Too much so, in fact, for some of Napa's residents, who strongly objected when a proposed wine train threatened to deposit compartment-loads of tourists on to their patch. As a result, the Napa Wine Train chugs, but the passengers stay on board to eat, drink and admire a perfect cross-valley vista, uncluttered by tourist and wine industry development.
Between the Mayacamas Mountains and the Pacific Ocean, Sonoma County is a rather different proposition from Napa. It is bigger, more diverse and more relaxed. Napa lifestyle demands a certain formality, and wants to know why if you turn up in shorts and a T-shirt. Sonoma wants to know why not if you don't.
Sonoma doesn't have to squeeze all its charms into one small valley, but sprawls north towards Mendocino and Lake County, blending ocean scenery, redwood forest, mountain and canyon before giving way to orchard, meadow and vineyard.
A meandering drive along the coastline up Highway One from the fishing village of Bodega Bay towards the more rugged coastline and primeval forests of Mendocino is a bracing experience. In summer, ocean breezes can dramatically lower temperatures on the beaches. The drive takes you through Fort Ross, the southernmost outpost of the Russian sea otter trade, and on to Salt Point State Park with its towering redwoods and pygmy cypresses.
Where viticulture dominates the Napa Valley in almost relentless, albeit multihued, monochrome, the vineyards of Sonoma recall the culture and diversity of Tuscany's hills.
Sonoma Valley is one hour's drive from San Francisco, tucked between the protective Sonoma Mountains and Napa's Mayacamas range. The last of the Spanish missions, San Francisco de Solano, was built in Sonoma in 1823, to christianise the natives. Here for 25 proud days, Sonoma was capital of the independent republic of California, three years before the gold rush of 1849. The history of the Spanish and Mexican eras is particularly well-preserved and numerous false-front buildings and Italian basalt structures form Sonoma State Historic Park.
North of Sonoma in the hills above the little town of Glen Ellen is the 800-acre Jack London State Historic Park, where Jack London tried to create his socialist utopia. More than 80,000 visitors drift annually through the tasting room of the family-run Glen Ellen winery, which nestles in the hills behind the town. Nearby, in Bouverie Wildflower Preserve, off Highway 12, M F K Fisher, the reclusive food writer, wrote How to Cook a Wolf, among other books.
In the heart of Napa Valley, vineyards carpet the alluvial valley floor in patchwork greens, occasionally creeping up the mountainsides to meet beech, sycamore and valley oak. Once off the valley floor, the pace grinds to a halt and the views become spectacular. Gliders circle at eye-level, passable imitations of the ubiquitous turkey vultures, which scavenge for the carcasses of rabbits and other prey.
Distances in Napa are small. So are the neat oleander and clapboard towns of Yountville, Oakville, Rutherford, and St Helena, bisected by Highway 29. Parallel to Highway 29, the Silverado Trail, which hugs the eastern floor of the valley, offers a quieter, more scenic route, and is easily accessible from any of the four towns.
Closest to San Francisco, the cool climate of the Carneros district, whose hour-glass figure spans both Napa and Sonoma County, funnels icy blasts from San Pablo Bay. The naturally high acidities of the area, which 20 years ago was used to farm sheep and cattle, make it a favourite for the early ripening grapes, pinot noir and chardonnay. Further north, Napa Valley gets hotter - here cabernet sauvignon comes into its own. Sonoma, however, closer to the ocean, stays cool.
Most Napa wineries are open for cellar-door sales, but many are either ill-equipped or ill-disposed to welcome visitors with open arms. They are more likely to be friendly if you ring first. Some of the smaller wineries double as homes, so they tend to announce their presence discreetly by a mere number on a mail box by the roadside. While trying to find one winery in the dark of the Sonoma backwoods, I discovered that knowing the precise address can mean the difference between having supper and going hungry.
If your time is limited to a single visit, the obvious choice in Napa is the Robert Mondavi Winery. Although the best known winery in Napa, it is no cliche. Cliff May's elegant mission-style winery, just off Highway 29 between Oakville and Rutherford, exudes the dynamic presence of its creator.
If you have longer, drop in to Sterling Vineyards at Calistoga, founded by an Englishman, Peter Newton - hence the name. To reach the winery and retail room, you take a toytown cable car up through towering redwoods to the top of the knoll opposite Diamond Mountain for a 'self-guided tour' (ie, you walk round the winery and admire the dramatic views of the valley). Sterling's whitewashed winery has the feel of a Mediterranean island monastery. The impression is reinforced by three giant arched bell towers which house St Dunstan's imposing re-cast bells, a sort of Bunuel meets Dada in Napa.
The degree of architectural diversity comes as something of a surprise to European eyes used to pigeon-holing winery styles to regions. At St Helena, the 'wine capital' of California, Beringer's Victorian gabled Rhine house was built in 1883 as a replica of the ancestral home in Mainz, Germany.
Some of Champagne's most prestigious houses, Mumm, Moet et Chandon and Taittinger have prospected the cooler parts of Napa, Sonoma and Mendocino in search of the golden bubble. Domaine Carneros, Taittinger's joint venture, sticks out from the side of a knoll, a Disneyland replica of a French chateau, with views of the rolling hills across Marin County towards San Francicso. Behind the facade, you can see the champagne process, American style, and sit sipping a glass of brut at dollars 4 a throw in the polished limestone and pink marble-floored tasting salon. Vivaldi's Four Seasons is piped to remind you - lest you forget - of the cycle of the vine.
Flights: Until the end of September Trailfinders (071-937 5400) is offering a London to San Francisco return with United Airlines for pounds 392: there is a minimum stay requirement of seven days, maximum stay of six months.
Accommodation: Napa and Sonoma have a range of accommodation, from B&B to luxurious country house hotel. Napa Valley Tourist Bureau Reservations, 6488 Washington St, Yountville, CA 94599 (707 944 1558/258 1957).
Car hire: In its Affordable USA programme, Hertz (081-679 1799) offers a week's car hire from San Francisco from pounds 79.
Packages: On 24 September Arblaster & Clarke Wine Tours (0730 266883) is running an eight-night wine holiday to California costing pounds 1,279.
(Photograph and map omitted)Reuse content