Rain — one of nature’s main elements

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“Rain, rain, go away,” went the rhyme in the house I grew up in, “Little Mattie wants to play.” This was, of course, in the olden days, just this side of the Jurassic, when kids weren’t wired to electronic devices and still did stuff outdoors.

Since I still do stuff outdoors, and really haven’t grown up much, rain still governs my schedule to a degree. But as I listened to the downpours that swept the Island last Saturday night, I reflected that I see rain as a much more complicated thing than I did as a child.

There’s something mildly amazing, for starters, in the fact that it rains at all. The atmosphere is mostly invisible to us, a planetary envelope of gasses that we can feel when in motion but otherwise have little reason to notice. And yet liquid water periodically materializes out of it (or, at this point in the season, frozen water, which is even more amazing).

I understand the physics of this well enough. Water molecules, dispersed into vapor form, dissolve in the atmosphere like salt into a glass of water. In the same way that a hot liquid can dissolve more minerals, warm air can hold more water; and when it cools past a certain point, the vapor condenses. If the condensation is light enough, the tiny droplets can remain suspended in the air as clouds or fog. But if enough droplets coalesce into heavier drops, voila — it rains.

The usual method for air to cool is an increase in elevation. Moist air can be lifted by cooler, denser air moving into the region (this is the impetus behind a storm like last weekend’s). Or air at ground level can be heated by the sun and, thereby rendered less dense, rise on its own (the mechanism behind a summer thunderstorm). In either case, as the moist air rises, it experiences lower pressure; and because of the relationship between temperature and pressure in a gas, it cools as it rises. And it’s a good thing that it does, because atmospheric transport leading to precipitation is where the Vineyard’s fresh water comes from.

During the growing season, a tangle of plant roots in the soil intercepts much of the rain that falls as it soaks into the ground; plants are effectively bundles of microscopic straws, sucking water molecules from the ground up to the leaves in order to support the plant’s metabolism. But in winter, plants are dormant, and without the upward suction in their root systems, much more water passes into our granular subsoils, percolating downward until it hits denser salt water that has infiltrated from the surrounding sea. The resulting pool of sand saturated with fresh water represents the aquifer from which virtually all our human water needs are met. So winter rains are a vital resource for the Island, and I find real elegance in such a simple mechanical system for storing a vital resource.

Above ground, especially in winter, animals are challenged by rain. Songbirds risk hypothermia if they get wet and their feathers cease to insulate effectively. These birds face a quandary. They can hunker down during storms, staying dry in a thicket or tree cavity but letting their caloric gas tank steadily empty; or they can brave the wet conditions to feed, ingesting energy but risking a lethal chill. I don’t envy them the need to decide, but it seems like instinct often guides them well; in cases where I’ve been able to keep an accurate, ongoing count of a stable flock of wintering sparrows, it seems like prolonged rain must occur before much mortality is evident. Snow cover, which can conceal a bird’s most important food sources for many days, is surely a worse threat.

If weather were theoretical, rain would arrive as pure, distilled water. But the atmosphere is a complex soup of chemicals, and rain descends containing a sample of what it has passed through. In our industrial times, this includes a healthy dose of nitric acid, a by-product of fossil fuel combustion. In its role as a component of “acid rain,” this compound, at any reasonable concentration, is largely shrugged off by our native vegetation, already well adapted to acidic conditions. But in our waters, added acidity poses a problem. It inhibits shell formation in aquatic animals ranging from diatoms to oysters, slowing growth and in extreme cases causing mortality. Meanwhile, the “nitric” part of nitric acid also serves as a fertilizer (acid rain is a major component of the excess nitrogen entering the Vineyard’s salt ponds).

So rain is critical but, for wildlife, potentially lethal. And the current composition of rainwater is altering the ecology of our aquatic systems, benefitting many plant species but harming anything that forms a calcium-based shell. It’s the typical paradox of ecology: nothing is either totally good or totally bad, and anything that happens produces both winners and losers. That’s a puzzle you can ponder during the next rain storm, as you lie warm in bed and listen to drops patter on the skylight.