In the weather app I use on my phone, there is a map view with configurable overlays. One of the most useful is the Doppler Radar overlay, which usually shows precipitation level on a colorful scale.
One thing I've noticed on the radar overlay is that there is an area in the north bay, near Fairfield and Rio Vista that appears to always have storm-level activity over it.
<a href="http://www.flickr.com/photos/thenobot/6954885862/" title="NEXRAD radar overlay by thenobot, on Flickr"><img src="https://farm9.staticflickr.com/8142/6954885862_9acf5ab335_z.jpg" width="640" height="547" alt="NEXRAD radar overlay"></a>
It's near the bay, so I had thought that it was possibly a swamp or marsh area with high levels of evaporation that was confusing the radar. Or perhaps it was the Bay Area's version of the Bermuda Triangle and there was indeed a raging storm over that spot. Searching the web didn't turn up anything useful. So I decided to take a closer look in Google Earth.
Here's a view of the overall area... <a href="http://www.flickr.com/photos/thenobot/6954897770/" title="Rio Vista - Far by thenobot, on Flickr"><img src="https://farm8.staticflickr.com/7108/6954897770_c89f403a7f_z.jpg" width="640" height="476" alt="Rio Vista - Far"></a>
Zooming in a bit shows some clues... <a href="http://www.flickr.com/photos/thenobot/7100966993/" title="Rio Vista - Medium by thenobot, on Flickr"><img src="https://farm8.staticflickr.com/7044/7100966993_4b8a674191_z.jpg" width="548" height="435" alt="Rio Vista - Medium"></a>
Zooming way in gives us our answer... <a href="http://www.flickr.com/photos/thenobot/7100966935/" title="Rio Vista - close by thenobot, on Flickr"><img src="https://farm9.staticflickr.com/8019/7100966935_b5fce8cf0f_z.jpg" width="639" height="478" alt="Rio Vista - close"></a>
Aha! Windmills! The spinning blades are indeed motion in the air. This is exactly what Doppler radar is designed to pick up. The weather radar apparently cannot distinguish between precipitation and spinning windmill blades.
Looking back at the weather radar, now I see that there is another similar area of perpetual storm near Altamont, CA, which is a more familiar wind-farm area.