CLIMATE OF NEW ENGLAND. 87 



In ;>uiuiiici' time, the prevailing southwest winds hrinj^; warm air from 

 the southern States, and when re-enforced on cyclonic gradients (that is, 

 by differences of pressure directed to the centre of a low-pressure area) as 

 is not infrc(|ucntly the case, they cause an op[)rcssive heat : at such times, 

 as well as on cloudless, anticyclonic — or high-pi-essure — days in «Tune, July 

 and August, maxinuun temperatures of 90° or 95° F. are reached in the 

 inland valleys, exce[)t well to the north ; hut at the same time, the diurnal 

 range of tem[)erature is strong, especially under anticyclones, averaging 

 twenty or thirty degrees, and often exceeding forty degrees, in the valleys 

 of the hilly or mountainous districts ; and the nights arc fairly cool. Im- 

 mediately on the coast, the sea-breeze of day-time moderates the heat and 

 decreases the diurnal range. Cool, fair weather with northwesterly winds, 

 or cloudy, rainy weather with southeasterly winds interrupt the warmtli 

 of sununer : thunder-storms with occasional destructive squalls and hail- 

 falls occur chiefly while cyclonic storm-centers are passing down the valley 

 of the St. Lawrence. Several tornadoes have been recorded in the Con- 

 necticut Valley and elsewhere, but they are rare. 



In winter, the prevailing winds are west or northwest, from the cold 

 interior of the country, but these are frequently reversed by cyclonic storms 

 that draw in the damp air from the Atlantic ; for few of these storms cross 

 the United States without giving New England some mark of their pas- 

 sage. As they move across Lower Canada, their cloudy southerly wind 

 brings rain and snow ; sometimes it is unseasonably warm, rising to a 

 temperature of 50° or 60° even at midnight in midwinter ; under such 

 conditions, abnormally high, foehn-like temperatures have recently been 

 noted in the White Mountain valleys. When the storm-center passes 

 south of New England, a chilling snow-bearing wind blows in from the 

 northeast. Closely following the Atlantic winds, come the cold, dry west 

 or northwest winds on the rear of the storm. A\'hen the pressure is high 

 in the far northwest, these are intensified into the violent cold wave, under 

 which the temperature may fall thirty or forty degrees in twenty-four 

 hours, in excess of the diurnal range ; indeed, in winter when the regular 

 diurnal range is small, it is almost supplanted by the irregular cyclonic 

 change of temperature, then at its highest value. During a cold A\'a\e the 

 temperature falls to 0° or — 20°, without distinct local variations ; l)ut on 

 the clear anticyclonic nights that follow, the minima in the valleys fall to 

 — 20° or even — 40°, and local variation from valleys to hills is excessive. 

 On ^It. "NA'ashington, the cold wave carries the temperature down to — 40° 

 or — 50°, with the wind blowing sixty or more miles an hour, but during 

 anticyclones, the mountain top may be ten or twenty degrees warmer than 

 the neighboring valley bottoms. 



The mean annual and the mean winter temperatures are shown by iso- 

 thermal lines on the accompanying map. These lines are drawn according 



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