Weatherless teams are most definitely not viable, they are just harder to play with and like the first post mentioned rain stall seems to be the only truly difficult team to play against in the hands of a good player. Abusing the core of tentacruel, jirachi, and ferro under the rain, decreasing damage from fire by 50% to ferro and rachi and tenta gaining 1/8 hp back every team, these teams are just hard to deal with. However, upon looking at the usage statistics for march you can clearly see stall is only used about 5% of the time. Weatherless teams have their advantages in that politoed and ninetales are terrible pokemon who wouldn't be in OU if not for their abilities. So in that regard you're essentially only dealing with 5 "true" threats, though some variants of the toad like specs/scarf can catch people off guard and deal some massive damage. Other than that these pokemon are easy to play around since they generally run the same sets. I have 1 team ranked in the top 10, two in the top 20, and another 2 in the top 50 right now all of which are weatherless.
Here's a weatherless team that hit #3:
http://www.smogon.com/forums/showthread.php?t=3480668
Pokemon that stand out on weatherless teams are kyurem-b, breloom, kingdra, and lando-I. Mixed Sub Kyurem-b absolutely shits all over rain teams without a spec def ferro with his insane bul. He can come in on many rain favorites such as politoad, ferro (without gyro), rotom-w, etc. sub and beginning wrecking havoc with fusion bolt+ice beam+ whatever coverage you need. RP lando shits all over sun teams as it destroys a large portion of sun favorites and outspeed venasaur after a boost. With psychic, earth power, and hp ice, lando beats venasaur, ninetailes, heatran, any other rando chlorophyll user (sawsbuck, shiftry, etc.), victini, etc. Kingdra obviously can use rain teams rain against them and abuse his swift swim ability becoming a huge threat. Breloom obviously has spore, fighting moves and bullet seed for waters, ferro, and the ability to incapacitate a pokemon is just too beneficial to pass up. Sand teams are another story because ttar is legit and hes usually paired with his pet pony whom together form an outstanding offensive core. Breloom shines here with priority mach punch as does scizor being able to use ttar to scout with u turn or KO/deal massive damage with bullet punch.
Teams I make now that are weatherless almost always have sub kyurem, terrak, and breloom. They form such an outstanding offensive core against weather teams especially if they arent running scizor. I'm always torn between lead terrak and sub salac terrak because lead terrak is such a good lead against sun and sand teams, but comes up short against rain. Sub salac can demolish all three if set up properly, yet that massive advantage gained on turn 1 if a sun team or sand team sends out their weather starter is hard to pass up. Another way to beat weather teams is to simply put hail on one of your pokes. Most players, when facing a weatherless team, play extremely loose with their weather setter as they believe their weather to up for the entirety of the game. Watch them ragequit when your starmie uses hail after ninetales has been disposed of, effectively making their venasaur useless.
To finish it off, know common threats that EVERY weatherless team should be prepared for in someway. Scarf keldeo under the rain and venasaur under the sun are two such examples you should have checks/counters for, as while laddering you WILL run into these pokes multiple times. Good luck and happy battling with your clear skies teams
PS thank god this thread got bumped as this is an interesting thread that should be discussed between us clear skies users on how to fully shit on all weather teams.