Most people are aware that Southern California enjoys a “Mediterranean Climate”; that is, our climate is sunny, warm, and rather dry, with our rainfall restricted to the cooler winter months. Most parts of the world that are dry in summer are even drier in winter, and thus are clearly desert or semidesert. We do not live in a desert because our rainfall considerably exceeds 10 inches (25 cm) a year. Our combination of cool wet winters and hot dry summers is so rare that it is encountered on less than 3% of the world's land surface, and only in 5 geographical locations: California, around much of the Mediterranean Sea, southern Chile, the tip of South Africa, and southwestern Australia. Note that these 5 areas lie between the 30th and 45th parallels of latitude, that they are all on the western borders of their continents, and that the world's hottest climates lie immediately inland — in our case, the Salton Trough and the lower Colorado River Valley, with temperatures higher than 115°F (46°C) on at least 15 days per year.
While our climate seems very benign to humans, for vegetation it combines the very worst features of arid and humid climates: the supply of water and the need for water are exactly out of phase. The growing season is limited by the cool temperatures of winter and by the summer drought. As a consequence, native vegetation begins growth with the winter rains, is lushest in the spring when there is temporary overlap between adequate water availability and warmer temperatures, and then becomes desiccated and dormant in the hot dry summer and fall. A characteristic vegetation type is found in all 5 Mediterranean climate areas — an evergreen shrub cover, well adapted to summer drought and the regular presence of fire. Indeed, some species seem to require fire for successful reproduction. The Coastal Sage Scrub community of the Bernard Field Station is characterized by summer-deciduousness.
While the regional climate of Southern California is Mediterranean, there are many local climatic variations, reflecting small-scale differences in slope, exposure, temperature inversions, cold air drainage, groundwater levels, etc. A third level of climate is the microclimate, that is, even smaller scale differences caused by vegetation and soil structure (see section on Microclimate).
Weather records for Claremont clearly demonstrate the nature of our Mediterranean Climate. Temperature records kept from 1959-1978 by the official U.S. Weather Bureau Station at Pomona College show that the average summer maximum did not exceed 90°F (32°C), and the average winter minimum did not drop below 40°F (4°C), though during this 20-year period temperatures as high as 108°F (42°C) and as low as 26°F (-3°C) were recorded. This recent 20-year period does not have as extreme temperatures as have surely occurred in Claremont: the great freezes of January 1937, when Redlands hit a low of 15°F (-9°C), and February 1949 are still spoken of with awe, particularly for the amount of smudge oil burned to protect the citrus. Riverside has recorded a maximum of 118°F (48°C), probably only a few degrees higher than in Claremont on the same day. There was a great temperature range during this period between day and night, averaging over 25-30°F (14-17°C), and often as great as 40-50°F (22-25°C). Claremont’s climate is called the Intermediate Valley type by Bailey (1966), who points out that this distinctive combination of mild winters and warm summers occurs only in the inland valleys, away from the coastal fringe with its summers tempered by the ocean.
Claremont receives over 90% of its annual rainfall in the 6 month period of November through April, and only 1% falls in the three months of June through August. Locally, rainfall is strongly affected by the San Gabriel Mountains, falling off very rapidly with distance from 30-50 in (75-125 cm) in the higher mountain slopes to 17.93 (45.5 cm) in Claremont, to less than 14 in (36 cm) in Chino. The higher rainfall areas are not characterized by a greater number of rainy days (which average about 40 per year), but by heavier rainfall per storm. There is a strong rainfall gradient just in Claremont, with a recording station north of the Infirmary at the BFS showing about 20% higher rainfall than at the Pomona College station on Sixth Street.
Rainfall in Claremont, as in Southern California generally, is markedly variable from year to year. The long-term average for the 86 years of record from 1892 to 1978 is 17.93 in (45.5 cm), but the standard deviation is very great: 7.1 in (18 cm), or 40% of the mean. This means that you can expect 1/3 of all years to have rainfalls greater than 25 in (63 cm) or less than 10.8 in (27.5 cm). This average rainfall of 17.93 in (45.5 cm) is a misleading figure, for the average is skewed upwards by the presence of a few very wet winters, most notably the high of 41.5 in (105.4 cm) in 1977-78. The median value, that is, the value for which 50% of all years are higher and 50% of all years are lower, is only 15. 64 in (39.7 cm). The mode (the interval with the highest frequency) is lower still, 14.5 in (36.8 cm).
Records show not only that Claremont’s rainfall is highly variable, but also that it has gradually, if irregularly, declined from the time records began in 1892. There were fewer wet years, and more dry years in the period from 1948-1978 than there were in the proceeding 40 years. The average rainfall for this 30 year period is only 16.58 in (42.1 cm), while the average was 19.23 in (48.8 cm) for 1908-48. In a very real sense, a drought of major magnitude occurred between 1948 and 1978, with a shorter drought period in 1894-1903. Earlier droughts are known less exactly, but from old documents it seems clear that rainfalls in 1785-1810 and 1819-1833 were below normal. It is reported that between 1828 and 1830, Los Angeles went 22 months without rainfall, meaning that one whole winter's rainy season must have been skipped. Another extremely severe drought occurred in 1862-1864. These severe droughts of the 1830’s and 1860’s resulted in a tremendous loss of cattle and horses, and coincided with the establishment of many exotic plants (European 'weeds' for the most part) in the State. Apparently these droughts, coupled with extreme overgrazing by cattle, gave the exotics (particularly annual grasses) an advantage over the native flora that has not been reversed. The 1915-1942 period was much wetter than normal. Extremely wet years do not seem to show any pattern: 3 of the 5 years since 1892 with rainfall greater than 30 in (76cm) occurred in the drought since 1948.
Evaporation is of greatest importance for plant growth and for temperature and water relations of animals, and is based not only on temperature, but also on wind patterns and relative humidity. Figure 2 presents evaporation data from a station at Puddingstone Dam, about 5 miles west of Claremont and slightly lower in elevation. Evaporation here was measured using a pan 24 in diameter by 36 in deep, set in the ground 33 in, with the water level maintained at ground level (3 in below top of pan). The pan was covered by a wire screen. For the 35 year period 1939-1974, average annual evaporation from this free water surface was 53.62 in (136.2 cm), or almost 4.5 feet per year. As you would expect, evaporation is highest in the summer and lowest in winter, but precipitation and evaporation are not completely mirror images of each other.
Claremont’s weather alternates from being dominated by the ocean (maritime weather) to being dominated by the land (continental weather), weather types being brought to us by the changing wind patterns, often from long distances. For the most part, winter rains, early summer fogs, late summer storms, and mild weather generally are all associated with maritime atmospheric circulations, while winter cold, Santa Ana winds, summer heat, and summer thunderstorms are continental.
Nearly all of Southern California’s rain comes from large, general storms (low pressure systems) that move off the sea to the land, storms originating in the Gulf of Alaska. Weak storms cut inland to the north, bringing, us at the most, clouds and light drizzles near the mountains. Stronger storms get further south, and may provide intense rainfall leading sometimes to floods. Major floods in the Santa Ana River watershed, of which Claremont is a part since it sits on the alluvial fan of San Antonio Creek, do not correspond completely to wet years. The most serious floods in the Santa Ana watershed since 1892 were in January-February 1969 and March 1938, but not in early 1978 or 1941. Even more severe floods have occurred in our watershed in the past, notably in February 1891, December 1867, and the biggest of them all in January 1862, with an estimated flow of 317,000 cubic feet per second in the Santa Ana at the Riverside Narrows, as compared with about 100,000 cubic feet per second for the memorable March 1938 flood, which caused much damage in the Claremont area.
Winter storms are usually followed either by a strong cold front derived from cold air over Canada moving southerly, or by dry and sometimes warm to hot Santa Ana winds derived from a high pressure system to the east. Santa Ana winds warm by compression as they drop in elevation through the mountain passes. At such times Long Beach may be as much as 20-30°F (11-17°C) warmer than the Mojave Desert. Between winter storms, gentle sea breezes may bring cool air and even dense fogs.
Summer weather is dominated by continental air masses, held in place by high atmospheric pressures over the ocean to the west. It is a surprise to many that early summer (April-June) may be very foggy with dull, gray, overcast skies giving rise to light drizzle near the seacoast and mountains. At the same time at higher elevations, such as at Baldy Village (4500 feet, or 1370 meters), the sky will be sunny and clear. Later in the summer these coastal clouds cyclically advance and retreat, and the weather changes to summer heat coupled with low relative humidities. The sea breeze weakens, and inland temperatures rise rapidly. In some inland valleys, such as ours, a weak sea breeze usually picks up by the afternoon, cooling the air (and bringing smog). Some summer days, especially in July, the hot air is also very humid; thunderheads form over the mountains setting fires on the ridge tops, and perhaps bringing brief thundershowers to valley areas close to the mountains. This air mass comes to us from the southeast, having gained its high water content from the Gulf of Mexico.
Sometimes, in the very late summer, periods of intense rainfall may occur, related to storms in the tropical Pacific called chubascos, a Mexican name for a cyclonic disturbance more generally called a hurricane. Apparently chubascos crossed the coastline fairly frequently in the 19th century, but they have not been common in the 20th century. However, one chubasco arrived in September 1976, and two in August 1977, bringing heavy rainfall to Southern California. These were the first chubascos on the mainland since the late 1930’s.
September may have the most unpleasant weather of the year, exceptionally hot with stagnant circulations which fail to disperse air pollutants. This horrid air mass comes from the southwest, hot because it is imported from a very hot region in southern Arizona and the lower Colorado River Valley. But sometime in October the first intrusion of cool winter weather comes in from the sea, usually too weak to give us rain, but then followed by the first Santa Ana winds from the east, often very hot and dry. It is these winds that have fanned the most serious brush fires in recent years. These autumn fires are almost always human caused, and may burn vast areas of dry brush and houses. Natural summer lightening-caused fires do not burn as fiercely, due to the high humidity and weak winds.
Claremont’s special version of the general Mediterranean climate poses particular problems for the organisms that live here. It is always very interesting to observe and understand the special adaptions of our biota for our climate.
© 2001-2013 Bernard Field Station Faculty Advisory Committee
Page last updated 23 August 2013 by Nancy Hamlett.