There are a lot more vineyards in the Petaluma Gap region than tasting rooms, so it can be hard to understand exactly what the area's wines are all about.

The region's grapes are sourced by such diverse producers as Kosta Browne, Arnot-Roberts, Bedrock, La Follette, Wind Gap and Williams Selyem, which each seek its mix of cool-climate aromatics and acidity with intensity of fruit.

These and other producers source from some 70-plus grape growers, with such vineyard sites as Gap's Crown, Griffin's Lair, Clary Ranch, Sun Chase, La Cruz and several Sangiacomo vineyards among the most sought-after.

"The Petaluma Gap is coming into a new maturity," said Ana Keller of Keller Estate, among the few wineries based in the region. "The biggest difference (from other parts of the Sonoma Coast) is that we're a bunch of growers mostly, but we are also clearly different in terms of weather and soils."

Marked by pronounced fog in the morning and late afternoon -#8212; Keller says she gets fog until about 11 a.m. -#8212; the Petaluma Gap enjoys midday sun and wicked afternoon wind that blows over the vines. As a result, the grapes tend to retain thicker skins, higher acidity and, thanks to substantial hang time, powerful flavors.

"Someone once pointed out to me that a lot of regions get fog, but the Petaluma Gap owns the wind," Keller said.

She thinks that wind contributes a unique set of tough conditions to the vines and that the combination of clay loam and sandy loam soils with a mix of gravel coaxes a minerality out of them, too. She also likes to remind people that many of the vineyards within the Petaluma Gap, specifically in and around Lakeville Highway, were once under water, part of the San Pablo Bay seabed.

"I think the quality of our vineyards has been moving forward," Keller added. "We can grow world-class pinot noir, chardonnay and syrah. But for the consumer, it's more about looking for vineyard designates at this point."

Pinot noirs from the Gap are known to exhibit flavors of rich red and blue fruit alongside savory spice and wild herbs, while chardonnay often is fairly bright in acidity, more tart than sweet. But it's the syrah that is most often revelatory, offering a cool-climate temperament that's as much about structure as power, with flavor highlights of white pepper ?la Northern Rhone.

More loose marketing association than official designation (there is no Petaluma Gap appellation, for now), most of what is considered the Petaluma Gap officially is within the Sonoma Coast appellation, a distinction with plenty enough cachet.

Still, conversations are beginning among members of the Petaluma Gap Winegrowers Alliance about what an AVA designation might mean. For now, it would follow the generally accepted borders of West Marin and Valley Ford to the west and Chileno Valley and Spring Hill Road to the east. Cotati is the farthest point north and Lakeville the demarcation to the southeast.

The Gap in Petaluma Gap is created by Pacific Ocean winds that flow between Tomales Bay and Bodega Bay through a 15-mile-wide gap in the coastal range mountains. That wind barrels through on its way to San Francisco Bay. Mornings are marked by fog, days by sun and early evenings by coolness and nighttime fog, with average temperature swing of 40 to 50 degrees F.

The grapes tend to hold on to a fair bit of acidity, with small yields the norm. Pinot noir, chardonnay and syrah are mostly planted, about 3,000 acres in all. Spending mornings and afternoons in the fog contrasts the Petaluma Gap from what many call the "true" Sonoma Coast and the newly defined Fort Ross-Seaview AVA, where vineyards rise mostly above the fog.

<em><strong>Wineries to visit</strong></em>

<strong>Azari Vineyard:</strong> Due west of Petaluma, Azari makes pinot noir, zinfandel and shiraz, opting to call it shiraz rather than syrah in homage to the founders' Persian roots. Visits by appointment only. 1339 Spring Hill Road, Petaluma, 658-0707, <a href="http://azarivineyards.com" target="_blank">azarivineyards.com</a>.

<strong>Cline Cellars:</strong> Cline is well known in Carneros but also farms a property in Petaluma, finding it cooler, with grapes able to hang longer on the vine to develop flavor. Its 2012 Cool Climate Pinot Noir comes from the Gap and at only $18 is a good one to know. Open daily 10 a.m. to 6 p.m. 24737 Arnold Drive, Sonoma, 940-4061, <a href="http://clinecellars.com" target="_blank">clinecellars.com</a>.

<strong>Enriquez Estate Wines:</strong> Specializing in pinot noir along with tempranillo and a white blend called Brisa, all from its estate-farmed 20-acre vineyard called the Flying Rooster, Enriquez also grows petite sirah, muscat and sauvignon blanc. Open by appointment only. 3062 Old Adobe Road, Petaluma, 347-9719, <a href="http://enriquezwines.com" target="_blank">enriquezwines.com</a>.

<strong>Keller Estate:</strong> Among the most visible wineries in the Petaluma Gap, Keller also is one of its finest, producing gorgeously made pinot noir, syrah and chardonnay from its estate vineyards, as well as selling some of its fruit to other fine producers, particularly from its La Cruz Vineyard. It also makes a small amount of dry, Alsatian-style pinot gris. Open Friday-Sunday 11 a.m. to 4 p.m. with cave tours and barrel tastings by appointment. 5875 Lakeville Highway, Petaluma, 765-2117, <a href="http://kellerestate.com" target="_blank">kellerestate.com</a>.

<strong>Saltonstall Estate:</strong> Saltonstall offers pinot noir, a sparkling wine and olive oil from its 170-acre estate five miles west of Petaluma with 40 acres planted to pinot. Grape growers primarily, Saltonstall began making a little bit of its own wine in 2008. Open Saturday 10 a.m. to 4 p.m. 2805 Spring Hill Road., Petaluma, 658-2581, <a href="http://saltonstallestate.com" target="_blank">saltonstallestate.com</a>.

<strong>Windy Hill Winery:</strong> Built on land once used by the artist Christo for his Running Fence art project, Windy Hill makes chardonnay and pinot noir. Open Friday-Sunday by appointment. 1010 W. Railroad Ave., Cotati, 795-3030, <a href="http://windyhillwinery.com" target="_blank">windyhillwinery.com</a>.

<em>Virginie Boone is a freelance wine writer based in Sonoma County. She can be reached at virginieboone@yahoo.com and followed on Twitter @vboone.</em>