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.
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.
The Santa Ana winds are strong, extremely dry downslope winds that originate inland and affect coastal Southern California and northern Baja California. They originate from cool, dry high-pressure air masses in the Great Basin.
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 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.
A pressure system is a peak or lull in the sea level pressure distribution. 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. Tule fog is the leading cause of weather-related accidents in California.
Berg wind is the South African name for a katabatic wind: a hot dry wind blowing down the Great Escarpment from the high central plateau to the coast.
Fog is a common weather phenomenon in the San Francisco Bay Area as well as along 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 occur simultaneously 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.
An actinoform or actiniform describes a collection of marine low clouds that takes a distinct shape. They are named after the Greek word for "ray" due to their radial structure. Actinoform clouds can spread out over 300 km (190 mi) across and thus cannot be easily seen with the naked eye. In addition, actinoform clouds can form "trains" that are up to six times the length of the original cloud field, yet they maintain their own distinct identity.
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 of the water on the surface layer of an otherwise warm air mass.
The climate of California varies widely from hot desert to alpine tundra, depending on latitude, elevation, and proximity to the Pacific Coast. California's coastal regions, the Sierra Nevada foothills, and much of the Central Valley have a Mediterranean climate, with warmer, drier weather in summer and cooler, wetter weather in winter. The influence of the ocean generally moderates temperature extremes, creating warmer winters and substantially cooler summers in coastal areas.
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.
The Catalina eddy wind pattern, also called the "coastal eddy" or "marine layer," is a localized weather phenomenon that occurs in the Southern California Bight, the mostly concave portion of the Southern California coast running from Point Conception to San Diego. The Catalina eddy leads to June Gloom, which is so much a part of the late spring and early summer weather in Southern California. The eddy is named for Santa Catalina Island, one of the Channel Islands offshore between Los Angeles and San Diego.
Thermal lows, or heat lows, are non-frontal low-pressure areas that occur over the continents in the subtropics during the warm season, as the result of intense heating when compared to their surrounding environments. Thermal lows occur near the Sonoran Desert, on the Mexican plateau, in California's Great Central Valley, in the Sahara, in the Kalahari, over north-west Argentina, in South America, over the Kimberley region of north-west Australia, over the Iberian peninsula, and over the Tibetan plateau.
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.
The climate of Los Angeles is mild to hot year-round, and mostly dry. It is classified as a Mediterranean climate, which is a type of subtropical climate. It is characterized by seasonal changes in rainfall—with a dry summer and a winter rainy season. Under the Köppen climate classification, the coastal areas are classified as Csb, and the inland areas as Csa.
The geography of southern California refers to the geography of southern California in the United States.
