California is known for both its cultural and geographical diversity. More than a quarter of Californians are foreign born, and no single ethnic group comprises a majority of the state's nearly 37 million residents. Geographically the differences are even more striking: California is home to the highest peak in the 48 contiguous United States (Mount Whitney, elevation 14,505 feet) and the lowest spot in the Western Hemisphere (Death Valley, elevation −252 feet). Not only are these two locations located in the same state, but they are actually only 76 miles apart! As a result, California has a climate that is just as diverse as its geography and population. Despite the conventional wisdom that the Golden State (so-called because of the Gold Rush in the 1850s) is blessed with sunny beach days 365 days of the year, the difference between the state's record extremes is actually the fifth greatest in the United States, with a range of 179 degrees between its record minima of −45°F at Boca Reservoir near Truckee and its blistering maxima, of 134°F at aptly named Death Valley, also a North American record high.
Most of the populated regions of California are graced with a Mediterranean climate (Koeppen type Cs), characterized by warm, dry summers and mild, wet winters. These areas include the coastal sections and coastal mountains near the Oregon border in the north, San Diego in the south, as well as the Central Valley. Within this area, temperatures typically trend from more temperate closer to the Pacific Ocean to wider seasonal extremes in the state's interior.
The two climatic exceptions occur in California's higher mountains and its deserts. The Sierra Nevada, the Northeast Plateau, and the Mount Shasta region are best characterized somewhere between a Continental climate and a Highland climate, while the Mojave and other desert regions in southeast California are dry desert climates.
Caption: The east face of Mount Whitney, the highest summit in the contiguous United States with an elevation of 14,505 feet, is located at the southern end of the Sierra Nevada Mountain Range.
The mitigating influence of the Pacific Ocean gives California coastal communities some of the most moderate temperatures in North America (and definitely factors into California's attraction). The upwelling of cold water along the coast leaves summertime water temperatures at northern and central California beaches colder than those in Washington and Oregon. For example, San Francisco only sees a range of 9 degrees between January's mean temperature of 52 degrees and July's mean of 61. It is this summertime mildness that has caught many out-of-state tourists in shorts and t-shirts when they should be wearing pants and sweaters. It also undoubtedly inspired the quote “The coldest winter I ever spent was summer in San Francisco,” which is widely, albeit inaccurately, attributed to Mark Twain. The range between summer and winter temperatures increases the farther one travels from the coast. It is not uncommon for inland locations to have seasonal temperature ranges that are more typical of locations across the central United States.
This means that during the summer, coastal communities are often 30 or 40 degrees cooler than cities sometimes only 30 to 40 miles farther inland. In the San Francisco Bay Area alone, it is not uncommon for downtown San Francisco to remain in the 50s and lower 60s, while just across San Francisco Bay, in the valleys east of the Oakland Hills, temperatures are near or above 90°F. As the crow flies, this variation occurs over a distance of about 15 miles.
California's location on the west coast of the continent and primarily at the latitudes of the westerlies means that it is almost exclusively impacted by the milder wintertime moist Pacific maritime air masses, and not by the colder continental air masses that impact most of the nation. However, there are exceptions: In December 1990, cold Arctic air invaded the state, with many areas recording all-time record minima. Ultimately this was the most costly weather disaster in the state's history, with $5.5 billion in widespread agricultural damage, primarily to citrus and avocado crops. This freeze also killed or damaged large numbers of trees in the Oakland Hills and ultimately was a contributing factor to the October 1991 firestorm (the state's third largest weather-related disaster), which devastated the area to the tune of $3.9 billion.
Death Valley holds the Western Hemisphere's record maximum temperature reading of 134°F, set in 1913. It also averages 138 days per year when the maximum temperature reaches at least 100°F, not to mention occasions about once every third year when the minimum stays above the 100-degree mark!
Rain, Flooding, and Drought
The same extremes in terrain, as well as juxtaposition to the Pacific Ocean, are responsible for widely varying rainfall across the Golden State. The highest annual rainfalls are in the northern Coast Range, where the town of Honeydew averages 104 inches per year and isohyetal analyses indicate upwards of 150 inches in remote locations. Locations in the Coast Range, Sierra Nevada, and other north-south oriented mountains see the greatest precipitation, as moisture-laden storms from the Pacific are forced to lift over the higher terrain. Conversely, areas in the lee of the mountains and away from the ocean see significantly less rain. Nowhere is this truer than Death Valley, which receives only 2.33 inches on average each year, the lowest annual total in the United States. Bagdad, in the Mojave Desert, holds the U.S. record for the most consecutive days with no rain; an impressive 767 days stretching from October 1912-November 1914!
“this was the most costly weather disaster in the state's history”
The impacts of too much rain are often mudslides and individual streams and rivers overflowing their banks. However, large-scale weather systems covering about one-fourth or more of the state—and often containing copious amounts of subtropical moisture and lasting several days—occasionally produce flooding of large river systems.
Caption: The upwelling of cold water along the coast leaves summertime water temperatures at northern and central California beaches colder than those in Washington and Oregon. As a result, summers in San Francisco are often characterized by fog have a mean July temperature of only 61°.
The most catastrophic California flooding of record occurred in the winter of 1861–1862, when there were record or near-record floods over the entire length of the state. During that December and January, many places received 200 percent to 400 percent of their average rainfall from an alternating series of strong warm and cold storms with heavy rain and heavy snow. As a result, the Central Valley became a large (5,000 square mile) inland lake. Sacramento was under several feet of water, and newly elected Governor Leland Stanford had to be rowed to his inauguration. Scientists estimate that the event was a once-in-a-30,000-year event.
“multiyear droughts have a widespread impact upon the state's economy”
Despite the popular misconception that El Niño invariably means flooding for California, an analysis of flood events since 1950 shows that only four of the 10 most costly floods occurred during El Niño. Of the remaining six events, two happened during La Niña and the remaining four during “neutral” years. The meteorological pattern that frequently results in California flooding is an “atmospheric river” (colloquially known as “the Pineapple Express”) that taps into subtropical moisture (often near Hawaii) and persists over a region of the state for several days, dumping copious amounts of rain.
Nearly as disastrous to the state as too much water is too little. A large part of the state's economy is dependent on water, including agriculture, hydroelectric power, industry, and human consumption. Further, water discharge is required to maintain fragile ecosystems, draining supplies needed for human needs. While California's water infrastructure of reservoirs and groundwater can sustain the state for a single dry year, multiyear droughts have a widespread impact upon the state's economy. The two-year drought of 1976–1977 saw parts of the state receive less than half of their normal rainfall, at an overall impact to California of $2.7 billion.
“It is this summertime mildness that has caught many out-of-state tourists in shorts and t-shirts when they should be wearing pants and sweaters”
The same winter storms that produce rain at lower elevations of California can dump prodigious amounts of snow in the mountains. Snowfall in many areas of the Sierra Nevada and Mount Shasta are best measured in feet. Compared to other parts of the country that typically receive light, powdery snow associated with cold, continental airmasses, the wet, heavy snow that falls in the mountains of California carries a very high water content. Sometimes referred to as “Sierra Cement,” six inches of snow depth can contain an inch of water; over twice the water content of fluffy powder. The very high water content that these snows provide is a critical source of water for California.
In February 1959, the greatest single continuous snowstorm in the world dumped 189 inches of snow at Mount Shasta Ski Bowl, with drifts burying entire ski lifts. And Tamarack, in the Sierra Nevada near Yosemite, holds the world record one-month (January 1911) snowfall with 32.5 feet! One of the most notorious snowstorms in California history occurred in late October 1846. This severe snowstorm is most well-known because it trapped the Donner Party of pioneers for nearly five months. Ultimately, almost half of the party of 87 died in the mountains during that terrible winter.
“newly elected Governor Leland Stanford had to be rowed to his inauguration”
Caption: One of the most notorious snowstorms in California history occurred in late October 1846. This severe snowstorm is most well-known because it trapped the Donner Party of pioneers for nearly five months at what is now known as Donner Lake, shown here. Ultimately, almost half of the party of 87 died in the mountains during that terrible winter.
Caption: Strong winds in both Southern Calfiornia and the San Francisco Bay area in winter fan frequent wildfires to deadly proportions.
Like many coastal areas of the world, California has many areas that are regularly impacted by the wind. Persistent westerly and northwesterly sea breezes result from differential heating between the state's interior and the Pacific Ocean and are the source of energy for some of the most expansive wind farms in the world. The same seabreeze pattern advects cool marine air inland on a regular basis during the summer months, allowing for periodic relief from the heat.
Winter storms are occasionally accompanied by strong winds that damage property and down power lines. These winds are most often from the south near sea level and from the southwest in the higher mountain areas. Some of the most memorable coastal storms have produced winds that gusted in excess of 100 mph at lower elevations and over 150 mph in the higher reaches of the Sierra Nevada.
On the morning of December 12, 1995, a very deep area of low pressure developed as a “bomb” off the northern California coast. Bringing wind gusts of 60 to 100 mph and torrential rains, the storm downed thousands of trees in exposed portions of the coast and inland valleys; nearly 2 million people lost power. That storm tore off the roof of the historic Cliff House in San Francisco and inflicted heavy damage on parts of the city's Golden Gate Park.
“warm dry winds have been a critical factor in the spread of many wildfires”
Fall and winter often see periodic shifts in the wind from onshore to offshore as high pressure builds into the Great Basin. The resulting winds, called Santa Ana winds in Southern California and Diablo winds in the San Francisco Bay Area, blow from the high deserts of Nevada downslope to the Pacific Ocean. As the air descends, it warms and dries; as the wind is channeled through gaps and canyons in the coastal mountains, it can also become extremely strong and gusty. Occurring at the end of the typical Mediterranean climate summer drought, these warm dry winds have been a critical factor in the spread of many wildfires in the state.
During spring and summer, the combination of a cool deep marine layer and persistent sea breezes results in extensive low clouds and fog that blanket much of the California coast and the adjacent coastal valleys. Referred to in Southern California as “June Gloom” and mistakenly identified in many areas as “high fog,” the blanket of stratus clouds has a persistent mitigating influence on temperatures near the coast. And as unbelievable as it may seem, the July average afternoon relative humidity in San Francisco (65 percent) is higher than that in Miami (63 percent), albeit with an average maximum of 68°F compared to 91°F in Miami.
In winter, “tule fog,” a California colloquialism for radiation fog in the Central Valley, can be equally persistent. Forming on clear, cool, and calm nights after recent rains, tule fog can cover large parts of the valley for days or even weeks. The very low visibilities associated with tule fog have been responsible for numerous tragic chain-reaction traffic accidents on highways when drivers fail to reduce speeds.
Caption: In winter, “tule fog,” a California colloquialism for radiation fog in the Central Valley, forms on clear, cool, and calm nights after recent rains. It can cover large parts of the valley for days or even weeks and cause very low visibilities that have been responsible for numerous tragic chain-reaction traffic accidents on highways when drivers fail to reduce speeds.
While not a common occurrence, severe weather can strike the Golden State. On average, a half dozen tornadoes, mostly of the F0 and F1 variety, are reported each year. Since 1950, only 21 California tornadoes have been rated F2, and two were estimated as F3. Surprisingly, Los Angeles County's total of 41 tornadoes is the most of any city in the state, many resulting from weak waterspouts that make landfall. In an oddity of nature, 2005 saw more tornadoes in California (27) than in Oklahoma!
Significant hail and severe thunderstorm winds are rare events in California. However, each year lightning triggers numerous wildfires that can be devastating to large areas of the state. In 2008, lightning touched off more than 100 fires that resulted in more than a billion dollars in damage to forests and property. The smoke from the fires was so severe that in June, officials canceled the Western States Endurance Run, a 100-mile-long ultramarathon that takes runners from the base of the Squaw Valley ski resort in the Sierra Nevada to the Placer High School track in Auburn, California.
“fog can cover large parts of the valley for days or even weeks”
California's location adjacent to the cool Pacific nearly precludes the possibility of tropical storms, although not completely. In the deadliest weather disaster in California history, a tropical storm made landfall near Long Beach in 1939. The storm came ashore with 50-mph winds, dumping more than five-and-a-half inches of rain on the area and killing 45 people on land and many more at sea. Historical records have also recently confirmed that a hurricane struck the San Diego area in October 1858.
Like so many other aspects of California, the weather and climate of the state are extremely interesting and diverse. There are few other places on the planet where someone can live and only be a couple of hours' drive from the beach, a lush redwood forest, a desert, or a world-class ski slope. Meteorologically speaking, California truly is the Golden State.
JAN NULL is a Certified Consulting Meteorologist with Goiden Gate Weather Services and an Adjunct Professor of Meteorology at San Francisco State University.
H. MICHAEL MOGIL is a Certified Consulting Meteorologist and science writer who lives in Naples, Florida.