"The skies have cleared up," said Lenarčič ahead of the first leg of his flight, to Tromso in Norway.

Lenarčič plans to complete the route in three to four weeks in eight to ten stages, over northern Europe, the North Pole, Canada and back via the Atlantic route first completed by Charles Lindberg.

The weather, which has already delayed his flight by three days, will be the key factor in his adventure, as ice easily builds up on aircraft at northern latitudes.

"The distances are long and the weather changes quickly," Lenarčič summed up what awaits him.

Lenarčič is piloting a slightly modified Virus-SW, an ultralight aircraft manufactured by Slovenian company Pipistrel.

He said he knows the plane well. "Pipistrel has made a supreme aircraft," he said.

Quite aside from being an adventure, Lenarčič's endeavour also has a scientific dimension.

His aircraft is equipped with sensors to measure the concentrations of black carbon, a key driver of climate change in the Arctic.

Pipistrel director Ivo Boscarol said he was happy Lenarčič's ventures were becoming more scientific, as the data he gathers will help understand climate change.

The project, dubbed North Pole 2013, is also sponsored by the Foreign Ministry, which has promised Lenarčič the assistance of Slovenia's entire diplomatic network.

The Slovenian adventurer was the first person to circle the globe in an ultralight aircraft without a copilot or additional air support in 2004.

Flying the Pipistrel Sinus 912 UL aircraft, which weighs a mere 284 kilograms, Lenarčič also set a world speed record in the under 500 kilo category.

During last year's flight, he became the first Slovenian to land on Antarctica and the first Slovenian to make a flyover of Mt Everest.

Lenarčič's progress will be tracked online at www.worldgreenflight.com.