Rain and the seasons

Starting on Friday, we began to get some significant amounts of rain in this area — this after the occasional brief (at most a few hours) and tiny (on the order of .02 inches) bits of rainfall during the past month. People complained about our being in a drought, and indeed we are, but it’s last year’s drought; what we were in until recently was just California’s “dry season”, the roughly seven months of the year when it virtually never rains.

We have two kinds of seasons out here: the more-or-less standard set of four seasons in the mid-latitude northern hemisphere, with a cyclic pattern of changes in day length and temperatures, and the accompanying changes in plant life (in particular, deciduous trees have their leaves change color and drop in fall, and leaf out in spring); and two rainfall seasons, related to the day-length seasons but distinct from them (the dry season, with its midpoint in summer, and the rainy season, with its midpoint in winter). With the onset of the rainy season, grasses and similar plants turn from brown to green: the hills turn green for Christmas! Meanwhile, there are plants that come into bloom in all four of the day-length seasons, including winter; we have quite a few winter-blooming plants, many of which I’ve posted about on this blog.

There’s considerable variation in all of this, some of it having to do with latitude, elevation, and closeness to the ocean, some of it with more obscure causes. In a perfect rainy season, we’d have steady rainfall most of the day for most days over several months. I’ve experienced a few of those. But much can go wrong.

Last year we had a terrible drought, so that creeks, lakes, reservoirs, and rivers were not replenished by winter rains in the lower elevations, and hardly any snowpack accumulated in the Sierra Nevadas.

Some years we’ve had the opposite problem, with torrential rains producing flooding and mudflows. My first winter in California was a year of disastrous rainfall, and we’ve experienced others since then.

Now for some weather overview, as we pray for a good rainy season, starting now.

Day-length seasons and rainfall seasons. Just surveyed. In addition to the variation already mentioned, there’s probably some that is actually random, though in a system with so many possibly relevant factors (more are soon to come) that’s hard to tell.

For example, though temperature varies with day length, the variation is usually within a small range, to the point that dwellings are built (with pipes running up the outsides of buildings) and vegetation (much of it Mediterranean or semi-tropical) planted on the assumption that hard freezes never happen. At least once in my experience here, that assumption was dramatically false: one December there was extensive property damage from burst pipes, and a truly massive die-off of trees and ground cover plants.

At the other end of the temperature scale, summer temperatures have been creeping up noticeably, no doubt as a consequence of global warming.

Jet streams and ocean currents. Two other contributions to weather patterns, both of them substantial and variable.

First the jet streams, in particular the polar jets flowing west to east. Sometimes they stay mostly in the higher latitudes, but they can also dip south, and they can stick in one place for a significant period.

Then the ocean currents. From Wikipedia:

El Niño is the warm phase of the El Niño Southern Oscillation (commonly called ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (between approximately the International Date Line and 120°W), including off the Pacific coast of South America. El Niño Southern Oscillation refers to the cycle of warm and cold temperatures, as measured by sea surface temperature, SST, of the tropical central and eastern Pacific Ocean. El Niño is accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific. The cool phase of ENSO is called “La Niña” with SST in the eastern Pacific below average and air pressures high in the eastern and low in western Pacific. The ENSO cycle, both El Niño and La Niña, causes global changes of both temperatures and rainfall. Mechanisms that cause the oscillation remain under study.

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