The temperature pattern within the County is profoundly influenced by the temperature of the sea-water immediately off the coast. Average values of sea-water temperature range from around 53° in winter to 55° in late summer and fall. Because of these water temperatures, air temperatures over the land remain very cool during the summer, particularly during the night hours, and the warmest part of the year is found in late summer or in the fall. This pattern is most pronounced near the coast. Warm season minimums average below 50° at most points and average less that 54° even in the Cloverdale area.

The greatest temperature variation across the County is found during the summer, a period when sharp contrasts may be found between the marine dominated coastal area and interior areas that are relatively continental in character. The mean daily maximum temperature is only 64° along the extreme north portion of the coastline in July, increasing to 65° in September. In contrast, the mean daily maximum is 92° in the vicinity of Cloverdale. This increase of more than 25° takes place in an airline distance of less than 25 miles. High temperature readings have exceeded 100° at all inland points and have reached 112° or higher at several stations within the County. Along the immediate coast, however, extreme temperature readings have never reached 100°.

Winter temperatures are generally mild, although occasional cold spells have been recorded. The mean minimum temperature in January ranges from around 42° along the coast to 36° or 38° over most of the cultivated area and on down to around 32° in the higher mountain areas. All-time lows have dropped to as low as 14° in the coldest spots in the mountains, and are generally in the range of 15° to 20° throughout the central part of the County. Coastal stations have reported lows of only 28° to 30°. Even during January, however, relatively warm temperatures are typical of the afternoons. The January mean daily maximum temperature ranges from around 57° at lower elevations to 55° in the mountains.