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The great tropical storms roaming the world's oceans and seas have earned the right for individual names: Andrew and Hazel, Keli and Jelawat. Extratropical cyclonic storms, those forming from the clash of polar and tropical air masses, have not yet been honoured with individual names. However, some extratropical storms take on such unique characteristics that they are often recognized with a unique name for the genre. The Gulf of Alaska storms, which I wrote about last month, fit within this category as do the Colorado Lows. But for two North American groups of cyclonic entities, their names are more descriptive: the Nor'easter along the Atlantic coast, the Alberta Clipper that races across the continent our of the Alberta plains.
The Nor'easterIn a cyclonic flow pattern, winds spiral around the low pressure centre in a push-and-pull struggle between pressure gradient, Coriolis and frictional forces. In a well-developed cyclonic storm with a proper placement of surrounding high pressure cells, this flow regime develops sectors with strong northeasterly, southwesterly and northwesterly winds. Therefore, any strong cyclonic storm has the potential to be called a southwester, a northwester, or a northeaster. However, only along the North American Atlantic coast, from Cape Hatteras northward to the Canadian Maritimes, can a storm truly be called a Nor'easter. By accepted definition, a Nor'easter should have gale force or stronger winds initially blowing from the northeast. These storms often bring heavy precipitation, falling as rain, snow, or at times, freezing rain. Along the coastline, heavy surf generated by offshore storm winds may cause extensive damage to the shoreline. Nor'easters are the most common, widespread severe weather events to worry New Englanders and Maritimers. The term Nor'easter arose in the colonial days before the concept of wind circulation around a low pressure center was established. Storms were then considered to travel from the direction of the wind. If a storm came with southwesterly winds, it originated from the southwest. A Nor'easter, therefore, originated, and arrived, from the northeast of those feeling its fury. Those beliefs began to change in the mid-eighteenth century, in part through the insights of Benjamin Franklin. Franklin had looked forward to viewing a lunar eclipse but had been denied the visage by clouds associated with a storm that rolled into Philadelphia that evening. Later, he was surprised to hear that his brother living in Boston had seen it. Franklin had expected the obscuring clouds had moved over Philadelphia from the northeast, around Boston, but Franklin's brother wrote the eclipse occurred before the clouds rolled in. From this and other correspondence, Franklin began to suspect that, contrary to then-current beliefs, large storms
The copyright of the article Nor'easters and Alberta Clippers in Meteorology is owned by . Permission to republish Nor'easters and Alberta Clippers in print or online must be granted by the author in writing.
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