MOST Washington-area runners will do anything to avoid the worst of summer's heat and humidity. Some run early in the morning; others run after dark. Their motivation is escaping the sizzle of the sun. On a couple of evenings every summer, however, a few thousand runners sizzle in the dark in popular evening races. The two largest of these are Saturday's Rockville Rotary Twilight Runfest 8K and the upcoming Moonlighter 8K in Alexandria.

Summer twilight races present unique challenges to runners. First, though run ostensibly under "twilight" or "moonlight," both race courses contain some sections where light of any kind is pretty scarce. People who run at night regularly will have already honed their pothole-spotting skills, but those who haven't should remember to tread carefully.

Second, the weather itself challenges even the fittest of athletes. Though not as high as it would be in the middle of the afternoon, the temperature typically hovers in the 80s, and the humidity is very high. The high humidity is actually more dangerous to runners than the temperature, because it inhibits the evaporation of sweat -- the body's principal method of cooling itself.

As if that weren't enough, the Rockville race offers hills. The race begins downtown near Courthouse Square on North Washington Street and proceeds past the Rockville Municipal Swim Center. The runners then wind through the neighborhood behind the center, return on North Washington Street past the courthouse again (great for spectators), and then head for the hills of Montgomery College (the longest hill). By the time most participants reach the top of the hill by the track and make the turn onto Route 355, they'll think they're in a sauna. From 355, though, the course runs flat to slightly downhill as the runners head back to the courthouse for the finish.

Of course, out of great suffering comes great rewards, and the post-race festivities on Courthouse Square make your body forget what you've just put it through. There are beer and fruit tents (both are free to registered runners) and food vendors (most notably, Armand's Pizza), so you can replenish those lost carbohydrates and electrolytes quickly (you'll want some water before you hit the beer tent, since alcohol dehydrates you).

Even the awards ceremony is unique, since it's usually preceded or followed by fireworks. The race is run by the Montgomery County Road Runners, who also put on the popular Fritzbe's 10K event. Besides individual awards, the race has team divisions. The three fastest times of all team members (each team is allowed a maximum of five runners) are added together to get the team's time. The Rockville race is also one of the few local races that features a separate corporate and government team division.

The Alexandria event has this year been moved from July to Aug. 11 so ambitious runners who want to do both will have a few weeks to recover from the Rockville race. This race has fewer hills than its Rockville counterpart, and most competitors will probably run a faster time here.

The Moonlighter begins in front of the Alexandria Tech Center on Eisenhower Avenue near its intersection with Telegraph Road. The out-and-back, out-and-back course is the race's most unique feature: Runners turn around at the 1-mile mark, run back past the starting line on the other side of the road, and then turn around again at about 3 1/2 miles and run back to the finish line. The doubling back doesn't make things particularly interesting for the runners, but it does allow spectators to see the participants in the beginning, middle and end of the race.

The post-race party is not nearly as good as the one in Rockville, but the Colonial Running Company, which puts on many local road races (including the popular Georgetown 10K), does provide a fast course that is interesting for spectators.

So don't let a little heat and humidity scare you off. If the post-race pizza and beer alone won't get you to toe the starting line or watch from the sidelines, remember that these are about the only races in Washington you don't have to get up early in the morning for.