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Climate of Los Angeles
Download the full "Climate of Los Angeles" (pdf)
Download charts and data used for this study (zip)
In the mid 1960s, the United States Weather Bureau's State Climatologist for California, C. Robert Elford, produced a series of papers which catalogued climatological data for several counties in the state--including weather summaries for Los Angeles County. The official Weather Bureau records for 1931-1960 were used in his analyses.
Since the 1960s, sweeping changes have occurred in the monitoring and collection of weather data in the Los Angeles metropolitan area. In addition to thirty-five more years of basic climatological data, these changes--which include vast improvements in meteorological equipment and the proliferation of new environmental study sites--have led to an overwhelming need for a new and broadly-based official survey of Los Angeles climate.
As we approach the beginning of a new century, a number of government and private agencies now maintain weather monitoring equipment in the Los Angeles area. The leading government agencies now involved in weather data collection include the National Weather Service, Federal Aviation Administration, National Aeronautics and Space Administration, Department of Defense, US Army Corps of Engineers, City of Los Angeles Department of Water and Power, South Coast Air Quality Management District, State of California Department of Water Resources, and Caltrans. In addition, educational institutions, private corporations and citizens also are engaged in weather and climate data collection for a variety of reasons. As a direct result of all these data collection activities, both the climatological database and the knowledge of meteorological processes within the Los Angeles basin have greatly expanded.
The study offered in this publication was conducted by members of the National Weather Service Forecast Office in Oxnard, California--which serves the Los Angeles area. Our goal was to assemble the latest available climatological data from as many reliable sources as possible for the city of Los Angeles and surrounding communities. Our effort was designed to provide a comprehensive view of Los Angeles climate, in a form and scope never before attempted in a publication of this type. Much of this work is original and is presented here for the first time.
We hope and trust that readers will find The Climate of Los Angeles, California to be both useful and informative, not only as a data source, but as an important document that broadens the understanding of weather and climate systems that affect southern California.
1. AN OVERVIEW OF LOS ANGELES CLIMATE
Los Angeles is noted for its moderate weather. Under the modified Köppen classification system, Los Angeles climate is categorized as Mediterranean. This climate type is characterized by pronounced seasonal changes in rainfall--a dry summer and a rainy winter--but relatively modest transitions in temperature.
In the dry season, the eastern Pacific high pressure area--a semi-permanent feature of the general hemispheric circulation pattern--dominates the weather over much of southern California. Warm and very dry air descending from this Pacific high caps cool, ocean-modified air under a strong inversion, producing a marine layer. This marine layer is the prominent weather feature for the Los Angeles Basin for much of the year--especially from late spring through early fall.
Daily variations in the strength of the Pacific high result in variations in the depth and coverage of the marine layer, which typically thickens and advances inland during the night and early morning hours, before retreating to the sea or "burning off" to hazy sunshine around midday. Surface pollutants trapped under the marine inversion result in smog--the infamous L.A. mixture of smoke and fog.
Due to the dominance of the stable marine layer, significant precipitation is rare between May and October. Any rain that does occur at this time of year is usually the result of isolated thunderstorms associated with subtropical moisture.
During the remainder of the year--from November through April--the eastern Pacific high pressure ridge is displaced and Los Angeles finds itself on the southern margins of the northern hemisphere polar jet stream. With cold air aloft, the marine layer breaks down and is no longer dominant. Pacific storms, sometimes fed with subtropical moisture, often push cold fronts across California from northwest to southeast. These storms and frontal systems account for the vast bulk of the area's annual rainfall. Such rainy season storms are migratory, with wet and dry periods alternating during the winter and early spring with considerable irregularity in timing and duration.
Average annual precipitation for the Los Angeles area is highly variable and terrain-dependent, ranging from twelve inches at the ocean to about twice that in the foothills. At downtown Los Angeles, the average seasonal rainfall is 14.77 inches. The annual average high temperature for the city is 75F, while the average low is 57F.
Winds are generally light, with frequent afternoon sea breezes of 10 to 15 miles per hour. While severe weather is uncommon, strong offshore winds, known as Santa Anas, can reach hurricane strength below passes and canyons. Also, passing winter storms can bring southeast winds to gale force. However, for the most part, damaging winds tend to be rare, or highly localized.
2.THE CITY OF LOS ANGELES:
A BRIEF HISTORY
[We found] a delightful place among the trees on the river. There are all the requisites for a large settlement.
--Fr. Juan Crespi
August 2, 1769
Perhaps no city in modern times has been so universally envied, imitated, ridiculed, and, because of what it may portend, feared.
--Encyclopedia Britannica, 1997
Less than 250 years ago, a primitive aboriginal village near the center of an obscure coastal plain on the west coast of North America was visited for the first time by European colonials. That agreeable Spanish campsite is now called Los Angeles. Today, the city has a population of 3.5 million people in an area of 464 square miles--extending from the south coast port of San Pedro through the sprawling San Fernando Valley to the northwest.
The Los Angeles metropolitan district, the second largest in the United States, is home to almost nine million people. The Los Angeles/Long Beach seaport is by far the largest in the United States, and the third largest in the world. In fiscal year 1997, the value of shipping imports through this busy seaport amounted to $80 billion. Los Angeles International Airport (LAX) moves more air cargo than any airport in the country--ranking third in the world in that category. If the Greater Los Angeles Area were an independent nation, it would have the world's twelfth largest economy.
The modern history of Los Angeles dates from the 1700s. A Native American settlement known as Yang-na existed on the banks of what is today the Los Angeles River. In August 1769, a Spanish expedition headed by Gaspar de Portola camped at what was to become downtown Los Angeles. A dozen years later, in 1781, forty-four Spanish settlers established a secular village--the first civil colonial town in California--at the old Spanish campground. The town was called El Pueblo de Nuestra Señora, La Reina de Los Angeles. It was referred to by locals as El Pueblo [The Town].
In the spring of 1822, word reached El Pueblo that Mexico had won its independence from Spain. Between 1839 and 1845, approximately 4,000 square miles of land surrounding El Pueblo were divided into 70 ranchos, each containing many thousands of acres. This led to a land boom, a developing cattle market, and an increase in the presence of Yankee traders.
In January 1847, after a brief skirmish, American John C. Fremont took possession of the Los Angeles area for the United States. The City of Los Angeles was incorporated into the State of California on April 4th 1850.
The city grew quickly. From a population base of 1,250 in 1835, the number of inhabitants grew to 11,000 at the time of the first official weather observation in 1877; to 50,395 in 1890; and to 320,000 in 1910. A nearby harbor, the railroad, aerospace and above all, the automobile, contributed to a dizzying growth rate. The world's first freeway, the Pasadena Freeway (I-110) was dedicated on December 30, 1940. There are now twenty major freeways in the Los Angeles area and 10.5 million registered motor vehicles.
Part of the elixir that attracted huge numbers of people to the Los Angeles Basin was the Mediterranean climate regime. An early drawback to the city's expansion was the lack of available water to support a large population. In the early twentieth century, shrewd businessmen and politicians arranged to import large amounts of water from the Owens and Colorado River valleys. That made the increase in population and human activity nearly inevitable, with the pace of growth continuing unabated ever since.
Diversity is Los Angeles' middle name. Its cultural, racial, and political diversity are universally acknowledged. And the diversity of Los Angeles extends to its geography and climate as well.
Los Angeles County has 70 miles of seacoast. The City of Los Angeles sits on a 20 to 40 mile wide coastal plain bounded on the north and east by relatively high mountains; on the south and west lies the Pacific Ocean. Elevations within the city range from sea level at its Pacific beaches to 5080 feet at Mt. Lukens. Some of the mountains in the San Gabriel Range north of the city exceed 10,000 feet in elevation (Figure 5). Transitions between vegetation and climate zones, highly dependent upon elevation and distance from the ocean, can be striking.
The diversity theme extends to Los Angeles' weather, which is normally about as benign as the inside of a shopping mall. However, the area has a lesser-known history of winter storms that can produce amazing rainfall rates and flooding. In fact, the 24-hour record rainfall for the entire state of California, 26.12 inches, occurred in the foothills of the San Gabriel Mountains, just a few miles north of downtown Los Angeles.
Download the full "Climate of Los Angeles" (pdf)
Download charts and data used for this study (zip)
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