Flight 447 and the ITCZ

I had never felt airsick before, or since. But now I was a nauseated rag doll flopping around in the middle row of a six seater prop plane and I was ready to hurl at any moment.

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BBC depiction of the path of Flight 447. I find it astonishing that the most important weather related feature on the planet is a “place where there are a lot of thunderstorms” or often not even identified at all. This is equivalent to a plane crashing into the Cascades and the news reporting that the aircraft went down in a “place with some hills” or not even noting the existence of the mountain range at all, as though it did not matter. (Note: We still do not know the cause of this crash. See text.)

The turbulence was epic, both of the plane and of my stomach, which fortunately was totally empty at the time. They told me that we’d stop in Uganda for lunch, and we were just now heading for Entebbe airport near Kampala, Uganda’s capitol. But I wasn’t sure I wanted lunch.

I was flying out of the Congo (then known as Zaire) for the first time, and since my route in had taken me to the capitol, Kinshasa, this was my first time flying over or into East Africa. There was just me … the passenger … and a pilot and copilot. That seemed strange, but this was a semi-regular international flight that would usually have five or six passengers on board, connecting Beni, Zaire to Nairobi, Kenya. But, for some reason, I was the only passenger on this particular day. Which is a good thing because it seemed pretty likely that I would throw up on someone any second now …

“That’s Entebbe down there, ahead and to the left,” the co-pilot said over his shoulder. “Normally we would land there, but we can’t today.”

“Hrmpph?”

“Museveni … the rebel leader …. his army is down there and they are fighting over the airport now. We have been instructed to stay away from the airport.”

“Grrgllph”

“The thing is, we’d normally refuel at Entebbe.”

“Huhg? Fuel?”

“But since we can’t, we have only two choices, We turn back now and rethink how to do this. You’d need to get a later flight, probably through Rwanda, in a week or two. We probably won’t be making this particular flight until further notice.”

“Ahh….,” (nausea giving way to moderate alertness)

“Or we fly through, instead of around all of the storms. But it will be rough.”

I glanced at the multi-colored on board radar screen where the co-pilot was now pointing, indicating the storms that we had not flown into yet. It was hard to focus on the screen since we were shaking up and down and back and forth like kids in a busted roller coaster ride.

“You see, normally we’d have enough fuel … with refueling and all … to drive back and forth between the storms. But without that fuel, we have to go in a straight line. We’ve done it before. It will be rough but with God’s grace ….”

Did I mention that this was a Missionary Air Fellowship flight?

“… we’ll make it to Nairobi, and ahead of schedule to boot. It’s up to you.”

I thought about it for all of one second. Going back would not be less rough, I’m so sick I don’t care if we crash, and I have almost no money left. Must. Go. Forward.

“Onward!” I gurgled, pointing in the general direction of the Indian Ocean.

The pilot and co pilot prayed briefly, and onward we flew, right into the storms. Oh, and it was much, much rougher going forward.

These storms were part of the weather pattern that is always here near the equator, a set of storms that move north and south with the seasons, but this time of year are always sitting right over Lake Victoria, and from there spinning out to make the rain forest wet and to light fires on the Serengeti.

The storms system I flew through that day over twenty years ago is still there, and it is the same storm system that may (or may not) be blamed for taking down Air France Flight 447 over the Atlantic the other day. This is the storm system that is always there, and it is the most important storm system on the planet. It is called the Intertropical Convergence Zone, or ITCZ. (Where I come from, this is pronounced “Eye Tee See Zee” or “Eye Tee See Zed” but these days the young-uns are calling it “Itch”)

What is the ITCZ? I’m glad you asked, and I’ll be happy to explain.

The earth is a big ball, and spins on an axis that is perpendicular to the plane of the sun. That means that the middle (equator) of the ball is more or less always either facing the sun (at noon on a given spot) or has just finished facing the sun (PM/evening) or is about to face the sun (AM). So, it gets hot around the middle of the planet. Here is a map of heat (more red equals more heat) that helps to demonstrate that the equator is extra hot.

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With that kind of heat, you can imagine that the heating air would rise dramatically at the equator. Like this:

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And if that happens, you can imagine that air will be sucked into the void where the heated air is rising, like this:

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And the air that has risen, will move away and cool off and drop down, again being sucked into the rising air column, like this:

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Which essentially causes a circle, when viewed in cross section, of air rotating in and out of the equatorial region. You are looking here at the cross section of a giant donut that encircles the earth, called a Hadley Cell. The rising part of this column is, essentially, a thunder storm generator.

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But of course, there are two of them, one for the northern and one for the southern hemisphere. So you get this:

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At this point, the air is actually pushed up higher than anywhere else on the planet, and the storms are bigger. This is a continuous process happening all the time. The ITCZ is this juncture of the two Hadley cells. The ITCZ moves north in the northern summer, and south in the southern summer, but is always near the equator. You can think of the ITCZ as being the “tropical equator.”

Because the air is higher and the storms thicker at this point, it is virtually impossible to fly over the ITCZ. Planes flying north or south across the equator always cross it, and normally there is turbulence at this point. The flight I describe above, over Lake Victoria, was notable because it was along the ITCZ, parallel to it’s central axis, staying within this zone of turbulence the whole time.

I have flown over the equator many times, and usually (but not always) it is at night. I always make sure I am awake while we cross the ITCZ because the sensation of flying through the Giant Turbulent Donut that encircles the earth is the closest thing you can get to realizing that you are on a huge spinning ball with a very thin atmosphere. The ITCZ is as persistent, and as important, as a major mountain range, but it is made mainly out of gaseous nitrogen and water vapor. It is a topographical feature of the landscape that is made out of energy. It is the engine that drives the formation of hurricanes. It is the place where the extra solar energy resides before it dissipates towards the poles, and it is this dissipation of energy that causes, ultimately, all of the weather on the planet Earth.

So when crossing the equator, I want to know — I want to feel — that I’m flying through the ITCZ, pictured here:

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And if I’m travelling with anyone else, and they are woken by the turbulence, I tell them, “Don’t worry, this is routine. It’s just the ITCZ.”

There is a part of me that wants there do be some other explanation for the crash of Air France Flight 447.

Comments

  1. #1 Paul Browne
    June 9, 2009

    Thanks for writing this, I’m rather ashamed to admit that I wasn’t aware that the ITCZ was such a permanent feature.

    While people often associate poor weather with crashes on or near to take off and landing phenomenon such as the ITCZ and “rotor” associated with mountain lee wave can be extrordinarily powerful and in their more extreme forms are fully capable of damaging large modern aircraft at cruising altitudes.

    This was one of the motivations behind the Perlan project to study mountain wave which saw the late Steve Fossett reach over 50,000ft in a glider with Einar Enevoldson.

    http://www.perlanproject.com/

  2. #2 Courtney
    June 9, 2009

    The most recent Ask the Pilot column at Salon covered why the pilots probably decided to fly through the storm (using a satellite-derived simulated weather radar image). The short version is that going around the storm would have been a huge diversion, and there were enough spaces between the really strong parts of the storm that the pilots likely though they could avoid them.

  3. #3 Courtney
    June 9, 2009

    Grr. I screwed up the link in the above comment. Here’s the fixed link.

  4. #4 ToSeek
    June 9, 2009

    It might be worth mentioning that India’s famous monsoon season is caused by the ITCZ moving north for the summer.

  5. #5 Alice
    June 9, 2009

    This convergence zone was not covered as part of the three (or is it four) R’s in school. Therefore it is new to most people. Thanks for pointing it out!

  6. #6 Nathan Myers
    June 9, 2009

    If I’m reading that temperature image right, the equator reads as less hot than immediately north and south of it. My understanding was that the storm system had a lot to do with counterclockwise air current systems in one hemisphere osculating with clockwise air current systems in the other, but I don’t doubt that the real circumstances are way more complicated than that.

  7. #7 Greg Laden
    June 9, 2009

    Nathan: that line of lighter orange is the rapid cooling connected to the ultra high cloud tops at the top of the convergence zone.

    The clockwise/counterclockwise thing you mentioned is not really a factor.

  8. #8 GBlivins
    June 10, 2009

    Do you find it curious that flight 447 also flew through the area of earth in which the magnetic field is the weakest?

  9. #9 Greg Laden
    June 10, 2009

    GBlivins: That idea is less crazy than it may sound, but I think that the damage that occurs to human-made machinery in this context is known to happen at much higher altitudes than this plane would have been flying.

  10. #10 GBlivins
    June 10, 2009

    Perhaps it’s not a direct cause. Perhaps it could have some effect on storms within the region; perhaps intensifying lightning or super charging storms in a way we have not yet seen. It’s a crazy long-shot guess, with no empirical data, but I can’t help wonder as the magnetic field weakens, what effect it will have on other meteorological phenomenon.

  11. #11 GBlivins
    June 10, 2009

    Perhaps it’s not a direct cause. Perhaps it could have some effect on storms within the region; perhaps intensifying lightning or super charging storms in a way we have not yet seen. It’s a crazy long-shot guess, with no empirical data, but I can’t help wonder as the magnetic field weakens, what effect it will have on other meteorological phenomenon.

  12. #12 anjel
    June 10, 2009

    wow…this is very interesting..never learned this in school and I dont even know if they actualy even do so now! Question is .. the airline flights have been going through that ITCZ for years now… arent there flights going thru that almost daily?. Why havent we had more crashes if this was the case? Thank God we don’t. My prayers go out to all the family and friends who are suffereing this tragedy. Thanks for all that info !

  13. #13 anjel
    June 10, 2009

    thanks for all that interesting info. I never learned in chool about this ITCZ. HOWEVER… planes fly constantly through that area on the routes as that flight did.. why hasnt this hapened more often in this area.. around the equator..

  14. #14 Greg Laden
    June 10, 2009

    Angel: The ITCZ is crossed by airliners many times a day, maybe many times an hour. It is generally quite safe, but these are mondo big storms, and storms are always an issue. It is not like they just drive through it without trying to go around the worst parts. I think.

    It is said that almost all modern aircraft crashes have multiple causes, because the planes are designed and flown so well. The storm may be one factor, strange stuff about how the Airbus auto pilot works may be another.

  15. #15 Dr Nagaraja
    August 14, 2009

    Hi,

    Thanks for the nice experience and documenting it. I am working on a problem to study the behaviour of parcel of air having greater ionization may pass to this trough and leads to the formation of cloud condensation nuclei, which causes clouds to have rich of charges and triggers the rain.

    Happy to receive more related articles.

    Thanking you

    Nagaraj
    Assistant professor

  16. #16 Dr Nagaraja
    August 14, 2009

    Hi,

    Thanks for the nice experience and documenting it. I am working on a problem to study the behaviour of parcel of air having greater ionization may pass to this trough and leads to the formation of cloud condensation nuclei, which causes clouds to have rich of charges and triggers the rain.

    Happy to receive more related articles.

    Thanking you

    Nagaraj
    Assistant professor

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