Page:Popular Science Monthly Volume 58.djvu/397

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known as the 'sirocco front' (shaded on the chart), while at the same time the cold northerly and westerly winds, blowing south in the rear, carry down the isotherms and mark the extent of the cold wave that follows. Hence around and about an intense early winter cyclone we may have warm, moist rains on the southeast, cool rains, turning to snow, on the east and northeast, with blizzard conditions on the northwestern flank and clear, cold weather on the extreme southwestern, as was the case in this instance. In consequence of this, the possible contrasts through the center of the average early winter cyclone are such as to jump any locality over which it passes from summer (60° to 70°) temperatures to winter (40° to 20°) in a few hours, and it is the passage of a typical cyclone over any given locality that gives the violent changes peculiar to American weather. Wholly independent of its own circulation of winds about its

center, the cyclone moves forward in the circumpolar drift at the rate of from fifteen to thirty-five miles and more an hour. If it passes north of a place, the locality is affected by its southeasterly, southerly and southwesterly to westerly winds and the weather that belongs to these quadrants. If it moves along a line south of any given place, the locality is affected by its easterly and northeasterly to northerly and northwesterly winds, which make up the coldest and stormiest side of the cyclone. As the barometer at its center is always low, the cyclone is called a 'low area,' or 'low,' for short, and as such appears in Weather Bureau reports. Storm intensity in a cyclone is in due relation to the minima of its own barometric pressures and to the maxima of the anti-cyclone nearest it. All forecasting is based on an effort to balance the probable paths that the cyclones and anti-cyclones will take with respect to the regions east of their point of origin.