The air is getting crisp, the sun is getting scarce, the clouds are coming on. It’s autumn in Clark County — time for a last little bit of outdoor exploration before gloom descends and sticks around for a while.Eye-popping colors can be found without too much trouble here, but Clark County isn’t exactly New England, where autumn “leaf peeping” is a regional pastime. While the right coast teems with explosively deciduous beauties, our Pacific Northwest is mostly home to towering evergreens. What we don’t have in rainbow colors we more than make up for in height.The following list of outdoor outings and sites to explore, right here in our own collective backyard, should keep you smiling for the next several weeks. Store up all that color — be it burning crimson-and-gold or cool forest green — for the gray months to come.Chelatchie Prairie RailroadNorth Clark County’s tourist train runs many weekends, spring through winter, but there’s never a better time than this weekend to enjoy its round-trip run through kaleidescopic scenery between downtown Yacolt and Heisson, with a stop at Moulton Station to visit Yacolt Falls. The 13-mile round trip also plunges through a 330-foot rock tunnel, so you’ll get one dark pause in all the glittering scenery.The train departs at noon and 2:30 p.m. Oct. 13 and 14 from Yacolt Station, 207 N. Railroad Ave.; you are strongly advised to buy tickets online ahead of time and arrive at the station 30 minutes prior to departure.