Landing in São Paulo recently, I was stunned by the intensity of the applause that rumbled through the plane’s cabin upon touching down at Guarulhos International Airport.

It’s a curious thing, that seemingly spontaneous ovation that often accompanies an aircraft’s safe return to terra firma. I call it the Landing Clap, a common yet enigmatic phenomenon of modern air travel.

The Landing Clap doesn’t follow every touchdown. In fact, with extremely few exceptions, it is rarely heard on domestic routes at all. The precious few times I have witnessed the Landing Clap on internal flights, it was at the end of a journey plagued by seriously heavy turbulence or obvious mechanical difficulties.

This leads me to conclude that, first and foremost, the Landing Clap is a joyful sigh of relief, a cathartic public celebration at the conclusion of a particularly disquieting flight.

Above and beyond the fear factor, however, the Landing Clap is a peculiar expression of love for one’s home and country. I have noticed that the salvo is always heartiest on national airlines and, with few exceptions, the applauders are always returning to their native soil. Spaniards on Iberia Air Lines who sit on their hands when the plane lands at New York’s John F. Kennedy International Airport will clap heartily when arriving at Madrid’s Barajas.

Scattered applause always means that a majority of a plane’s passengers are on an outbound flight. Hundreds of hours of subjective study have also proved that the length of any given flight is directly proportional to the heartiness of the applause. For example, the Landing Clap at the termination of a journey from London to Hong Kong (13 hours) is distinctly heartier than, say, on a flight from New York to Prague (8 hours).

Furthermore, the relative enthusiasm of any given Landing Clap is commensurate with the “cultural passion” of the clappers. Extroverts like the Spanish — or in my recent case, the Brazilians — clap with far more zeal than the relatively restrained English or Argentines.

But no culture is immune from participating in this curious ritual. Jordanians, Indians, Kenyans — I have heard ovations from them all. Even the famously unemotional Japanese engage in the Landing Clap.

The heartiest clap? That distinction goes to El Al passengers from New York touching down at Israel’s Ben Gurion International Airport. Because the plane is often filled with Brooklynites, this is one of the rare occasions where the majority of passengers applaud at the end of an outbound flight.

As for myself, I must admit to having literally kissed the ground when returning from particularly wretched places, but I have never participated in the Landing Clap. To me the display suggests a lack of sophistication that only serves to identify the infrequent flyers on board.

More importantly, the Landing Clap has a displeasing nationalistic sound, as unpleasant as Olympic medal counts and just a notch above soccer hooliganism.

Passionate applause is de rigueur when planeloads of Russians, Germans, Italians and others from flag-waving cultures return safely to their motherland.

Never mind that, statistically, it’s far more risky to drive a car — and landing actually signifies that the most dangerous part of the journey is about to begin.

Author Bio: Daniel Levine is a trends expert, keynote speaker and former writer of a dozen Frommer’s travel guidebooks. He is currently the Director of The Avant-Guide Institute, a global trends consultancy based in New York City.