While hurricane Irene bore down on New York City last night, so steady in her course and yet spinning more wildly than a runaway top, I considered reminding my esteemed readers that both this blog and the storm it was writing about share a common origin. But then I thought that might be bad luck and I went to bed instead.

This morning though, lazily getting up around nine and noticing that the rain had stopped and the wind was no more than a steady breeze, I realized luck was no longer a factor. Hurricane Irene, it appeared, had run out of fuel. Its center was passing right atop the city and yet, the tiger had turned into a mere tired kitten.

It’s now almost eleven. The house smells of coffee, our sliding door is wide open on a terrace-full of wet plants in their pots taken down yesterday from the roof and edges, and the sun is shyly trying to peak through an ordinary cloud clover. Radar reveals a strangely torn Irene, having now been downgraded to a weak tropical storm and lost most of its rotation and power. Reminiscent of what was a very large storm, a huge cloud bank remains, stretching from Washington D.C. to Toronto, past Montreal, all the way to Gaspésie, down to Halifax and along the coast to Boston, but it stops short of NYC.

According to the few damage reports I’ve read, our metropolitan area weathered the whether almost impeccably. There are trees down, of course, some flooding, and conflicting accounts of power outages, but nothing widespread, nothing major. The MTA must already be assessing its boot-up procedure, a probably lengthy procedure.

So it would seem that I now have time to tell you about those origins.

You see, all things earth-linked are bound by a curious effect born from the furious rotation of our planet through space. Coriolis is the name, pseudo force is the game, they’ll play right here on this Earth. And then they’ll bet for the biggest stakes yet, the souls of the… Yeah, I just can’t make it rime. It was stolen from an old Chris de Burgh song.

But let me explain.

Only three men in history have held an absolute apple patent: Sir Isaac Newton, Wilhelm (William) Tell and Steve Jobs. The former was the father of the three famous laws of motion that form classical mechanics; his first law dictates that an object in uniform motion tends to stay in uniform motion unless acted upon by an external force. In other words, an air mass moving from A to B would go straight from A to B at an even speed.

However this only works well in an inertial frame of reference. But our planet is spinning on itself and becomes a rotating reference frame. Suddenly, the fictitious – or pseudo – Coriolis and centrifugal forces appear.

The Coriolis pseudo force is one that makes an object in actual uniform motion through space appear to deviate to the left or right because of the observer’s location on the surface of a rotating reference frame.  If you’re in the Northern Hemisphere, the deviation will appear to be to the right. Too complicated? OK, you’re on a merry-go-around, ridding a horse but facing out. You throw a ball straight out, away from you and the carousel. The ball is going to travel essentially straight out through space, from where you were when you threw it to where it lands. But you are on a spinning horse and as the horse takes you away, the ball appears to deviate away from you in a curve. That’s the Coriolis effect.

The same applies to a hurricane. Far from us, a tropical wave causes a disturbance off the Western shores of Africa. This is near the equator and the ocean is warm. The disturbance feeds on hot, moist air from the sea and grows slowly as it drifts westward pushed by the trade winds. Hot air being lighter than colder surrounding air, it starts rising. If the conditions are right, outside wind influence and an unstable air mass further promote the climb. All that warm air is humid and as it ascends and cools off, it starts to condensate, releasing tremendous amounts of heat and energy.

The pressure at the center of this phenomenon drops because of the warmer temperatures and rising air. More air is drawn in from the outside, creating an inward momentum. That’s exactly when the Coriolis effect kicks in and diverts all that flow of air to the right, initiating a counterclockwise rotation of air around the central low pressure. A cyclonic storm arises.

Eventually, with the help of ideal upper level conditions which, way up in the troposphere, establish a good outward flow of all that rising warm air without much shear from strong winds that would disorganize the pattern, the disturbance gathers so much momentum and strength that it is promoted to the status of tropical depression, then tropical storm. Its ego grows tenfold while its central pressure drops further.

With a strong counterclockwise rotation established, as long as the three key elements – warm water and moist air, proper low level wind and instability, and a good upper outward flow – are maintained, the storm strengthens. It slowly builds up. Bands of rain form along its spiral, galaxy-looking arms. Close to the center, a wall appears where the winds are strongest and the warm air’s ascent maximum. But the system has to somehow balance itself, and if all that warm air is going to rise, something must replace it: a flow of cold dry air coming from the upper layers begins to plunge right through the center of the storm, clearing the clouds and cancelling the wind. It’s the eye of the hurricane, often calm, sunny and windless.

But don’t let the eye fool you; a killer was just born.

It will keep growing and rushing westward on an increasingly northbound trajectory itself affected by the Coriolis effect as long as the conditions allow it to, potentially growing to a potent category 5 hurricane, its central pressure dropping below 900 mb and eye walls featuring winds in excess of 155 miles per hour.

But sooner or later, the hurricane makes landfall or runs over colder water, and the chain reaction is broken. Without the presence of warm water to evaporate into warm moist air, the heat engine stops working and the storm weakens.

The violent warm updraft slows, and with it, the storm’s rotation. The central pressure creeps up. From then on, unless a rare event throws the storm back down over better water conditions, it’s all downhill. Within hours, sometimes up to a day or so, the hurricane has been tamed to a mere annoyance. It usually dies alone and ignored, far from its birthplace and deep into frigid northern latitudes.

But based on the damage and suffering it inflicted, the hurricane might long be remembered and its simple name will suffice to trigger shivers and sorrow for many years to come. How could we forget the greatest, and meanest of them all, Wilma, Katrina, Ivan, Floyd, Hugo, Andrew…

Today, New York was lucky. Let’s not complain about the boarding up and the closures and the lost business and the evacuations and the hang over. It was all worth the trouble. Much better safe then sorry. Let’s look at it like a training exercise. Irene was a city-wide emergency preparedness drill. A costly one, but they always are. So that, down the line, when the time comes and real trouble shows its ugly face again, we will be prepared.

[Update] And to all who ARE affected, flooded or without power, the many thousands of you, my sympathies!

[Update 2] And yes, despite the training drill, hurricane Sandy nailed us real good a year later…