Contra Costa County Weather

 

When is the best time to have a picnic or reroof a home or stage an outdoor wedding? Although generally delightful, the local weather is fickle but some predictions can be safely made. For one, rain almost never falls in the summer. For another, the wettest months are December and January.

Contra Costa Public Works Department figures show that the true rainfall in July and August is about one hundredth of an inch. You’re thinking, “I’ll have the wedding, but watch — on that day that little bit of rain will fall.” Take a chance.

Mild, Diverse Climate

The Bay Area enjoys one of the mildest climates on earth and one of the most diverse. While Antioch and Byron residents swelter in 90-degree sunshine, fishermen at Stinson Beach in Marin County may shiver in parkas. On the Contra Costa side of the Caldecott Tunnel, the sun shines. On the Alameda side, during the summer, the sun often hides behind ocean fog and the air becomes chilly. In the early winter, this pattern reverses: Contra Costa County is often chilled by “valley fog” while Alameda County, shielded by the hills, enjoys winter sunshine.

The Extremes

Temperatures rarely drop below freezing, but almost every winter Mt. Diablo dons a mantle of snow. In the 1975-76 season, the county endured one of the worst droughts of the century. (“Endured” is not quite precise. We had a glorious Indian Summer throughout the winter, but water had to be rationed.) For the 1990s and 2000s, it has been extra rainy in some years and normal or a little dry in others.

Understanding Contra Costa weather is easy, if you know the roles played by five actors: the sun, the Pacific, the Golden Gate, the hills and the Central Valley.

The Sun

In the spring and summer the sun moves north, creating a mass of air called the Pacific High. The Pacific High blocks storms from the California coast and dispatches winds to the coast. In the fall the sun moves south, taking the Pacific High with it. The winds slough off for a few months, then in bluster the storms. Toward spring, the storms will abate as the Pacific High settles into place.

The Pacific

Speeding across the Pacific, the spring and summer winds pick up moisture and, at the coast, strip the warm water from the surface and bring up the frigid. Cold water exposed to warm wet air makes a thick fog. In summer months San Francisco often looks like it is about to be buried by cotton candy.

The Golden Gate

This fog would love to scoot over to the East Bay but Mt. Tamalpais and the hills running up the San Francisco peninsula stop or greatly impede its progress except where there are openings. Of the half dozen or so major gaps, the biggest is the Golden Gate.

The fog shoots through the Golden Gate in the spring and summer, visually delighting motorists on the Bay Bridge, and bangs into the Berkeley, Kensington and El Cerrito hills. Without the hills, Lafayette, Orinda and Moraga and much of Central Contra Costa would be decidedly cooler.

As it is, some fog does spill over, giving the Lafayette-Orinda area and the San Ramon Valley cooler summers than the East County. El Cerrito, almost opposite the Golden Gate, catches some fog, but Richmond and other towns in the West County usually escape with just the cooler air.

The Carquinez Strait and Sacramento River are at sea level and allow the clouds to penetrate inland, until the Valley sun burns them off.

Frequently in the summer, a long finger of fog from the Golden Gate reaches out and touches the top of Mt. Diablo. On some days, fog forms over the Carquinez Strait and runs like a wide road straight into the Delta.

The Central Valley

Also known as the San Joaquin Valley and located about 75 miles inland, the Central Valley is more influenced by continental weather than coastal. In the summer this means heat. Rising hot air pulls in cold air like a vacuum. The Central Valley sucks in the coastal air through openings in the East Bay hills, mainly the Carquinez Strait, until the Valley cools. Then the Valley says to the coast: no more cool air, thank you. The suction gone, the winds taper off for a few days.

With the winds down, the fog stays offshore and San Francisco enjoys some sunny days. Meanwhile, lacking the cooling fog, the Valley heats up again, creating the vacuum that pulls in the fog and renews the cycle.

This cycle has its daily counterpart. As the sun’s rays weaken in the late afternoon, the fog will often steal across the Bay to be burned off the following morning by the robust sunlight.

In the fall and winter the temperatures are reversed. The Central Valley grows colder and the Pacific Ocean, which is warmer than the land in winter, sends its balmy breezes over the coast.

Again the hills impede the coastal flow. San Francisco, in the winter, is warmer than Berkeley and the West Contra Costa communities, which are warmer than Orinda and Walnut Creek and Danville. These towns are warmer than Antioch and Brentwood.

Also in the winter, “Valley” fog, attracted to warm air, moves toward the Bay Area but, except for openings like the Carquinez Strait, is blocked by the hills.

The Hills

Besides blocking the fog, the hills greatly influence the rain pattern. When storm clouds rise to pass over a hillside, they cool and drop much of their rain. Orinda and Moraga, which stand against the Oakland-Berkeley hills, get the most rain in Contra Costa, Brentwood and the East County the least.

Records kept over 100 years put the average annual rainfall atop Mt. Diablo, 3,849 feet, at 25 inches. But about 1,000 feet lower, at the junction of roads from Walnut Creek and Danville, the annual rainfall is 22 inches.

Minor variations like this are common throughout Central Contra Costa County. The rain dances to the tune of the hills. In the San Ramon Valley, rain gauges placed on the Las Trampas ridges catch a few more inches annually than those placed on hills east of the freeway. The clouds that pour 32 inches on the hills above Orinda can manage only 28 inches at the Lafayette Reservoir and 22 inches at the outskirts of Walnut Creek (100-year records).

Lastly, the hills buffer some towns against the wind. Martinez and Benicia are built on the Carquinez Strait, one directly across from the other. Benicia has a great view of the strait but suffers when the wind comes whistling through the inland passage. Martinez, nestled in the Alhambra Valley, sees less in the way of views but suffers less from the winds (although the waterfront gets gusty).

That basically is how the weather works (see, it wasn’t hard) but, unfortunately for regularity’s sake, the actors often forget their lines or fail to show up. Rainfall figures at the Walnut Creek station show how undependable nature can be. Totals ranged from 7 inches in 1976, the height of the mid-Seventies drought, to 39 and 43 inches in 1982 and 1983, respectively, when El Niño acted up.

Even when erratic, however, the weather is almost always mild. Rainy winters cause slides and road washouts and bring many complaints but, as much as Contra Costans grumble, they shiver with delight when they sit in front of the television and see what mischief Nature raises in the rest of the nation.

Sunshine

Like sunshine? You are in the right place. Records show that during daylight hours the sun shines in New York City 60 percent of the time; in Boston, 57 percent; in Detroit, 53 percent; and in Seattle, 43 percent. In San Francisco, the sun shines during 66 percent of daylight hours. Atop Mt. Tamalpais, where conditions are comparable to those in Contra Costa County, the sun shines 73 percent of daylight hours.

Humidity

Although moist, the coastal climate is rarely muggy. The fog keeps the temperature too low. Summer heat is usually dry.

Fire Danger

In late fall, usually October, winds whistle down the San Joaquin Valley, losing their moisture, and blow hot and dry into the Bay Area. Locally, these winds are called Diablos and they bring extreme fire danger. The 1991 Diablos fanned an ember into a roaring fire that killed 25 in the Oakland and Berkeley hills and destroyed 2,500 homes and apartments. Be careful.

Allergies

They often kick in during the spring and in October. During the spring, the grasses pop their buds and many trees release pollen.

In the fall, the Diablos (see fire danger) dry out the trees and cones and pollen fills the air. Hanky time.