A lone cluster of clouds gathers demurely behind the Con Edison Building as Claire Kuhn and Jessica Watson spread their striped beach towels into a pristine pool of sun that has formed on the roof. Un-self-consciously, they splay their bikini-clad bodies toward the light.

On a quest for that ultimate summer badge — casually sun-tinged skin — Claire, 15, and Jessica, 16, are here on the blacktop nearly every day on East 10th Street in Manhattan. If they remember, on the half-hour, they flip onto their stomachs to make sure they brown evenly. They usually can stand an hour or so before it gets too hot.

As long as there have been sun worshipers in search of the perfect tan in the city, there has been the tar beach. Roofs have long been the urbanites’ slightly hotter, slightly gooier answer to the backyard pools and lawns of the suburbs — like private little plots without bothersome trees to throw shade.

Jessica even insists that she likes this urban substitute better than the real beach; she cites the view, the pleasurable sense of being part of a members-only world, and of course the fact that “there is no sand to get stuck to your skin.”

But in this she may be a vanishing breed. This time-honored summer escape is a diminished, perhaps even dying habit. This has been noted by those who have a bird’s-eye access to the city: helicopter pilots, water tank repairmen and occupants of tall buildings in otherwise low-lying neighborhoods.

There are many explanations: security and insurance concerns since 9/11, real estate prices so high that roof space has become a lucrative commodity, and the rise in popularity of summer beach shares among young people.

From the skies above New York where he reports on accidents and fires in a helicopter for WCBS-TV, Joe Biermann has noted that rooftop tanning is a declining pastime. “Since 9/11 we don’t see a lot of people on the roofs,” he said. “Maybe it is a security issue. We think the landlords must be keeping the doors locked.”

Landlords and tenants have long skirmished for the rights to the roof, but since the fall of 2001 the landlords have been winning, according to Joseph Strasburg, president of the Rent Stabilization Association, the landlords’ group.

“If we are talking pure tar, like those wonderful days when we were growing up,” said Mr. Strasburg, who as a teenager used to sunbathe on the roof of his family’s apartment building in the Bronx. “Landlords are not going to allow access, not with insurance rates like they are.”

Andrew Rosenwach, president of the Rosenwach Tank Company, which builds and repairs water tanks throughout the city, agreed that he sees fewer tanners. But he has another explanation.

“People used to be up there sunbathing,” he said, “but there is just a lot of other infrastructure up there now.”

And not just infrastructure, but more like a miniskyline building boom.

Once rooftops were, well, rooftops: tar-covered yardage with a water tank or chimneys and little else.

The patch shared by Claire and Jessica remains very much of that era, and they have a routine for their visits. First, they wake up; they aim to meet at noon, though they often oversleep and actually get together about 2 p.m. (No, they do not have summer jobs.) Jessica always comes to Claire’s house, a tenement off Avenue B, where they climb five stories to the roof. Then they plug an iPod into its speaker. These days they like to listen to Kings of Leon and the soundtrack from “Disturbia.”

But with real estate prices soaring, most building owners and developers have figured out how to award roof rights, for everything from patio space to cellphone towers.

David Von Spreckelsen, a vice president for Toll Brothers, a luxury developer, said that almost all of the new construction and renovation his company is working on include the option to buy rooftop space. At a Manhattan apartment building just opening to tenants on Third Avenue at East 13th Street, residents may buy “cabanas.” For $150,000, the buyer gets 300 to 400 square feet and a water and electrical hookup. Also, the floor is concrete, not tar.

There is, Mr. Von Spreckelsen added, a small public deck that is shared, for those who cannot afford cabanas.

But if rooftop access is evolving from an alternative beach escape for the lower classes to a privilege of the wealthiest, it has not quite happened yet.

Richard Casciato and April Dinsmore, married artists who live off Flatbush Avenue in Prospect Heights, Brooklyn, resist the trend.

The living room window of their four-story walk-up opens onto 600 square feet of roof space, and their use of it effectively doubles the size of their apartment for six months of the year. They have covered part of the roof with beige AstroTurf, but they leave a fringe of honest-to-goodness tar (silver-coated to reflect heat).

They insist that the breeze from the harbor, visible in the distance, keeps them cool even in hot weather and that the sound of traffic on Flatbush is a little like the roar of the surf.

They are out here every day for breakfast, reading the paper and, of course, soaking up the rays. Mr. Casciato, 39, who is Irish-Italian, is particularly devoted to the task. “He sweats a ton,” Ms. Dinsmore, 37, explained, “and eventually he turns a different ethnicity.”

And they are amazed that while they can look out and see a dozen roofs, they are the only people using one.

“Honestly,” Ms. Dinsmore sighed, “I don’t know what everyone else is thinking.”