Honestly, I'd rather ride when it's cold (well, maybe not 30 degrees, but in the 40 range, perhaps) than in 100 degree heat. You can always dress for the weather, but it's hard to undress for the heat! I melt in the heat (could be that I'm so sweet and made of sugar! ).

But I don't agree that for a recreational cyclist who slows down in cold weather versus summer rides is "buying excuses" for poor performance. If you're riding consistently, your fitness shouldn't have changed that much. Sure, we all have off days and wonder why we left an anchor out to drag behind us some days - or gosh, I think I have a flat! - but it can be more difficult to ride in the cold. Not every body performs optimally in wide temperature ranges. Many things could slow a body down in the winter. No foul. No shame.