(FORTUNE Magazine) – Feeling blue because football season is drawing to a close and baseball is months away? Join the millions of other armchair jocks who are increasingly tuning their TVs to ever more popular alternative sports. Among them: supercross, which features motorcyclists charging hills at speeds high enough to jump their bikes 30 feet into the air. Or, if you prefer sun and sand, you can watch professional beach volleyball, a sport that attracts bikini-clad fans known as ''volley dollies.'' Supercross starts its season January 12; pro beach volleyball, February 1. Both so-called niche sports are getting more television time, and advertisers are hopping aboard. Cable network ESPN grossed an estimated $768,000 in ad revenues last year from 16 one-hour supercross events -- up from about $624,000 it took in for 26 half-hour slots in 1989. On March 9, ABC's Wide World of Sports is set to include supercross from the Tokyo Dome. The network is selling commercial time at $30,000 per 30-second spot. To watch pro beach volleyball, also tune in to ESPN. It will show 16 tournaments this year, up from 14 in 1990. Corporations, aware of the increased number of viewers, are increasing their sponsorship of events. RJR Nabisco's tobacco subsidiary puts $200,000 into the Camel Supercross Series. Japanese motorbike makers American Honda Motor, + Kawasaki Motors, and American Suzuki Motor sponsor various top riders. Miller Brewing, a subsidiary of Philip Morris, is laying out some $2.1 million in prize money this year for beach volleyball players and will increase that to $2.2 million in 1992.