Last November, Qantas launched a major advertising campaign called “Feels Like Home”. It shows five real Australian travellers reuniting with their families after extended periods away, to the strains of Feels Like Home, a Randy Newman song performed by Australian artist Martha Marlow.

The two-minute montage version made me cry. Indeed, Qantas handed around boxes of tissues in its focus groups. The ad sparked so many tears that the Daily Mail dubbed Qantas “The Crying Kangaroo”.

But perhaps the ad is so successful because it evokes the well-known phenomenon of crying on aeroplanes.

Floating far above the ground in a padded chair, in a dry, sealed aeroplane cabin that hums gently, facing away from other passengers, with no phone or email interrupting our thoughts, we travel inward as well as onward. It’s a sentimental journey. We indulge in emotional thinking, watch melodramatic movies on the in-flight entertainment system, read light novels purchased in airport bookshops, and listen to maudlin music. And they make us cry.

In a 2011 episode of This American Life, Brett Martin likens the experience of flying to “some sterile, infantile travel purgatory. You’re strapped in, given a blanket, a sippy cup, and tiny silverware, forced to do what you’re told and borne away at speeds we can’t conceive, without seeing where we’re going”. No wonder we turn into big crybabies.

Particle physicist and TV presenter Brian Cox cries on planes. So does actor Jake Gyllenhaal. Richard Madden, who played Robb Stark on Game of Thrones, wept over the Red Wedding all the way home to London: “I was the crazy boy on the plane crying at about midnight.” Even civil aviation enthusiasts find themselves crying.

In 2011, Virgin Atlantic issued a jocular “emotional health warning” before some movies after a highly scientific survey on its UK Facebook page revealed 55% of respondents had experienced heightened in-flight emotions. Some 41% of male respondents had disguised their tears by burying their faces in blankets; women tended to feign “something in their eyes”.

Interestingly, the song used in the Qantas commercial also features prominently in the tear-jerking 2009 drama My Sister’s Keeper, this time performed by Edwina Hayes. It plays during a montage scene in which terminal 15-year-old leukaemia patient Kate (Sofia Vassilieva) visits the beach with her family for the last time.

When Virgin Atlantic polled its Facebook fans on which in-flight movies made them cry most, My Sister’s Keeper came in at number four. Toy Story 3, a sentimental tale of putting away childish things, led Virgin’s list of “top ten tearjerkers”.

Emotional tears contain hormones that differentiate them from the eye’s regular lubrication, or tears shed in response to an irritant such as dust or onions. Photographer Rose-Lynn Fisher discovered these stark differences in 2013 when she captured dried tears under a microscope.

“Much as only a few people can witness our toilet tears, we can cry on a plane knowing only one or two other people — our seating row neighbours — will directly observe us.”

Jeffrey Kottler’s 1996 book The Language of Tears suggests crying has an evolutionary function: as infants, we can signal our distress to nearby adults without loud cries revealing our vulnerability to predators. Dutch psychologist and crying expert Ad Vingerhoets, author of the 2013 book Why Only Humans Weep, adds that in early human communities, crying in adults could have signalled mutual trust and connectedness.

Cultural historian Thomas Dixon reminds us, however, of the range of emotions that provoke tears: joy, pride, pity, frustration and more. He also points out that the bad reputation of public crying as “emotional incontinence” stems from late-19th-century psychoanalytic literature: “that a similar shame should attach to a public stream of tears as to a public stream of urine”. (Some nutty Freudians even suggested women’s crying revealed their subconscious desire for male urination.)

The percolation of psychoanalytic ideas into mainstream discourse has led to two broad theories of crying: that it signifies a healthy release of repressed emotion; and that it represents a regression to a childlike state.

But a study in 1997 by Vingerhoets and colleagues discovered that most adults don’t cry openly in public; they wait until they’re at home, alone or in the company of one other person. Crying can be a response to stress, but it’s governed by the body’s parasympathetic nervous system — the so-called “rest and digest” functions that kick in during the aftermath of the sympathetic nervous system’s more immediate “fight and flight” reactions. It’s when we relax and reflect on our situation that we give ourselves permission to cry.

When asked to explain recent crying jags, Vingerhoet’s study participants far more frequently chose “separation” or “rejection” than other options including “pain and injury” and “criticism”. And of those who answered “rejection”, the most common subcategory they selected was “loneliness”.

Flying represents our separation from those we love. If you want to get both evolutionary and psychoanalytic, you could view separation crying as a vestigial echo of our former babyish vulnerability. In the cloistered environment of the cabin, we’re forced to confront our existential aloneness.

But while Virgin Atlantic film critic Jason Solomons suggests we cry at in-flight movie scenes that reflect our own melancholy at leaving happiness behind, or yearning for its return, Brett Martin of This American Life notices that we’re less likely to cry at the sad parts of a movie than the happy ones: “The parts where everything turns out all right.”

“Flying and films is a heady cocktail,” Solomons said, “the images and feelings so close to your eyeballs, so intimate.” And it’s the intimacy of the flying space — which Martin associates with a regression to childhood road trips — that makes a plane seat perhaps the most private kind of public space.

Think of that other hallowed public crying venue: the toilet cubicle. Like a plane, it’s small, quiet and impersonal. Much as only a few people can witness our toilet tears, we can cry on a plane knowing only one or two other people — our seating row neighbours — will directly observe us.

Aeroplanes, however, are unique among liminal public spaces because they are unmoored from the earth. As air travel has become more ordinary and frequent, we’ve stopped feeling so overwhelmed that we wingless creatures may travel at such heights and speeds. Only those afraid of flying still contemplate its singular un-earthliness.

Other travellers, however, experience this alienation from our ordinary lives as an emotional release. As Martin put it: “Something happens up there, the space between where you’re going and where you’ve left … some strange overhead compartment of the heart opens up.”

A famous Bible verse, now more often used as an epithet of exasperation, is John 11:35, “Jesus wept.” Back when humans only roamed the earth, we imagined the clouds as the domain of gods and supernatural beings. Now, we can float magically there; and like the gods of old, our change in perspective enables us to feel not just for ourselves, but also for humanity.

As W.H. Auden wrote in The Age of Anxiety (1947):

“Sob, heavy world
Sob as you spin,
Mantled in mist
Remote from the happy.”