Rising Air

This is the wettest place I've ever lived--over 44 inches of rainfall per year on average. Even a year with only half of average rainfall here would be wetter than the average rainfall of any place I've ever lived. This is the farthest west that I've ever lived and the farthest north I've ever lived in this hemisphere.

On the isohyetal map of West Marin in the "Natural History of the Point Reyes Peninsula" by Jules G. Evens, it shows 50+ inches of rainfall in the Kent Reservoir watershed. It really increases dramatically from Pt. Reyes, with only about 12 inches of rain a year at the lighthouse. I've moved from a rain shadow, where a reversed-but-similar-magnitude gradient exists east of the Sierra crest (close to 40 inches near the crest to 12 inches near Lee Vining), to an orographic hotspot. I'm now living on the wetter end of that gradient.

Peter was telling me about how the wettest area in Marin County is
just east of Mt. Tamalpais, around Kentfield, because of the way the rising air moves around the mountain. Olema Valley and the eastern slope of Inverness Ridge are wetter than the western side of the ridge. Typically we think that orographic precipitation is heaviest on the western slopes of mountains and a rain shadow exists to the east, however by the time the rising air at the top of the ridge produces precipitation, the precipitation doesn't reach the ground until the east side. See this Webpage for a detailed discussion of this in the San Jose area.

Two weekends ago we were at Limantour Beach. It was a sunny day with scattered low clouds. The fog had burned off, but humidity was high. There was a very strong west wind. Uncle David pointed out that the clouds were all dissipating at the same point in front of us, where you would be looking if you looked straight out to sea from where the path to the parking lot reaches the beach.

At first the mechanism causing the clouds to dissipate wasn't clear. The wind was blowing along the beach and there was no obvious reason for clouds to appear or disappear. But as we walked back to the parking lot over the dunes, I realized what was happening. The fast-moving ocean-saturated air coming off the sea to the west of the Point Reyes Peninsula was rising over the peninsula--forming clouds--and as the air descended on this side (the east side) of the outer peninsula, the clouds dissipated. The direction of the wind just happened to be parallel to the beach at this point. Without an understanding of the larger topography of the region, the reasons for the clouds would be mysterious.

It reminded me of a time when I was at the Guadalupe Dunes, part of the largest expanse of undeveloped coastal sand dunes in California, near the mouth of the Santa Maria River in Santa Barbara County. It was also a sunny day, with a strong offshore wind carrying moisture-laden air from the ocean and rising up over the dunes. The rising air caused a cloud to form overhead and dissipate inland. The cloud was stationary but the air was moving fast. This is also like the lenticular clouds that form over and in the lee of mountain ranges, a common sight during the cooler months in the Eastern Sierra. The clouds appear stationary while some of the most intense updrafts imaginable are occurring within them.