Winter is over. Thank Goodness! Spring is beginning. Oh, dear!

The winter of 2009 - 2010 was shaped by three major factors:

1. Heavy volcanic activity - Over the last 3 years we have seen unusually heavy volcanic activity in the Arctic. In 2009, Mt Redoubt in Alaska and Sarychev Peak on Russia's Kuril Island were so large that they penetrated into the stratosphere. In mid-January Russia's Mt. Klyuchevskoy exploded, sending debris from 4.5-8 km (2.8 - 5 miles) high.

The ash and chemicals in the air blocked out incoming sunlight, cooling the Arctic air mass. This, in turn, changed air pressure patterns, altered the direction of the prevailing winds. Polar winds turned south, penetrating deep into eastern North America, Europe and East Asia.

2. El Niño - The waters of the Tropical Pacific have been an average of 1.8°C (3.2°F) warmer than normal. This has heated the air above it, altering weather patterns. The changed ocean and air patterns have brought warmer weather to the West and most of Canada.
3. The warm phase of the Atlantic Multidecadal Oscillation - The Gulf Stream and other Atlantic currents are flowing faster, bringing warmer water to the northern latitudes. This has created a pool of warm air and water that has energized winter storms.

The combination of these factors created an unusually cold winter throughout most of the nations in the Northern Hemisphere. A persistent high-pressure system in Eastern Canada steered Arctic air southeast into the US, across the Plains, Midwest and Southeast. Nor'easters blasted up through the East Coast. The weather broke 18,201 temperature and precipitation records. It was the 10th coldest winter on record for most of the Southeast and the 10th wettest along the Mid-Atlantic and Southeastern coastlines. Huge storms swept through the eastern two-thirds of the continent, stranding Christmas travelers in December. January saw snow in every state but Hawaii and killing frosts in Florida. February pounded the East with a "snowacane".

As winter continued, the global volcanic activity and the moderate El Niño in the Pacific showed signs of ebbing. Now both are strengthening. Renewing the forces that shaped this stormy winter will create a very extreme and stormy spring.

David Fuller's view This issue was released before the Eyjafjallajoekull volcano eruption in Iceland occurred, creating havoc with Europe's airline industry. Here is the BBC's latest report.