Just about now,
in early spring, pollen starts to show up in the gutters.
In between the bursts of rain, early flowering trees—where I live it’s the plums, almonds, and acacia—all start producing pollen in abundance. It all washes down from the stamens, over petals and into the drains, leaving long strands of cadmium yellow along the rainwater’s edge. Fecundity’s promise, denied and discarded, but leaving brilliant streaky color along the way to sea.
In the San Francisco Bay area, those rains visit only between November and March, just five months of the year. Maybe because they’re so rare, they seem to bring wonderful moments of pure delight.
As I walked down the street just after a long rain, the sun emerged for a moment drenching the wet surfaces in warmth and light; a momentary pause in the rain that filled with brilliant sunlight. I turned around, looking away from the sun, hoping to catch a rainbow in the droplets reflections, but missed seeing anything. I kept walking. As I passed a tall pine tree, one drop fell off the tree limb into the sunlight, an aerial jewel. Then another, and another… until an entire shimmering cascade came off the long needles and into the shaft of light with a hush of quiet velvet sound. No rainbows in the moment, but there’s a curtain of falling crystals to see.
It was just an avalanche of raindrops; it happens constantly, nothing special in particular. But in that moment, in that quintessence of a fragment of time, I missed the rainbow, but saw diamonds in mid-flight, surging to the ground.
Earlier that morning I’d gone for a run at the Stanford Dish, it’s an open area featuring a gigantic radio-telescope, a landmark in Silicon Valley. But for locals, this is one of those standard runs you do often, just because you know exactly how far it is, what the trail is and what to expect. It’s not an exploration so much as it is a familiar ritual.
In rainy weather, though, it can be surprising. The Dish is in a hilly region of Palo Alto, with spectacular, open views in all directions. When it’s clear, you can see from San Francisco to San Jose (thirty miles to the north, thirty miles to the south) up and down the bay. To the west, the Santa Cruz mountains form a woodsy barrier wall. And when it rains, the clouds seem to always be especially dramatic from this hilltop.
Naturally, being a curious guy, I was thinking about that as I ran on the crestline trail. It’s where the views are best, and where the view of rain, clouds and sky are always most impressive. Why is it so consistently impressive? Is it just the openness of the viewshed?
On this run, this time, I noticed two patches of rain off in the distance. One was to the north, roughly over San Mateo, where there’s a gap in the western coastal mountains that lets the fog through in the summertime. The other rainy patch was to the south, roughly over Los Gatos, where there’s another gap in the mountains. An insight begins to dawn… Those two patches of rain are coming from the sea to the west, and are coming through the gaps in the mountains.
Of course! The reason the clouds are always so spectacular is that the Dish happens to sit between two major mixing points between the marine layer of air (coming through the gaps) and the air that sits in the bay (or in the summertime, that’s coming from the east, off the Central Valley).
In essence, this is the eddy between the mixers. Swirls, bumps, and lumps in the clouds happen preferentially here. Shafts of light that come through the breaks happen here as well, for just that reason—it’s an accident of geology.
The San Mateo gap is well-known. It’s what lets the summer fog over the mountains, often causing SFO to shut down. The gap at Los Gatos is also well-known—it’s where Highway 17 takes the low pass over the mountains to reach Santa Cruz.
When I get home I do the obvious thing and search for a rainfall density map of the Bay area, looking for confirmation that my gap theory is actually true. And after searching for a while (the key insight was to realize I needed a “precipitation” map, not a “rainfall” map), I found just the diagram I sought. And it’s true. Extending eastward from both mountain gaps are regions of increased rainfall. Los Gatos averages about 33% higher rainfall than the Dish, with similar numbers for San Mateo. Even better, the map shows a plume-like structure to the rain distribution: it’s the gaps causing the focusing of precipitation.
And now, naturally, being a curious guy, I wonder if there is a difference in the types of pollen you’d find in the gutters and waterways of Los Altos (to the south) and San Mateo (in the north) as compared to the location of the Dish. I suspect so.
That search continues. I will tell you that in the process of searching for differential pollen patterns in the Bay area, I learned a new word: palynology—the study of pollen… and its distribution. The word dates from 1944, and was coined in the Pollen Analysis Circular (a small journal that published from 1943 to 1954 and then merged with the larger publication, Micropaleontologist).
The question is, can I link geological features to pollenfall patterns? What kind of research would I have to do to get the answer? That’s the joy of finding stuff out—the chance to look at the world, do a bit of deep looking and digging in the literature, pulling together ideas, linking them together to learn something brand new.
I love doing research both professionally and at a small, personal scale. And I now know why running the Dish on a rainy day is almost always rewarding—it’s fertile ground for observing the world and launching into small personal research quests.
The next step is just connecting literatures between geography, meteorology, and palynology—a cross pollenization of academic disciplines, if you will.
