High above the sweaty streets lies Manhattan’s most hidden luxury: the rooftop pool.

In New York City, it’s always about numbers. The Department of Environmental Protection has picked some 1,700 municipal-owned properties — 500 schools, 600 comfort stations, 10 housing projects, 400 spray showers and 87 parks among them — to help the city cut back on water use. For locals nobly struggling to conserve resources, there is also this number to make them steam: $7.5 million. That’s the asking price for a four-bedroom apartment in Franklin Place, a luxury condo development in TriBeCa with a rooftop pool.

You wouldn’t know it, but they’re up there — those turquoise oases, invisible to those of us who cope each day with sour summer smells, sweltering subway platforms and scorching sidewalks. More than any other city, New York converts the graph of its income inequality into a vertical urban plan, with most people spread out at street level — conniving to linger for just one extra second before an air-conditioned storefront when its door swings open — and the lucky few in their secret aeries and tiny triangle bikinis, lolling poolside.

Once upon a time, relief from summer in the city meant a vandalized fire hydrant or a snooze on the fire escape. When I was growing up in New York, the closest thing to a rooftop pool was dropping water balloons onto friends from my second-story window, before trading places so they could drop them on me. Rooftops were deserts of sticky blacktop, the last places to which any sane New Yorker would retreat. And rooftop pools were as exotic as soccer fans. But now they’re proliferating as come-ons for condos and hotels — whose developers, truth be told, would probably prefer erecting more lucrative penthouses but must occasionally meet bothersome green requirements. Landscaped pools help turn those requirements to their advantage.

Are we jealous? The pools are utilitarian, occasionally clumsy architecture, mostly devised to maintain an aura of exclusivity. The real estate market thrives on amenity envy. And yet, envy aside, there is something deliciously voyeuristic about helicopter photographs of a suddenly unfamiliar, upturned cityscape dotted with David Hockney bathers in dappled water and lounge chairs. Those chairs come with their own numbers. The Dream Downtown, a hotel in the Meatpacking District, charges $175 a day to use the pool, Monday through Thursday. A cabana on the weekend will set you back at least $2,500.