Outdoors this summer, the sun was hot enough to pop corn. Inside the multiplexes, though, it was noticeably milder, in what industry analysts call a "tepid" season.

As you're reading this, summer movie grosses are being scrutinized as the Romans once studied animal entrails to divine the future.

Although rising admission prices have given the box office a slight uptick over last summer, the number of tickets sold appears to be the lowest since the summer of 1997.

Estimates are that the 2010 summer box office will be $4.35 billion, a 2.3 percent increase over the 2009 figure of $4.25 billion, says Paul Dergarabedian, industry analyst for hollywood.com. He projects that 552 million moviegoers will have bought tickets, a 2.5 percent drop from 566 million in summer '09.

What accounts for the box-office blahs? Hollywood analysts disagree, but the most frequently cited causes are:

Too many sequels.

Industry dependence on 3-D effects over three-dimensional characters.

Costlier tickets coupled with lower-quality films.

Social networks accelerating the speed of negative word-of-mouth.

Despite fare such as the heady Inception and heartwarming Toy Story 3, "it's been a lackluster summer," says Brandon Gray, president of boxofficemojo.com, which tracks Hollywood revenues.

He ascribes the box-office blues to "an overreliance on sequels and remakes" like Shrek Forever After and The A-Team.

With the exception of Toy Story 3, he says, "most were sequels for the sake of sequels rather than movies people wanted to see." Sex and the City 2, anyone?

For Dergarabedian, sequels are an asset, not a liability. "Last summer there were 10 sequels released, this year only seven," he says. "Four of them - Toy Story 3, Iron Man 2, Twilight: Eclipse, and Shrek Forever - were in the top five grossing films of the summer. Maybe there weren't enough sequels," he joked.

Asked to write an equation to explain the decline of summer ticket-buyers, Dergarabedian offers: "Higher ticket prices plus lower-quality fare equals unhappy moviegoers."

Although the national average for a movie ticket is $7.88 (which takes into account reduced prices for children and seniors), a ticket for a 3-D film carries an added surcharge of $3 to $5.

Where Dergarabedian and Gray agree is that 3-D movies enrich the studio but not necessarily the moviegoing experience. "Technology is great, but the best special effect is a great script," Dergarabedian says. (In the wake of the 3-D Avatar, which wowed moviegoers with its artistry, 2-D films like Alice in Wonderland and The Last Airbender were given 3-D enhancements to cash in on the craze.)

Still, four of the Top 10 summer movies - Toy Story 3, Shrek Forever After, Despicable Me, and The Last Airbender - were available in 3-D, boosting the revenues of each by as much as 25 percent, estimates Len Klady, the box-office guru at moviecitynews.com.

Even when filmgoers weren't paying the 3-D surcharge, summer movies of 2010 didn't give them much bang for their buck.

"Iron Man 2 is a perfect example of a studio resting on its laurels," says Gray. "Essentially, the marketing was, 'Iron Man is back, go see it.' There wasn't much development of the characters or conflicts, which is why Iron Man 2 made less than the first."

Much of the summer roster felt like "a seemingly endless supply of films that felt like video-rental candidates," says Dergarabedian.

He declined to name names. But just consider duds like The Sorcerer's Apprentice, Disney's live-action rethink of a sequence from Fantasia starring Nic Cage and Jay Baruchel. Or Killers, starring Katherine Heigl and Ashton Kutcher as husband-and-wife hit men.

Inception, with Leonardo DiCaprio worming his way into other people's dreams, and Salt, with Angelina Jolie speeding into other drivers' lanes, were the rare star vehicles that connected with audiences. For the most part 2010 was a summer where star-studded projects such as Knight and Day, with Tom Cruise and Cameron Diaz, and Robin Hood, with Russell Crowe and Cate Blanchett, underperformed.

While more than a third of summer releases - 13 out of 37 - grossed more than $100 million, "we didn't see as many hits, and the misses fizzled faster," observes Klady. (By comparison: In 2009, 15 summer films grossed over $100 million, and in 2008 and 2007, 17 passed the century mark.)

Dergarabedian observes that "back in the old days" - say, 2008 - "negative audience reaction could take days to affect a movie."

Social networking has accelerated negative word of mouth. Says Klady: "About 18 months ago we started seeing the phenomenon that after the 'avids' saw a picture on opening day, they killed it the next on Twitter." What Dergarabedian dubs the "big-on-Friday, dead-on-Sunday" phenom had a chilling effect on the fortunes of 2010 summer releases like Sex and the City 2 and Cats and Dogs.

Apart from Toy Story 3 and Inception, the popular and critical successes of the summer, did the movies do anything right?

"Even in a summer where Hollywood's not doing its best, PG-rated films like The Karate Kid and The Last Airbender really clicked with audiences," says Gray. Dergarabedian cites the success of indie and foreign titles like Letters to Juliet, The Kids Are All Right, and The Girl Who Played With Fire and the quality of the documentaries Restrepo and The Tillman Story.

If Gray were a dream pirate like DiCaprio in Inception, he'd implant a simple idea in the minds of moviemakers in advance of next summer: "Don't rely on 3-D and franchise branding. Focus on telling good stories."

Contact movie critic Carrie Rickey at 215-854-5402 or crickey@phillynews.com. Read her blog, "Flickgrrl," at http://www.philly.com/philly/blogs/flickgrrl/