Island weather lately has been a series of lurching mood swings, as if March suffered from some meteorological bipolar disorder. We careen from balmy days to threats of snow and back — now the trill of the red-winged blackbird on sunny afternoons, now the early-morning search for snow boots just in case. Amid this annual stumbling toward spring, we look skyward for the sun’s steady progress toward our hemisphere, the days lengthening by a reliable minute at each sunrise and sunset.