The changes from season to season in pop music are not as sudden or pronounced as in other arts. Like the weather, pop music shifts: from squint-inducing songs to more contemplative, autumnal fare. Already, Ashlee Simpson, Jimmy Buffett, and “NOW, Vol. 16” are passing from the top of the charts like the last pink ribbons of sunset. (Their like won’t disappear altogether, of course – if God sold sunshine, it’d be summer all year.)

The fall hip-hop slate is led by releases from two of its most talented and articulate MCs. Stealing time from his rock-rap project, Black Jack Johnson, and his busy acting schedule, Mos Def has completed “The New Danger” (Geffen, September 28), the much-anticipated follow-up to 1999’s “Black On Both Sides.” Nas is also back with the double-album epic “Street’s Disciple” (Sony, September 14), which takes inspiration from his father’s era jazz by way of A Tribe Called Quest’s “Low End Theory.” And one-time Nas sparring partner Jay Z continues his busy retirement this fall with a double-bill tour alongside R. Kelly. It’s called “Best of Both Worlds,” but “A Fighter and a Lover” would be more accurate. The tour swings through New York in late October.

Indie rock fans solemnly await the release of “From a Basement to the Hill” (Anti, October 19), the final studio album from the late, great Elliot Smith, who died of an apparent suicide last October. The album captures Smith at the height of his songwriting powers and in the depths of his considerable despair. It is, as he sings, “a fond farewell to a friend who couldn’t get things right.” Fans will inevitably view it as a suicide note, and comb the lyrics for clues; they won’t have to look very hard, as it reads like one. “I can’t prepare for death anymore than I already have,” he sings, “give me one good reason not to do it, so do it.”

With the November election fast approaching, the music community will also weigh in on the presidential race with “Vote For Change Tour,” the biggest and most politically savvy partisan event in the history of pop. Sponsored by MoveOn.org’s political action committee, it will features Bruce Springsteen, Pearl Jam, R.E.M., Death Cab for Cutie, Bright Eyes, John Mellencamp, the Dixie Chicks, the Dave Matthews Band, and others barnstorming through swing states from October 1 through 7.

Leading the effort is Springsteen, who, with his patriotic, working-class image, is a potent spokesman, and apparently also a reluctant one. He has been dragged into the political fray before: In 1984, Reagan used his song “Born in the U.S.A.” in the campaign and evoked his name at campaign stops. But while Springsteen distanced himself from Reagan, he stopped short of endorsing Mondale.

This time around he’s gone from politic to blatantly political, laying out his case against Bush in the New York Times and on Nightline. The Bush camp isn’t likely to fret much about dissent from Pearl Jam or Bright Eyes, but opposition from the likes of Springsteen, Bonnie Raitt, and Dave Matthews hits where it hurts most: in the swing vote.

The election also anchors the orbit of R.E.M.’s latest, “Around the Sun” (Warner, October 4). Michael Stipe calls the music “pretty hardcore, and fairly political.” The album will be out the first week of October, but a track called “Leaving New York” can be heard now on the R.E.M. Web site, remhq.com.

MoveOn.org types will be back in NYC in late October for the annual CMJ Music Marathon. And while there won’t be any Republican delegates to harass this time, with Al Franken serving as keynote speaker, politics is sure to be in the air. They’ll be coming to see a lineup that includes Sonic Youth, TV On the Radio, and the Faint, among hundreds of others, but the trip should also allow some of them to make overdue appearances in New York City court.

For those sorry to see summer go, there’s former Beach Boy Brian Wilson’s long-abandoned opus, “Smile” (Atlantic, September 28), to look forward to. After a 37-year delay, the most famous unreleased album in history will finally see the light of day in a new recording put out by Nonesuch Records. Like a warm ray of sunshine arriving from a distant star, Wilson’s self-described “teenage symphony to God” is a little wave of Indian summer.