Flight 447 and the ITCZ

"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.

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.