Helicopters working in the North Sea oil and gas sector which are struck by lightning sometimes help cause the atmospheric effect in the atmosphere, according to a new study.

The civil aviation industry asked scientists at the Met Office to examine lightning strikes in the hope of coming up with a forecasting system to help pilots avoid them.

Several factors are needed to create the right conditions for helicopters triggered lightning.

Positively charged cumulonimbus clouds need to be present and these are formed by cold air from the Arctic moving south during winter months.

Improvements in design and technology mean a strike is unlikely to knock the aircraft out of the sky, although they cause damage.

Dr Helen Wells, head of the civil aviation research and development group at the Met Office, said: "In a typical winter season there are about 10 cold air outbreaks in the North Sea operating area, each lasting around three days.

"This means that there are typically 30 strike risk days per season."

Pilots whose helicopters have been hit by lightning have reported there was no sign of it present in the area before they were struck.

A negatively-charged helicopter acts as a conductor for the lightning, aiding its passage to the earth.

Typically, in studies, it would enter one of the main rotor blades and leave at the tail.

Strikes can damage instrument panels, heat up components or leave burn marks.

They also may magnetise navigational equipment.

Although studies have been ongoing for several years, the 2014/15 winter was the first in which a forecasting system - trialled the previous year - was fully introduced.