PORTLAND — Vampires aren’t supposed to see the light of day. But the sun keeps shining on the set of “Twilight,” an adaptation of Stephenie Meyer’s popular teen vampire-mortal love story.

The book is set in Forks, Wash., and filming is taking place around the usually rainy Pacific Northwest.

But the weather isn’t cooperating, so fake rain drizzles down the window at Madison High School in Portland, enhancing Robert Pattinson’s chalky complexion as he sits in a classroom inside.

Amid microscopes, dictionaries and biology posters, Pattinson’s character, Edward, appears to be an average student. Looking all jaw and cheekbones, with an intense gaze, he is the paramour of classmate Isabella, played by Kristen Stewart (“Into the Wild”).

Smitten with Edward’s good looks, Isabella fails to immediately notice a key flaw — he’s a vampire.

“Edward is the reluctant 180-year-old vampire stuck in the body of a 17-year-old boy who falls in love with a mortal and destroys both of their lives, in a romantic and loving way,” Pattinson said.

“Twilight” is among four books by Meyer that has stirred a slightly fanatic base of preteen fans.

The series has sold more than 5.5 million books domestically, and has been on the best-seller lists of The New York Times, USA Today and Publisher’s Weekly.

“Twilight” has been translated into 20 languages and published in 34 countries, and Meyer’s book signings have a “Harry Potter”-esque feel to them. But Pattinson, who played Cedric Diggory in the “Harry Potter and the Goblet of Fire” and “Harry Potter and the Order of the Phoenix” films, dismisses that comparison.

“Twilight” does aim for a similar age bracket, and the movie is being directed by Catherine Hardwicke, who directed teen-friendly films “Thirteen” and “Lords of Dogtown.”

“I was really drawn to voice of Bella in the books,” Hardwicke said. “You really felt like you were back in your teenage days again and feel what it’s like to fall madly, obsessively in love with some guy that you’d do anything for him. Even turn yourself into a vampire.”

But these vampires aren’t your dark and dreary undead type. They are more James Dean cool with a dash of Superman strength.

Pattinson said that wasn’t easy.

“I’m atrociously unfit,” he said.

“Twilight” is expected to be released Dec. 12.