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.

A timely repost


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."


"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."


"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.


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


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


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:


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.


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


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:


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.

More like this

There are some ideas out there as to why flight 447 went down. The challenge will be to test those ideas and see which ones are indeed likely. Whether or not the actual cause of the crash can be determined remains to be seen. With thunderstorms the 'storm scope' can help dodge the turbulence; it is clear air turbulence which remains one of the biggest nuisances - pilots typically find out about them from communication with pilots who have just flown through the stuff.

By MadScientist (not verified) on 12 May 2011 #permalink

The reason that rising air always causes thunderstorms is because of adiabatic expansion. As air expands, the volume increases and it takes energy for the volume elements to push against atmospheric pressure and generate work while it is expanding. That is why expanding gases always cool.

The vapor pressure of water is exponential with temperature, so as the air cools, the vapor pressure of water goes down and the water condenses, liberating its heat of vaporization.

If the temperature gets below the freezing point, the water freezes and you get hail. That might be what happened with flight 447, there was hail, a piece of hail hit one of the pitot tubes used to measure air pressure and wind speed and they lost that data. Water can also get supercooled, so it is liquid but at -20. Then when it hits surfaces it can freeze. That can block pitot tubes too, and can accumulate on wings and reduce lift.

The reason that it usually becomes clear at night is because the air near the ground cools, the air above it falls, by falling the air is compressed, it heats via adiabatic compression and the little drops of water that comprise clouds evaporate.

I've always considered the ITCZ to be misnamed. It should be the Intertropical Nonconvergence Zone, because the trade winds generated by the Hadley circulation don't mix. The "gap" in between them can be a place of eery calm. I've read accounts of sailors (and I mean real sailors, on sailing vessels) get becalmed there for weeks at a time due to migration of the thermal equator. Sometimes, they starved or went mad.

When I make my multi-million km2 floating plantations to make biofuels from plants growing in sea water, I will start by putting them at the equator to avoid hurricanes.

Once they get big enough to reach far enough from the equator to be hit by hurricanes, I will be able to control the weather sufficiently that they will not be hit.

Wow, I'd never even heard of the ITCZ!! I have flown near the equator (in a prop plane, pre-jet flight), but never across the equator. I'll keep that in mind next time I decide on a travel destination.

@gwen: It's not always rough everywhere when crossing the equator (and the ITCZ can drift a few degrees N and S as well). However, when it *is* rough, as Greg pointed out, commercial aircraft can't dodge it. Near the equator the turbulence can extend to over 16km altitude as opposed to jet aircraft ceiling (max) altitudes of 12-14km depending on the aircraft type, nor do aircraft typically cruise at their max. altitude. Another hazard to aircraft are 'gravity waves' which occur in the vicinity of mountains (the Rockies are notorious) and I'd rather have a nice ride in the ITCZ. I flew through gravity waves around New Zealand once and it was the roughest ride I'd ever had in a heavy aircraft. As in teh case of the ITCZ, flying over/near a mountain won't necessarily give you a rough ride.

By MadScientist (not verified) on 13 May 2011 #permalink

You wrote "The earth is a big ball, and spins on an axis that is perpendicular to the plane of the sun."

Er, no, that's wrong. The axis of spin is 23° off the plane of the ecliptic. That's why we have seasons :-)


Actually, that isn't correct either. The tilt of the axis varies over time and is almost never exactly 23 degrees. That's why we have ice ages!!!

Ah, a little pedant to pedant battle does one good...

But seriously, yes, you are almost exactly correct. Indeed, there is a thing called the "Climatological" or "Climatic" equator, which reflects the offset of the equator against the sun's plane and tracks the seasonality.

Actually the axial tilt is about 23.5 degrees relative to a plane normal to the ecliptic. It's 66.5 degrees off the ecliptic itself. The variation in tilt is why the north star is not always the north star. I once heard a theory that the Arthurian legend was rooted in the movement of the axis. At one time, celestial north was in the constellation Draco, the dragon, but it has shifted to polarism which is in Ursa minor (the small bear). Arthur means "the bear", and Arthur's father was Pendragon, which means "head of the dragon." Can't remember where I heard all of this, but I always tell my students about it in my intro met class.

"Can't remember where I heard all of this, but I always tell my students about it in my intro met class. "

There should be a wiki of just stuff we all tell our students in intro classes even though we don't really remember how we know them.