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Boston Weather: 56.0°F | Mostly Cloudy

Weather: Cool and Clear

By Greg Lawson and Bill Ramstrom
Staff Meteorologist

The cool weather we are experiencing today is setting the stage for some frost tonight outside the city. Here in Cambridge, we will have a somewhat longer growing season thanks to the "urban heat island." The asphalt, concrete buildings, and other man-made surfaces in the city absorb more solar radiation during the day than does the vegetation in the suburbs. At night, this reservoir of heat is slowly radiated back to the atmosphere, keeping the temperatures 5-10 degrees warmer in the city. This effect is most pronounced on clear nights with light or calm winds, when the air near the surface can cool rapidly towards the dew point.

However, the urban heat island does not just affect how late into October our tomatoes will grow, but also fuels controversy over claims that global warming has already begun. Some studies based on surface temperature readings over the last century have shown an upward trend in observed temperature, but often the readings were taken in expanding cities where the heat island effect may be the true cause of this change.

For the rest of the week, expect slow warming as a blocking high protecting us from the bad weather over the Midwest through late Thursday moves offshore. The large cutoff low over the Dakotas will only be able to move eastward very slowly, and will thus be forced to go around the high, most likely to the north into Canada.

Today: Cooler but sunny. Light north wind. High 57F (14C).

Tonight: Quite chilly. Winds calm down after sunset. Low 40F (4C).

Wednesday: Mostly sunny, pleasant. High 60F (15C). Low 49F (9C).

Thursday Outlook: Warmer with some sun. High: mid 60s (17-20C).