Alcides Moreno was hurtling downward at a speed up to 124 mph, clinging to a 1,250-pound scaffold that acted like a surfboard in the sky.

Those grappling with why he miraculously survived his plunge from the roof of a 47-story high-rise say the aluminum platform added air resistance that slowed his descent – and blunted the tremendous force of hitting the concrete pavement in an alley below.

Maybe, experts said, a random air current rising between the Upper East Side buildings where Moreno plummeted slowed him the extra bit that spared his life.

The 37-year-old window washer – who remains in critical condition but has moved his arms and legs and tried to open his eyes, relatives say – has amazed trauma doctors and physicists alike.

“Fifty percent of people who fall four to five stories die. By the time you reach 10 or 11 stories, just about everyone dies,” said Dr. Sheldon Teperman, director of trauma and critical-care surgery at Jacobi Medical Center in The Bronx.

“This guy absolutely should have died.”

The tragedy occurred on Dec. 7 at about 10:15 a.m. as Moreno and his brother Edgar, 30, employees of City Wide Window Cleaning, got ready for a day’s work at the Solow Tower, a luxury apartment building at 265 E. 66th St. with a black-glass surface.

The workers – not yet wearing required safety harnesses – fell or were dragged off the roof when the 16-foot-long “swing scaffold,” which could slide around the building, collapsed.

Officials suspect the cause was improperly secured cables, but investigations are ongoing.

Physicists theorize it was Alcides’ training, presence of mind – or luck – that he remained atop the Louisville Ladder scaffold as it plummeted that saved him.

He landed in a tangle of cables and bent railings that may have broken his 500-foot fall or absorbed some of the shock.

His brother slipped off the scaffold and smashed onto the top of a brick wall, which cut him in half.

In a free fall, both men would quickly have hit a maximum speed of 124 mph, or terminal velocity – the point when gravity pulling a person down is balanced by upward air friction, said James Kakaklios, a physics professor at the University of Minnesota.

But the scaffold’s platform likely slowed Alcides’ descent “significantly” by pushing the air, he said.

His landing position also could have made a huge difference.

“If he was lying flat, not only would the scaffold act as a shock absorber, but the force to stop him would be spread more evenly over his body. If he came down on one point, the sudden shock could easily break the back or neck,” Kakaklios said.

He compared Alcides to people who surprisingly survived the I-35W bridge collapse in Minneapolis on Aug. 1. Some “rode their cars down” the splintered bridge.

Supports underneath added air resistance and slowed its fall into the Mississippi River.

Alcides suffered blood clots in the brain, collapsed lungs, numerous broken bones, damaged kidneys and other internal injuries, relatives said.

Stefan Bright, safety director for the International Window Cleaners Association, never heard of a case like Alcides’, calling it “extreme luck.”

“Window washers have fallen six feet and died,” Bright said.

susan.edelman@nypost.com