The ITCZ And The Crash Of Air Algérie AH5017

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The ITCZ in July. Courtesy of NOAA

The ITCZ in July. Image courtesy of NOAA

Though bad weather has been blamed for the crash of the Air Algérie flight in Mali, that cannot be the whole story.

Thunderstorms are to be expected at this time of year, as the Inter-Tropical Convergence Zone (ITCZ) shifts north over the area.

A plane that cannot fly through or around a large thunderstorm has no business flying through these unfriendly skies.

The Global Zones Of Weather

The global circulation is characterized by several persistent zones of wind and weather that circle the earth. Most prominent for those living in the middle latitudes is the jet stream, a fast-flowing ribbon of air between ten and thirty thousand feet which blows from west to east. Beneath the jet stream, both turbulent and tranquil weather occur. Fair weather alternates with storms.

Equatorward of the mid-latitude westerlies is a zone of very peaceful weather at around 30 degrees latitude. Most of the world’s major deserts are found in this region, which is characterized by high pressure and dry, sinking air.

Finally, near the equator, winds are invariably easterly, with waves that set off different kinds of stormy weather, depending on the underlying terrain.

The ITCZ: Inter-Tropical Convergence Zone

The easterly winds near the equator blow with an equatorward component. As the hemispheric winds converge, the air is lifted and storminess is frequent: tropical storms over ocean; thunderstorms over land.

The ITCZ moves poleward in the summer of each hemisphere, centered almost directly under the sun’s highest point. Right now, the ITCZ lies around fifteen degrees north latitude. Air Algérie flight 5017 crashed near 16 degrees.

Waves In The ITCZ

The atmosphere likes wavelike patterns. Waves in the jet stream set off mid-latitude surface low pressure centers that can produce winds up to 100 miles per hour.

In the zone of easterlies, waves are associated with tropical storm development over the ocean. Over land, the waves create areas of instability where enhanced thunderstorm activity takes place. Individual thunderstorms can be on a scale of a few miles to as much as 100 miles across.

The Structure Of Large And Dangerous Thunderstorms

A thunderstorm is characterized by a rapidly rising column of air that can reach to 50,000 feet. As the air rises, it cools, and moisture condenses.

A thunderstorm can produce a lot of rain; if the updraft is strong enough and drops of rain are near the freezing level, hailstones can grow to baseball size.

What Goes Up Must Come Down

The vertical movement of air over the entire earth has to balance to zero — otherwise our atmosphere would be lost to space. So the powerful updraft in the center of a thunderstorm has to be balanced by downdrafts somewhere else. These downdrafts occur on the edges of the storm, and this is where an aircraft can get into trouble.

A powerful downdraft, if the pilot doesn’t account for it, can drive the aircraft into the ground.

The Crash Of AH5017

A large thunderstorm is evident on satellite images around the time of the crash of AH5017. The pilot communicated that he wanted to change course. Though this is a dangerous situation, nothing could have occurred that pilots and controllers don’t routinely handle. If planes could not fly through or evade thunderstorms, no plane would ever fly near the ITCZ.

No possibility has been ruled out as a cause of the crash of AH5017; al-Qaeda has been active at times in the area. But it seems most likely that someone did not perform properly and the accident could have been avoided: Either traffic control failed to notify the plane of impending bad weather, or the pilot failed to take proper evasive action with a storm in the vicinity — possibly both.

Will We Find Out The Cause Of The Crash?

Both black boxes have been recovered from the aircraft. The final words of the pilots should be definitive in determining the cause of the crash.

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