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Boston Weather: 34.0°F | Fair

Weather: A Taste of Winter

By Rob Korty
STAFF METEOROLOGIST -- With the loss of daylight savings time and some snow on Sunday, the onset of winter seemed to be imminent. Fortunately (for those who are not ready for icy mornings quite yet) temperatures will bounce back to more seasonal levels by the latter part of this week. But with the sun setting before quarter to five (it will be as early as 4:12 in mid-December) and brisk winds, Bostonians have been quickly reminded that winter is not long away.

A strong gale that is sitting offshore of the Canadian Maritimes continues to funnel clouds and light snow across portions of New England. This is because in the Northern Hemisphere air circulates counter-clockwise about an area of low pressure. Why? Fundamentally it is simply the balance of two forces. Left to its own devices, air naturally flows from regions of high to low pressure. But since we live on a rotating sphere, motions in the Northern Hemisphere are deflected to the right (this is named the Coriolis “force”). Thus as air evacuates a high, it is turned to the right, and a clockwise rotation ensues. Similarly, as air channels toward a low, it too is turned to the right, and a cyclonic flow develops. If you include friction in the balance of forces, it is not hard to show that rather than simply rotate about the pressure anomalies, air actually diverges from the center of a high pressure system, and converges into a low. This convergence leads to vertical motion, which supports the development of precipitation in the vicinity of a low pressure system.

So as this low sits to our east, northerly winds will continue to advect cold air into the region. By mid week, the low should have passed far enough east to allow the skies to clear, and temperatures to warm.

Extended Forecast

Today: Occasional rain and drizzle. High near 44°F (7°C).

Tonight: Lousy, chance of drizzle. Low near 38°F (3°C).

Wednesday: Becoming partly cloudy by afternoon. High near 50°F (10°C).

Thursday: Partly cloudy and milder. A seasonable high near 58°F (14°C). Low near 35°F (2°C).