense fog. Often associated with marine layers are stratus clouds, which are lumpy, often uniform clouds that form at low elevations (less than 3000 feet). Since marine layers are pushed ashore by westward winds, wind will almost always be present in one. Ultimately, the marine layer is a medium within which clouds may form under the right conditions; it is not the layers of clouds themselves. [2]
As air cools in a temperature inversion, the surface air becomes denser than the warmer air above it, and thus becomes trapped below it. The lower layer will also gradually increase its humidity by evaporation of the surface water, as well as by the effect of the cooling itself, since cooler air holds more moisture. [1] [2]
Clouds and potentially fog can form within a marine layer when the water-saturated air is cooled and reaches a humidity of 100%, where it will then condense and turn into water droplets. [3] Stratus and stratocumulus can form at the top of a marine layer in the presence of these conditions. A marine layer can often be dispersed by sufficiently strong winds, which essentially blow it away. The sun can also evaporate the marine layer, as it warms air and land which decreases the relative humidity and causes marine layer clouds to evaporate. [3]
In the case of coastal California, the offshore marine layer is enabled by the cold relative sea surface temperatures of the Pacific Ocean. California receives this unusually cold water through a process called the California Current, where cool polar water is transported from the Gulf of Alaska to the California coast. [3] This process creates anomalously cool water temperatures given the latitude of California, [1] and causes a strong temperature inversion. Surface waters in coastal California are kept cold through a process called upwelling, which involves cooler, deeper waters rising to the top of a water column. [3]
Occasionally the marine layer becomes particularly deep, and the clouds on land can persist all day. This can happen at any time of the year, inspiring colloquialisms such as "May Gray" or "June Gloom". An approaching frontal system or trough can also drive the marine layer onshore. [1]
Though California is a notable example of the marine layer, these atmospheric phenomena exist in several other places around the world. Marine layers often form on the west coasts of continents, where upwelling usually occurs, and where there is a relatively warm air mass moving over a cool body of water. [2] They can also form on the coasts of cool lakes, such as the Great Lakes in the United States. [1]
In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may compose the droplets and crystals. On Earth, clouds are formed as a result of saturation of the air when it is cooled to its dew point, or when it gains sufficient moisture from an adjacent source to raise the dew point to the ambient temperature.
Cumulus clouds are clouds that have flat bases and are often described as puffy, cotton-like, or fluffy in appearance. Their name derives from the Latin cumulus, meaning "heap" or "pile". Cumulus clouds are low-level clouds, generally less than 2,000 m (6,600 ft) in altitude unless they are the more vertical cumulus congestus form. Cumulus clouds may appear by themselves, in lines, or in clusters.
In meteorology, an inversion is a phenomenon in which a layer of warmer air overlies cooler air. Normally, air temperature gradually decreases as altitude increases, but this relationship is reversed in an inversion.
Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Fog can be considered a type of low-lying cloud usually resembling stratus and is heavily influenced by nearby bodies of water, topography, and wind conditions. In turn, fog affects many human activities, such as shipping, travel, and warfare.
A stratocumulus cloud, occasionally called a cumulostratus, belongs to a genus-type of clouds characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumulus, and the whole being at a lower height, usually below 2,000 metres (6,600 ft). Weak convective currents create shallow cloud layers because of drier, stable air above preventing continued vertical development. Historically, in English, this type of cloud has been referred to as a twain cloud for being a combination of two types of clouds.
Stratus clouds are low-level clouds characterized by horizontal layering with a uniform base, as opposed to convective or cumuliform clouds formed by rising thermals. The term stratus describes flat, hazy, featureless clouds at low altitudes varying in color from dark gray to nearly white. The word stratus comes from the Latin prefix Strato-, meaning "layer". Stratus clouds may produce a light drizzle or a small amount of snow. These clouds are essentially above-ground fog formed either through the lifting of morning fog or through cold air moving at low altitudes. Some call these clouds "high fog" for their fog-like form.
An anticyclone is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as viewed from above. Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure.
A sea of clouds is an overcast layer of stratocumulus clouds, as viewed from above, with a relatively uniform top which shows undulations of very different lengths resembling waves on the sea. A sea of fog is formed from stratus clouds or fog and does not show undulations.
In meteorology, the synoptic scale is a horizontal length scale of the order of 1,000 km (620 mi) or more. This corresponds to a horizontal scale typical of mid-latitude depressions. Most high- and low-pressure areas seen on weather maps are synoptic-scale systems, driven by the location of Rossby waves in their respective hemisphere. Low-pressure areas and their related frontal zones occur on the leading edge of a trough within the Rossby wave pattern, while high-pressure areas form on the back edge of the trough. Most precipitation areas occur near frontal zones. The word synoptic is derived from the Ancient Greek word συνοπτικός (sunoptikós), meaning "seen together".
A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than cold fronts, and move more slowly than the cold fronts which usually follow because cold air is denser and less easy to remove from the Earth's surface. This also forces temperature differences across warm fronts to be broader in scale. Clouds ahead of the warm front are mostly stratiform, and rainfall generally increases as the front approaches. Fog can also occur preceding a warm frontal passage. Clearing and warming is usually rapid after frontal passage. If the warm air mass is unstable, thunderstorms may be embedded among the stratiform clouds ahead of the front, and after frontal passage thundershowers may continue. On weather maps, the surface location of a warm front is marked with a red line of semicircles pointing in the direction of travel.
Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of atmospheric clouds. These aerosols are found in the troposphere, stratosphere, and mesosphere, which collectively make up the greatest part of the homosphere. Clouds consist of microscopic droplets of liquid water, tiny crystals of ice, or both, along with microscopic particles of dust, smoke, or other matter, known as condensation nuclei. Cloud droplets initially form by the condensation of water vapor onto condensation nuclei when the supersaturation of air exceeds a critical value according to Köhler theory. Cloud condensation nuclei are necessary for cloud droplets formation because of the Kelvin effect, which describes the change in saturation vapor pressure due to a curved surface. At small radii, the amount of supersaturation needed for condensation to occur is so large, that it does not happen naturally. Raoult's law describes how the vapor pressure is dependent on the amount of solute in a solution. At high concentrations, when the cloud droplets are small, the supersaturation required is smaller than without the presence of a nucleus.
June Gloom is a mainly Southern California term for a weather pattern that results in cloudy, overcast skies with cool temperatures during the late spring and early summer. While it is most common in the month of June, it can occur in surrounding months, giving rise to other colloquialisms, such as “Graypril,” “May Gray,” “No-Sky July,” “Fogust”, and “Summer Bummer”. Low-altitude stratus clouds form over the cool water of the California Current, and spread overnight into the coastal regions of California.
A pressure system is a peak or lull in the sea level pressure distribution, a feature of synoptic-scale weather. The surface pressure at sea level varies minimally, with the lowest value measured 87 kilopascals (26 inHg) and the highest recorded 108.57 kilopascals (32.06 inHg). High- and low-pressure systems evolve due to interactions of temperature differentials in the atmosphere, temperature differences between the atmosphere and water within oceans and lakes, the influence of upper-level disturbances, as well as the amount of solar heating or radiationized cooling an area receives. Pressure systems cause weather to be experienced locally. Low-pressure systems are associated with clouds and precipitation that minimize temperature changes throughout the day, whereas high-pressure systems normally associate with dry weather and mostly clear skies with larger diurnal temperature changes due to greater radiation at night and greater sunshine during the day. Pressure systems are analyzed by those in the field of meteorology within surface weather maps.
Drizzle is a light precipitation which consists of liquid water drops that are smaller than those of rain – generally smaller than 0.5 mm (0.02 in) in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation rates from drizzle are on the order of a millimetre (0.04 in) per day or less at the ground. Owing to the small size of drizzle drops, under many circumstances drizzle largely evaporates before reaching the surface, and so may be undetected by observers on the ground. The METAR code for drizzle is DZ and for freezing drizzle is FZDZ.
Tule fog is a thick ground fog that settles in the San Joaquin Valley and Sacramento Valley areas of California's Central Valley. Tule fog forms from late fall through early spring after the first significant rainfall. The official time frame for tule fog to form is from November 1 to March 31. This phenomenon is named after the tule grass wetlands (tulares) of the Central Valley. As of 2005, tule fog was the leading cause of weather-related accidents in California.
Fog is a common weather phenomenon in the San Francisco Bay Area and the entire coastline of California extending south to the northwest coast of the Baja California Peninsula. The frequency of fog and low-lying stratus clouds is due to a combination of factors particular to the region that are especially prevalent in the summer. Another type of fog, tule fog, can occur during the winter. There are occasions when both types can coincide in the Bay Area. The prevalence of fog in the San Francisco Bay Area has decreased, and this trend is typically attributed to climate change.
The climate of San Diego, California, is classified as a hot-summer Mediterranean climate. The basic climate features hot, sunny, and dry summers, and cooler, wetter winters. However, San Diego is much more arid than typical Mediterranean climates, and winters are still dry compared with most other zones with this type of climate. The climate at San Diego International Airport, the location for official weather reports for San Diego, as well as the climate at most beach areas, straddles the border between BSh and BSk due to the mild winters and cool summers in these locations.
Marine stratocumulus is a type of stratocumulus cloud that form in the stable air off the west coast of major land masses. The Earth spins on its axis, which results in the Coriolis force pushing the ocean surface water away from the coast in the mid-latitudes. This results in upwelling of cold water from below that creates a pool of cool water at the surface, which in turn cools the air directly above it. The surface cooling results in a large temperature inversion at the top of the marine layer. As the temperature is cooled to the dewpoint, water vapor condenses upon available cloud condensation nuclei, and forms a cloud. The stability of the marine layer prevents deep convection, and thus stratiform clouds are formed.
This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.
Trade wind cumulus clouds are formed by cooling and moisture absorption of the dry trade winds over the relatively cold sea surface in the eastern parts of the oceans. These are clouds, typically Cumulus humilis or Cumulus mediocris, which are considered as fair weather clouds.
