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Climate data presented here are based on historical records for the same Cheeseboro (CHE) weather station used to report weather data in my gardening diary. (CHE is approximately 2 miles ENE of my house.) Thus, quantitative data on this page relate to the immediate vicinity of my garden and the community in which I live. However, qualitative and descriptive aspects of this page apply generally to southern California excluding our mountains and deserts.
Excluding the mountains and deserts, much of southern California has a Mediterranean climate.
Beyond the Mediterranean area, this climatic type prevails in much of California, in parts of Western and South Australia, in southwestern South Africa, in isolated sections of Central Asia and in parts of central Chile. … The climate is characterized by warm to hot, dry summers and mild to cool, wet winters.
Sunset's Western Garden Book classifies Oak Park within its zone 21 — an interior area with some ocean influence, on a slope away from which cold air drains in the winter — very near zone 19 (which gets more winter chill). This is USDA zone 10a. The National Weather Service (NWS) places Oak Park in forecast zone CAZ045 (Ventura County Coastal Valleys); we are very close to forecast zones CAZ046 (Santa Monica Mountains Recreational Area) and CAZ547 (Los Angeles County San Fernando Valley).
Note that a detailed discussion of humidity is not found here. Relative humidity fluctuates wildly and widely during each day. In May 2012, relative humidity at the CHE weather station had an average change from the daily high to the daily low of 55%, with a maximum change of 92% and a minimum change of 25%. Humidity is also strongly affected by whether a garden nearby was recently irrigated and thus fluctuates significantly across very short distances. Therefore, humidity data from a weather station approximately 2 miles away do not provide a really meaningful picture of the climate in my garden. The humidity that I report in my diary is from the CHE weather station — where artificial irrigation does not exist — and not from my garden.
Over the nine-year period from 2006 through 2014, the record high was 110°F; (3 September 2007); and the record low was 32° (13-14 January 2007). The following table shows monthly data over that same period, plus partial data for 2015. Note that there is more nighttime cooling in the summer (often more than 30°) than in the winter (often less than 15°).
In much of southern California, two miles — the distance from my garden to the CHE weather station — can mean a noticeable difference in micro-climates. While this table indicates that temperatures never dropped below 32°F, pockets of sub-freezing air might become trapped in my back yard by my house. Thus, I do occasionally see frost damage to plants in my garden.
In many areas of southern California 2004-2005 was the wettest rain year of all time, exceeding any records kept for more than a century. During a storm in that rain year, My Hill failed. On the other hand, 2012-2013 was the driest rain year for many areas (not Oak Park), drier than 2006-2007.
East of the Rocky Mountains, summer is the wet season. Despite all the snow that falls in eastern winters, there is much more moisture in eastern summer rains. On the other hand, west of the Rocky Mountains, winter is the rainy season or at least the "not-so-dry" season. Since the rainy season in California straddles the change of calendar years, annual rainfall totals here are measured across a rain year that officially begins on 1 October and ends on 30 September of the following calendar year.
Most rain falls in southern California in the months of December, January, and February, which together often account for more than 75% of the annual total. While measurable rain has been recorded in every month, it is not unusual for more than 200 days to elapse in the summer without any rain at all. Twice since the rain year 2003-2004, no measurable rain fell at all during the months of May through September.
The average annual rainfall in Oak Park over the rain years 2003-2004 through 2013-2014 is 11.7 inches. However, the annual total rainfall can be quite variable as seen in Table 2. Because of this variability, the probability is 90% that the rainfall will be within the broad range from none to 24.0 inches in any given year. Thus, the average may be a very poor predictor of expected rainfall.
Strong, dry winds from the north-east — the dreaded Santa Ana winds — plague southern California every year, generally from mid-fall through the winter (although they can occur at any time of the year). These are caused by cold, high-pressure air masses extending over the Great Basin area of Nevada and Utah. As the air descends into lower altitudes, it warms by compression, which increases its capacity for holding moisture. However, this air is descending through an arid region and thus does not gain any significant moisture. Thus, by the time it reaches southern California, it can be very dry. In October, relative humidity below 10% (or even too low to measure) and temperatures over 100°F are possible.
Santa Ana winds are strong and constant. Sustained winds of 30 mph are common. Where funneled by canyons and passes, gusts of 50 mph or more are not unusual. During the 24 hours of 6 January 2003, a Santa Ana wind maintained an average speed of over 34 mph at the CHE weather station, never dropping below 25 mph. Several gusts on that day exceeded 75 mph, which defines a hurricane; the maximum gust was 92 mph. Yet actual hurricanes are unknown in the area. A major difference is that hurricanes blow in a circular pattern and bring heavy rain while Santa Ana winds blow in a generally straight line (except where diverted by mountains and canyons) and are very dry. While my garden is partially shielded from the Santa Anas by My Hill, I still must use bird mesh to hold the mulch in place around my oak tree in front.
During a Santa Ana condition, the wind may blow steadily from the north-east for several days without interruption. However, the wind sometimes seems absent, with hot, dry air creeping towards the ocean so slowly that that it seems calm. In the winter, the reverse is possible: Strong winds blow, but compressional heating is insufficient to warm air that was sub-freezing. In any case, the extreme low humidity sucks moisture out of plants and soil — and out of my skin.
In most places — including southern California — a season does not begin on the precise date of an equinox or solstice.
If winter begins with the first nighttime frost, it might begin in November, before Thanksgiving. Or it might not begin until January. Sometimes, we have a mild winter without any frost. Note that hard surfaces may radiate much of their heat into a clear sky at night and then be coated in frost even though the air temperature does not drop to 32°F or below.
Since I moved into my current house in 1973, I have seen snow here only three times. Never did the snow linger; it always melted within a couple of hours after falling. Only once have I experienced a killing freeze, in which I only lost a few tender perennials. Although my dwarf citrus and eugenia (Syzygium paniculatum) are occasionally damaged by winter cold, I have never had a woody plant die from a freeze. While I have some subtropicals, I have no true tropical plants in my garden
Excessive winter chill limits the use of tropical and subtropical plants. On the other hand, a lack of winter chill is a limiting factor for growing other plants, especially deciduous fruit trees and some bulbs. Over the 15 winters from 2000-2001 to 2014-2015, the annual average of winter chill — cumulative hours below 45°F from November through March — was 333 hours. Table 3 shows the chill for those years. Although temperatures have dipped to 45°F or below in October, April, and even May in some years, those months are not considered when computing winter chill because plants are not sufficiently dormant then in southern California to benefit from any chilling. Note that chill hours accumulating in February or March might not be effective if a warm December or January causes plants to "break" their dormancy; this happened in the winter of 2011-2012, when significant chill hours accumulated in the second half of March (almost 25% of the winter's total) after my roses and peach tree were already in leaf because of a warm January and February. Also note that how much below 45°F the temperatures might drop is irrelevant; two hours at 44° gives the same amount of winter chill as two hours at 30°.
Winter is the time for cool-season annuals. This includes cole vegetables such as cabbage and broccoli and flowers such as pansies, stock, snapdragons, and Iceland poppies. These do best if they are planted in October or early November so they become established before the first frost.
With frosts possible in March, spring generally begins in April. However, my garden sometimes acts as if it were spring as early as the end of February. This merely reflects the significant variability in spring weather. Often, a heat wave (temperatures exceeding 90°F) will occur in April or May, followed by overcast, cool days. Indeed, April can experience both nighttime frosts and heat waves.
In any case, spring ends with June gloom, which might start in May and last into early July. June has many days that are cloudy, gray, and cool. The sun might not appear until afternoons, when it might be hazy and tempered by high fog. June sometimes brings drizzles and mists enough to require using wipers while driving but not enough to be measurable precipitation.
Through much of the summer, each day is the same: clear, sunny, and warm or hot. Note in Table 1 that July, August, and September are very much alike in temperatures. Thunder storms may enter the area from Mexico to the south, but measurable rain is rare. With daytime temperatures averaging at least 90°F, "heat wave" in summer means temperatures reaching 100°; such heat waves do occur every summer.
In the summer, late-afternoon breezes are sometimes generated when the rocky south face of Simi Peak bakes in the sun. Around 3:30-4:00 in the afternoon, the resulting thermal updraft draws cooling ocean air from Oxnard, 25 miles away to the west. Although the Pacific is only 10 miles away to the south in Malibu, intervening ridges of the Santa Monica Mountains block breezes from that direction. Unfortunately, the breezes fail too often. Fortunately, temperatures are generally 27°-30°F lower at night than in the daytime.
Summer is the time for hot-weather annuals such as marigolds, zinnias, cosmos, and salvia and vegetables such as tomatoes, zucchini, and melons. But these do best if planted in the spring so they can become established before summer heat places a major demand on roots for moisture.
Autumn has warm or mild days and cool nights. This season might be short if frosts occur in November. Actually, autumn might consist of only October plus a week or two of November. Summer weather might even extend into the beginning of October.
October is the prime time for planting and transplanting in southern California. The soil is still warm from summer, but the air has started to cool. This means that disturbed roots are encouraged to recover and grow while foliage places only a moderate demand on those roots to supply moisture. However, summer annuals and tropical and some subtropical plants are better planted in the spring, when frost is no longer a danger to new growth stimulated by planting. Of course, a few plants are best planted when they are dormant, in December or January.
23 February 2011
Updated 1 November 2014
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