The Climate of Sonoma County

The climate of Sonoma County is characterized by moderate temperature and precipitation. Along the coast temperatures remain cool throughout the summer and seldom drop below freezing during the winter. Inland there is a wider temperature range, with high readings occasionally exceeding 100° and lows sometimes falling several degrees below freezing. Even during the warm period of the year, however, the night temperatures usually drop into the lower 50s. Precipitation is concentrated in six months of the winter period with only light amounts reported during the rest of the year. Along the immediate coast low clouds and drizzle at night during the summer may provide enough moisture to keep pastures green. Inland, however, the summer dry period is long enough that stored moisture in the soil is depleted and range dries up. Winds are relatively light most of the time, although they blow rather persistently during the summer, particularly during the summer, particularly in coastal areas. Sunshine is abundant during the summer over most of the county, except for considerable cloudiness along the immediate coast.


Sonoma County extends approximately 40 miles northwest and southeast along the Pacific Coast just a short distance north of the Golden Gate. It is about 40 miles in width from east to west, extending from the coast inland to the crest of the coast range. The county is elongated southeastward into San Pablo Bay, a northern extension of the San Francisco Bay. Elevations range from sea level along the western edge and in the southeast to 3,000 to 4,000 feet along much of the eastern boundary. The Russian River flows southward into the sea. The northwest corner of the county, between the coast and the river is mountainous, with elevations ranging upward to around 2,000 feet. East of the river elevations increase again; Mt. St. Helena, on the eastern border reaches a crest of 4,344 feet. Sonoma Creek drains into San Pablo Bay through the Sonoma Valley in the southeastern corner of the county. Much of the southeastern part of the county is relatively flat farmland near sea level.

Effect of Terrain on Climate

The coast range of mountains, east of the Russian River, provides a barrier that protects Sonoma County from the very hot weather of the central valley of California during the summer months. At the same time low elevations within the County receive enough sunshine during the summer without any import of hot air from the interior. The nearby Pacific Ocean, however, provides a source of cool, moist air during the summer, and the steady inflow of marine air holds temperatures at low levels over that part of the County through which it moves. As a result, the warmest area tends to be the Russian River valley near the north end of the county, where mountains deflect the marine air and diminish the cooling effect it provides elsewhere.

Rainfall tends to increase with elevation and also to increase toward the north. These are the principal factors that influence the distribution of precipitation within the County.


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.

Freeze Data

The average date of the last 32° temperature reading in the spring ranges through the month of March in most of the cultivates areas of the county, but may be as early as February 1st along the coast and as late as April 15th in the mountains. When freezes of 28° or colder are considered these dates range from January 1st along the coast to late March in the mountains.

The average date of the last 32° temperature reading in the fall is around the middle of November over most of the County, early November in the mountains, and later than December 31st along the coast. In the case of 28° readings the dates range from early December in colder spots to later than December 31st in warmer areas.

These dates give a mean growing season length, as determined by the 32° temperature readings, of 230 to 260 days in the central part of the County, as little as 200 days in the mountains, and as high as 340 days along the coast. For the 28° growing season the length ranges from 260 days in the mountains to 290 to 340 in much of the area to 365 days along the coast.


The average seasonal precipitation ranges from less than 20 inches in the extreme southeast corner of the county through 30 and 40 inches over much of the central part of the County. In the mountains of the northwest portion totals increase to more than 70 inches at some points, and in the northeast they increase to more than 80 inches.

Totals vary from season to season. In 1 season out of 10, on the average, the total ranges from near 10 inches in the dry, southeast corner to around 20 to 35 inches in the central area and to 40 to 50 inches in the mountains. At the other extreme, in 1 season out of 10, on the average, the dry corner receives more than 30 inches of moisture, the interior 40-50 inches, and the mountain 100 to 110 inches of rainfall.

Rainfall intensities also vary considerably across the County. The greatest 1-hour rainfall is expected to amount to .50 inch to .80 inch about once every 2 years, increasing to 1.10 inches to 2.00 inches once in 100 years. The maximum 6-hour precipitation may be 2.00 to 3.00 inches as often as once in 2 years, increasing to 3.8 to 6.3 inches with a

frequency of once in 100 years. Twenty-four hour totals are 3.00 to 6.00 once in 2 years and build up to 5.70 to 12.60 inches once in 100 years.


Most of the County receives but little snowfall, average seasonal totals of less than 1 inch being characteristic of all low elevation stations. In the mountains, however, snow falls with some regularity, and annual averages amount to 5 inches or more in the higher elevations of the northwest portion, and totals of more than 25 inches are reported from some points in the mountains of the northeast.


The term evapotranspiration refers to the total loss of water from the soil as a result of evaporation from the soil and transpiration from a crop growing in the soil. Thornthwaite has established empirical relationships between temperature and evapotranspiration. 1 Using these relationships it is possible to estimate the potential and actual evapotranspiration from a field well covered by a growing crop.

Since plant production of dry matter is approximately proportional to the transpiration of moisture through the plant, a comparison of potential and actual evapotranspiration provides a first approximation of the plant production that can be anticipated under conditions of irrigation and dry farmed conditions. The computed value of potential evapotranspiration (PET) provides information about the amount of moisture a full crop might utilize under existing conditions of temperature, provided the supply of moisture is not limited. This might be the case under irrigation. PET values for the entire year are presented in tabular and in chart form; they show around 25 inches of moisture utilization as a potential along the coast, increasing to 32 inches in warmer areas. This annual figure is of significance in the case of range or timber crops that grow under favorable conditions the year around.

If the same figure is limited to the growing season, as defined by the latest and earliest 32° temperature readings in the spring and fall, respectively, these figures become 25 inches along the coast and 28 inches in warmer areas. Change from the previous values is slight because the growing season covers much of the calendar year. The new figures are applicable to frost sensitive crops that can grow only between frosts.

In computing the amount of moisture a crop might use under dry farmed conditions (Ea) it is necessary to establish the amount of plant-available moisture that can be stored in the root zone of the soil. This will vary from one soil to another, but for purposes of comparison from place to place a figure of 4 inches is used in these computations. Highest readings of 4Ea for the year as whole are found along the coast, where 18 inches of moisture can be utilized by plants growing without the addition of irrigation. This figure drops to around 13 inches in some of the drier areas of the County.

A further reduction in potential plant use of moisture is noted when the period is reduced to the 32° growing season (4Ea32). For this value a total of 17 inches is indicated on the coast and only 8 inches in some interior points. A comparison of these figures suggests the possible production variation that may result from temperature, precipitation, and irrigation differences..

1. Thornthwaite, G. W., and J. R. Mather, The Water Balance. Publications in Climatology, Vol. VIII, No. 1. Drexel Institute of Technology, Laboratory of Climatology. Centerton, New Jersey, 1955.

On the basis of these computations it is possible to estimate the date when stored moisture supplies in the soil will be exhausted in a typical year. If we assume that the crop uses soil moisture at the maximum possible rate as long as moisture remains available, the earliest drying of range can be expected around May 15th in some of the low level points in the interior, while coastal points may find soil moisture adequate until early in July.


There are no records of evaporation within the County, but an analysis of existing data from nearby points suggests that the annual evaporation from a Class A 4-foot pan would amount to about 50 inches along the immediate coast, increasing inland to around 80 inches in the warmer, drier area. Evaporation from ponds and reservoirs would be about 75% to 78% of this amount.


Summarized wind data within the County are extremely limited. Records covering a 33-month period at the Santa Rosa Army Air Force base show the predominating wind direction to be from the south and southeast. Only 1/2 of 1% of the time were winds of 25 mph or higher reported, and winds in excess of 12 mph were observed 10.5% of the time. Winds were less than 4 mph 37.8% of the time.

Wind direction is profoundly influenced by local topography. In general it may be assumed that winds tend to blow inland from the ocean during summer afternoons in areas that are not protected from such incursions of marine air. Under similar conditions marine air may also flow northward from the San Pablo Bay Area over the southern part of the County. Winter storms bring strong southerly winds over most of the area.

It is estimated that wind speeds may reach 40 mph over most of the area as often as once in 2 years, increasing to 80 or 85 mph as often as once in 50 years.

Relative Humidity

The average relative humidity near the coast probably remains near 80% the year around, while inland areas range from 75% during the winter down to around 60% during the summer and fall. There is a steep humidity gradient during the summer between the marine air and the drier and warmer air of the inland locations.


During the winter months the sun may be expected to shine about 50% of the time between sunrise and sunset, and the figure seldom goes above 60% during the rest of the year along the immediate coast. Inland, however, sunshine increases during the summer

to 80% or more.

Most of the winter cloudiness is associated with migrating storms that move inland from the Pacific. The cloud patterns of these storms are essentially the same over all parts of the County. In the summer, however, the cloud patterns are more localized. Low clouds or fog persist during much of the summer in a narrow band offshore. Under typical summer conditions this cloudiness moves inland during the late afternoon and spreads across much of the county during the evening; higher elevations observe it as fog. By mid-forenoon the cloudiness starts to dissipate over inland portions of the County, and the cloud deck gradually receded toward the coast. By early afternoon the entire County is enjoying sunshine, except possibly a narrow band close to the ocean where the cloudiness may persist throughout the day.