YOU'VE NO DOUBT heard that droll line attributed to Mark Twain: "The coldest winter I ever spent was a summer in San Francisco." Though some doubt that he actually said this, there's no question that he knew that San Francisco - and indeed much of the central coast - gets foggy in summer, and that fog doesn't so much tiptoe in on little cat's feet as roll in and envelop, sucking the warmth out of a body. Mark Twain may even have been enough of a natural scientist to know what, at least in part, causes this fog: upwelling.

In a nutshell, upwelling is a process by which cold water is brought up from the depths of the ocean. Along California and Oregon, wind from the northwest pulls warmer surface waters away from shore, and cold water moves up to take its place. The reason the wind doesn't just push that water on down south has to do with physics - friction, to be specific, and the Coriolis effect, which combine in a process known as Ekman transport. But more on that in a bit: the point is, cold water appears off our shore from about April to September.

When that cold water comes in contact with the moist warm air flowing off the land, fog results. And a beautiful sight it is - especially from a distance.

But fog is only one aspect of the upwelling phenomenon. That cold water also brings nutrients, which nourish the very bottom of the food web: phytoplankton and zooplankton. And plankton, in turn, supports some very large animals indeed.

I once went whale watching during the summer on Monterey Bay. Or rather, it turned out to be a "whale-listening" trip, since we were quickly shrouded in fog, and the only way we could tell if whales were around was to shut off the boat engines and . . . listen. Every so often we'd hear the huffing sound of a blow, and we'd fire the engines up and chase off in that direction. Occasionally we got close enough to see the long, long back of a blue whale break the surface and go under. That time, the fog made for a special experience.

You might be thinking, Blue whales? In June? Isn't it just grays that ply our coast, and then during the winter months?

Well, no. In fact, blue whales, along with humpbacks, come during summer, and they come specifically to feed. (The grays, by contrast, probably don't feed as they pass by - but that's another story.) They hover above the edge of the continental shelf, where masses of small shrimplike creatures known as krill converge, and they strain these animals through their hairy plates of baleen by the ton.

So why is the krill here? Let's get back to upwelling to tackle that question.

Krill is a zooplankton - a tiny drifting animal. And a zooplankton eats, well, other zooplankton to an extent, but also, and significantly, phytoplankton: microscopic plants.

Phytoplankton, like any plant, whether a baobab or a daisy, needs a few things to survive. It needs sunlight and carbon dioxide, for photosynthesis. But it also needs certain nutrients, for growth - the same 10-5-5 minerals that we buy in bags to strew over our flowerbeds.

In the ocean, when organic matter dies, it sinks and decomposes. When that happens, the carbon and other minerals in its cells dissolve - but in lower regions of the water, not up at the surface where microscopic plants are busy photosynthesizing. So nutrients such as nitrate and phosphate (dissolved forms of the elements nitrogen and phosphorus - the first two numbers on our fertilizer bags) are relatively inaccessible to the plants that need them.

Enter upwelling: when the winds come and push the warm water away from shore, the cold water rising from the depths brings those nutrients to the surface. The plant life thrives - plankton and all the other seaweeds. As they convert light and nutrients into food and oxygen, they manufacture most of the organic compounds required by marine animals: zooplankton first, which are eaten by fish, which are eaten by larger fish and birds and marine mammals. And of course, the oxygen given off by the marine plants benefits us land animals as well.

The reason upwelling occurs has to do, as I mentioned above, with Ekman transport. The flow of most surface currents in the oceans is driven by wind. When wind blows over water, the surface of that water is not pushed directly in front of the wind, but moves at about 45 degrees to the right of the wind's motion in the Northern Hemisphere, or to the left in the Southern, thanks to the Coriolis force, an effect of the rotation of the earth. As one descends in the water, the direction of flow continues to be deflected rightward (or leftward), until ultimately a three-dimensional spiral is formed vertically in the water. The net transport of water, as explained by Ekman transport, is at an angle of roughly 90 degrees to the direction of the wind. In short, water, to a depth of a few hundred meters, is pulled directly off the coast - leaving room for deep water to rise and replace the displaced water. (Want to know the nitty-gritty of Ekman transport? Check this web site: http://ekman.sr.unh.edu/course/intropo/IPO20.dir/IPO20.html, which is replete with diagrams and messy formulas.)

If you look at a map of the major cold ocean currents along coasts, you'll see that the California Current, for example, is sweeping past North America from north to south; water that is pulled to the right of this current is swept out to sea, leaving room for deep water to rise and take the warmer surface water's place. The same occurs off South America, where the Peru Current flows from south to north and the water is pushed leftward. High pressure zones off the areas of major upwelling appear to be determining factors in the upwelling process as well.

These areas of strong upwelling are, not surprisingly, associated with active fisheries. In fact, although they constitute only about one percent of the surface of the ocean, they account for some 50 percent of the catch worldwide. Let me repeat that, since it's taken me a while to get here: About one percent of the ocean surface accounts for 50 percent of the fisheries catch worldwide. For such a small area to be the haven for such productivity is, I think, mind-boggling. It also bespeaks the need to treat those areas with respect.

For one thing, the fact that fish exist in such concentrations may suggest a greater need for fisheries management and water quality monitoring in those areas, especially when reproduction occurs there - as in many species it does, because the larval animals are presented with such a bounty of tiny food morsels.

Fortunately, humans are a weak match against such strong earth processes as Ekman transport and upwelling. We're unlikely, though hubris might tempt us to try, to stop the flow of the winds or the currents, and so keep the cold water from rising and enriching the plant and animal life off our shores. The ENSO (El Niño-Southern Oscillation) phenomenon is much more able to disrupt upwelling, in South America particularly, by changing the depth of the uppermost layers of water - often with disastrous consequences for birds and fish.

But then there's global warming. . . . Could it be that our hubris will get the better of us yet? Let's just hope we're smart - or should I say wise - enough not to allow that. 

Anne Canright, a geographer, writer, and photographer, is a contributing editor of Coast & Ocean.