In the Smith River watershed, summers are relatively dry and mild, and winters are cool and wet. In general, variations in climate in the Smith River watershed are caused primarily by distance from the coast and elevation. The Smith River watershed combines climatic characteristics of California and Oregon. The main climatic types present are California Mediterranean climate and the temperate oceanic climate typical of coastal Oregon and Washington (Trewartha and Horn 1980). Near the coast and/or at lower elevations, the climate is Mediterranean. The montane temperate oceanic climate is found farther inland and/or at higher elevations.

Temperature:

Because the entire Smith River watershed is within thirty two miles of the ocean, marine air flow moderates temperatures and increases humidity. Influence from continental air masses is subdued. Coastal areas at low elevation have high humidity and little variation in either daily or seasonal temperatures. Summers are mild and relatively dry although fog is often present. Winters are mild and wet. Due to the infrequent occurrence of cold temperatures, this coastal climate is classified as Mediterranean (Elford and McDonough 1974).

As distance from the ocean increases, marine influence decreases. Therefore extreme temperatures occur more frequently, giving inland areas more variation in temperature. There are larger daily and seasonal temperature fluctuations with more frequent frost (Elford 1970, Tables 8 and 9). Due to the occurrence of cooler temperatures during a larger portion of the year, the climate in inland areas is most similar to the montane temperate oceanic climate of coastal Oregon and Washington. Summer temperatures in inland areas often reach 80 to 100° F. Due to the high summer temperatures and reduced relative humidity, these areas become dry and prone to forest fires in the summer.

Moisture:

Annual rainfall in the Smith River watershed ranges from 65 inches (165 cm) on the coast to over 150 inches (381 cm) at Ship Mountain and Bear Basin (Table 10). Precipitation is highly seasonal with about 90% occurring between October 1 and April 1 (Table 11, Winston and Goodridge 1980, California Department of Water Resources 1970). During this period, storms come off the Pacific Ocean and move through the Smith River area, typically remaining for several days.

Distribution of precipitation is influenced by the interaction of air flow and topography. Greater amounts of precipitation fall on slopes that face southwest into the wind. Precipitation increases with elevation, but also decreases with distance from the coast (Tuffly 1995). Therefore the interior river valleys receive less precipitation. Below 4000 feet elevation, most precipitation falls as rain, although snow occasionally falls below 1700 feet. Above 4000 feet, snow typically remains for long periods. Average annual snowfall is 27 inches (69 cm) at Elk Valley, elevation 1711 feet (522 meters), and 126 inches (320 cm) at Monumental, elevation 2,420 feet (738 meters) (California Department of Water Resources 1970).

Particularly in the summer, fog is generated as warm moist air flowing from the Pacific Ocean is cooled by cold water welling up along the California coast (Warrick and Wilcox 1981). The moisture contribution of fog is significant in coastal areas especially during the summer.

During the winter, relative humidity is high throughout the watershed, ranging from 100% in the morning to 75-90% in the afternoon. During the summer, relative humidity in coastal areas tends to remain around 90% while relative humidity farther inland may drop to 40 or 50% (Elford and McDonough 1974).

Wind:

Although winds are generally light, high winds sometimes occur during winter storms and summer thunderstorms. In Eureka, a locale with a very similar wind regime, winds were calm 22% of the time, from the northwest or north 29% of the time, and from the southeast or south 19% of the time. The prevailing wind direction from November through March is from the southeast, and during April to October winds usually come from the north or northwest. On the average, wind speeds reach 40 to 50 mph once every two years and 80 to 90 mph every 100 years. Higher winds are expected in exposed areas while reduced winds are anticipated in sheltered areas (Elford and McDonough 1974).

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