The area just north of the equator where Air France Flight 447 disappeared late Sunday is known as the Intertropical Convergence Zone, an area of alternately calm and stormy weather where the tropical weather cells of the Northern and Southern Hemispheres meet and sometimes clash.
In this zone, which changes position according to the season, huge thunderstorms can form, some towering to 60,000 feet, much higher than a normal thunderstorm and which airliners cannot fly above.
The missing Air France Airbus 330-200 was flying at 35,000 feet during its last radio contact at 9:33 p.m. EDT Sunday evening. It then entered a "dead zone" where it was too far from land to be seen by radar, so its exact position was unclear.
Forty-five minutes later, and five minutes before the plane was due to make radio contact with air-traffic controllers in Dakar, Senegal, its computers sent a four-minute burst of data via satellite, indicating electrical-systems failures and loss of cabin pressurization.
At the time, a line of thunderstorms was moving across the Atlantic between west Africa and northern Brazil. The Airbus would have flown right through them during the 50-minute radio gap.
Severe bursts of lightning and even tornadoes are associated with huge thunderstorms, but modern aircraft are designed to withstand lightning strikes, and tornadoes generally form close to the ground.
More dangerous to airplanes is a condition known as wind shear, when a sudden gust of wind suddenly pushes a craft down. But that should not have endangered a plane tens of thousands of feet in the air.
The area's thunderstorms rise so high because the Intertropical Convergence Zone is essentially where the warm surface trade winds of the Northern and Southern Hemispheres blow right into each other.
When they meet up, there's nowhere for the huge currents of air to go but up. Warm air rises high and quickly cools, forming thunderstorms under the right conditions.
A vessel sailing through the area, having been propelled toward the equator by the trade winds, would suddenly find itself stilled as the air starting moving vertically instead of horizontally -- it would be caught in what sailors call the doldrums.
The rising air then spreads out and reverses course, traveling away from the equator for hundreds of miles, the opposite of surface winds.
Between 30 and 35 degrees north or south, varying by season, the air suddenly descends again as it runs into countervailing winds of the temperate zones, creating yet another wind-free zone feared by sailors: the horse latitudes.