Wednesday, March 27, 2013

Blame the NAO



It is really cold.



Not just a little cold.  It is inches of snow, wind cutting through the coat, fingers numb on the bike cold.   And it just drags on, sub-zero day after slippery day.  Dutch snow on Sunday, Sheffield snow today:  More snow everywhere by Easter.

It seems like more than just a cold snap, a ‘northwester, or series of fronts.   And there is an answer.


This is the Arctic Oscillation (AO),  a measure of air pressure over the Arctic.  When it is negative, the low pressure trough centered over the north pole is weaker than normal.  This, in turn, leads to weaker westerly winds across the North Atlantic from Canada towards Europe and allows cold Arctic air to slide farther south than usual.  Cold.


Allied with this is the North Atlantic Oscillation (NAO), a complementary pair of balanced pressure centers over Iceland and the Azores.  Negative value indicate a weakening of the center over Iceland and slackening of the jet stream, letting cold air slide undisturbed even further south.  It also drives precipitation patterns and storms.  Snow.

NAO-NegThese are big, climatic air masses which determine the air circulation that drives the daily weather.  At he moment, the graph and projections indicate that the cold weather will continue well into April.

There are lots of places to read more, but this presentation is a pretty good start.

Stay warm!

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