Periodically, southern California experiences extreme foehn-type winds locally called Santa Ana winds. Throughout the year, winds are primarily from the west-northwest, tending to increase throughout daylight hours and becoming east-northeasterly at night. Preliminary data from Santa Cruz Island suggests that geographic variation in the presence and duration of the fog layer has a profound influence on the temperature and humidity regimes. Fog frequently is thicker and lingers longer into the day offshore than along the mainland coast. The marine layer fog flows down the coast with the prevailing NW wind, and bends around Point Conception, usually blanketing San Miguel and Santa Rosa, and often the western portion of Santa Cruz Island. Fog is most common in spring and summer, and west of the Santa Cruz Channel. Annual rainfall is about 14” per year but depending upon the topographical features of a particular location, rainfall on the islands might range from less than 10 inches to more than 20 inches annually.įog is a common weather feature, especially at San Miguel and Santa Rosa Islands. Rainfall patterns also vary geographically within the Channel Islands National Park. Extended multi-year droughts punctuated by moderate to extremely wet years are not uncommon. Long periods may occur between storms in a single season, and enormous variation exists in yearly rainfall totals. Regional rainfall patterns are highly variable and unpredictable. Evaporation exceeds precipitation from April to November. June, July and August are the driest months (1% of annual mean rainfall). The dry season generally extends from May-October. Most rain comes from large storms that last for several days. January and February are the rainiest months (45% of average totals). On average, 95% of the rainfall in the Channel Islands occurs between November and April. In the afternoon, readings on average reach around 60%. At night and in the early morning relative humidity often reaches 100%. Relative humidity in and around the Channel Islands varies diurnally. This microclimate experiences freezing temperatures most years, higher average summer temperatures, and greater diurnal temperature variation. The one exception is the central valley of Santa Cruz Island where its inland location and surrounding high mountains create a microclimate more characteristic of a warm Mediterranean climate (Csa) such as that found on the mainland away from the immediate coast. The Channel Islands are frost free for the most part. As with rainfall, fog duration, and relative humidity there are significant variations in temperature between the islands. The exception to the norm of relatively cool temperatures and high humidities occurs occasionally in the fall with Santa Ana wind conditions when temperatures dramatically increase and humidities decrease. Diurnal temperature differences are small with cool days and warm nights. Both winter and summer temperature extremes are moderated by the moist ocean air with generally high nighttime humidities and frequent fog. The average mean temperature in January ranges from 53-59 degrees F summers are a little warmer with an average mean temperature in July that ranges from 62-70 degrees F. Unfortunately data collected from these stations has been irregular so that the historic weather analysis provided below is derived from other published records of Channel Islands climate data.ĭecember-March are the coolest months and July-October the hottest months in the Channel Islands. Historic data from San Miguel Island are also located at the WRCC website. Current and historic data from these stations are available at the Western Regional Climate Center (WRCC) website. There are currently five Remote Automated Weather Stations (RAWS) stations at Channel Islands National Park. In the Köppen system of climate classification this is a cool Mediterranean climate (Csb) or a Mediterranean Dry Summer Subtropical climate type, with a cool summer regime. On the Channel Islands the climate is moderated by the marine influence with the result that summers are milder, there is generally higher humidity, and there is more frequent nocturnal fog compared to the mainland. These climate regions occur in only five locations throughout the world including parts of California, along the Mediterranean Sea, central Chile, parts of southwestern Western and South Australia, and the southwestern Cape region of South Africa. This climate type is characterized by cool, wet winters with average annual rainfall of 8-40” and hot, dry summers. Channel Islands National Park has a Mediterranean-type climate.
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