Weather: Unusually NormalBy Robert Lindsay Korty
STAFF METEOROLOGIST -- Weather reporters commonly dramatize the severity of a day’s weather by commenting on how far a temperature or a precipitation total deviated from the normal value. In Boston on Feb. 24, the normal high temperature is 40°F (4°C), but this nomenclature is misleading. Climatologies are generated by a variety of means, but one common method uses a recent 30 year period, say, 1961-1990, from which an average of all the recorded temperatures may be computed. The normal temperature is formed from these average values after the data are smoothed with a running mean. From this base, current weather can be measured against some climatological expectation. Climatologies of standard deviations can be computed just as easily, but are seldom reported.
Large departures from average are far more common during winter than summer because the hemispheric temperature gradient is stronger and mixing across that gradient is more efficient during winter. The average high temperature in February is in the upper 30s°F( 3-4°C), but it is common for temperatures to vary markedly between days in the 20s°F (-6 to -1°C) and others in the 50s°F (10-15°C). Large temperature swings ahead and behind of storms are customary during New England winters. With the storm track split to our north and to our south for much of this month, Boston temperatures have been near normal, and have not deviated far away. That stability is unusually tranquil for February, and it will continue through the end of the month.
Today through Thursday: Sunny days, clear nights, and seasonably cool temperatures. Highs 35-40°F (2-4°C), lows 20-25°F (-6 to -4°C).