Climate Sonoma County has a Mediterranean climate meaning wet relatively mild winters and warm dry summers. This is only a generality however because there are many micro-climates. Summer is where the micro-climates come into play. Because the Pacific Ocean's current comes from the north the water is always about 55 so coastal areas may be much cooler than inland areas protected from the ocean by the coastal hills. A layer of low clouds and fog often moves inland during the night and retreats in mid-morning.
Summer evenings are usually quite cool.
It's the warm days, cool nights and lack of rain during the growing season that makes for premium wine. Don't bother with ties or dresses. A few years ago a new Holiday Inn moved into Santa Rosa. They instituted a dress code for their restaurant. Luckily they sold out to someone else.