PARIS, June 2, 2009 (AFP) - The grim hunt for the missing Air France jet is concentrated on a zone of the Atlantic Ocean that early sailors once called The Doldrums, where an eerie calm can quickly become a violent storm.

With little hope of finding survivors, reconaissance planes from Brazil, France and Spain are hunting for wreckage in the Intertropical Convergence Zone which Flight AF 447 entered early Monday and then disappeared from radar screens.

The crew on the Rio de Janeiro to Paris flight reported hitting fierce turbulence and 14 minutes later emergency automatic messages were transmitted indicating an electrics failure. Air France says it could have been hit by lightning.

The convergence zone is the technical name for an area of the huge ocean straddling the equator where trade winds from the northern and southern hemisphere collide.

The low pressure zone moves according to the season. It can be seen on weather maps as huge bands of cloud, mostly made up of thunderstorms.

"You have a lot of moist ascending air due to the heat from the sun on the sea," said one meteorology expert.

"Where the trade winds meet, they have to go somewhere and so they go upwards. It is rather like a car in a crash which crumples up. The result is violent storms."

Jean-Marie Carriere of the Meteo-France weather service said jets are normally "well used to crossing the zone" as their radar systems enable them to avoid the most turbulent spots by going through gaps in the bad weather or by climbing higher.

Inside the zone the winds are usually light, which is why early sailors called it "The Doldrums". Many feared death because their sailing ships barely moved.

Based on the last signal from Flight AF 447, officials have identified a zone around 1,100 kilometres (680 miles) off northeastern Brazil and 2,000 kilometers (1,240 miles) off the coast of Senegal in West Africa for the hunt.

A Brazilian pilot for TAM airlines who was in the zone at the same time reported seeing orange glimmers on the surface of the ocean which he said could have been fire or a distress buoys.

The zone is officially in Senegalese airspace but it is so remote that a French military reconnaissance plane took four hours to arrive from its base in Dakar, Senegal before it could start a sweep of the ocean.

French military spokesman Christophe Prazuck said a French plane had flown from the Cape Verde islands back along the missing jet's expected flight path.

"We didn't find anything, but the weather was terrible," he said.

The French government has said that relatives of the missing passengers may be taken to the search zone if they wished.

"The search will continue as long as necessary. All means are deployed in the area and we'll put as many assets at their disposal as necessary," French Defence Minister Herve Morin said Tuesday.