That Kate Bush: She's so formulaic!

Okay, not really, given the British songstress's well-known penchant for unconventionality.

But still: Not even five minutes into her long-overdue double album, "Aerial," we find the idiosyncratic artist singing tenderly about a man "with an obsessive nature and deep fascination for numbers /and a complete infatuation with the calculation of pi." To illuminate her point, Bush then transforms the digits that make up the ratio of a circle's circumference to its diameter into one of the most atypical if strangely compelling choruses in recent memory.

"Threeeeeeee /Point-one four one five niiiiinne," she purrs in that swooping, otherworldly three-octave voice of hers. "Two six five three five eight nine seven nine three twooooo," etc., ad (nearly) infinitum.

Indeed, by the time the jazzy, atmospheric song, "Pi," ends, Bush has carried the titular mathematical constant out to more than 100 decimal places. And yet, you somehow find yourself wanting to hear more.

For those about to math-rock, Kate Bush salutes you.

Here's more math: "Aerial" is basically half-good.

After a 12-year hiatus -- a long period during which her fans might have suffered in complete silence if not for all those new Bush-y albums from Tori Amos -- the real Kate Bush has finally presented the world with 16 new tracks. Alas, nine of them make up "Aerial's" not particularly rewarding second disc, "A Sky of Honey," an odd, uneven song suite about art, the sun, the moon and the stars, and, quite naturally, birds that "sound like they're saying words."

Though the drum-machine-driven "Somewhere in Between" is a standout, the "Sky" disc is largely loaded with dated instrumentation, chirping-bird sounds and other artistic flourishes that conspire against greatness, or even goodness, and it features one of the weakest songs of Bush's otherwise impressive career: "Sunset," which starts as an elevator-jazz number and then gets worse, abruptly morphing into some sort of pop-flamenco track.

Perhaps the second disc is some sort of a cruel joke. After all, somebody who sounds an awful lot like Bush spends a good deal of the title song laughing loudly and somewhat manically. Theoretically, she's guffawing at birds, but you never know.

And no, there is no "Pi" in the "Sky." Instead, the song appears on "Aerial's" vastly superior first disc, "A Sea of Honey," which would've made for a fine release on its own.

"Sea's" best song is the lead single, "King of the Mountain," on which Bush wonders whether Elvis has really left the building, or if the King has maybe just punk'd us all. "Another Hollywood waitress / Is telling us she's having your baby," she sings in a quivering voice. The cinematic, slightly pitch-twisted song serves as a reminder that Bush, a noted studio perfectionist, is capable of fits of self-production brilliance, not unlike Brian Wilson circa "Pet Sounds." (Which may explain why "Aerial's" cover prominently features a digital soundwave image.)

But Bush can also create beauty out of sonic simplicity: "Mrs. Bartolozzi," a gorgeous song about housework, of all things, features just a piano and Bush's vocals -- at least until the lilting sound of a girls' choir sweeps through the back part of the mix. For the majority of "Bertie," an elegant valentine to Bush's son, her voice is accompanied by nothing more than a harpsichord, though other elements, including strings and tambourines, are added and subtracted at various points.

Of course, a lot of Bush's lyrics are a long way from simplicity. A literary songwriter whose first hit was the Emily Bronte-inspired "Wuthering Heights," she suffers sometimes from an acute case of obtuseness. Even after a dozen years, she still knows how to confound listeners with the best of them; thus, you get lines like this, from the throbbing "How to Be Invisible": "Eye of Braille / Hem of anorak / Stem of wallflower / Hair of doormat."

Thump of conundrum.

After 12 years of silence, Kate Bush has finally released 16 new tracks on "Aerial."