Agree with Yojimbo - 50*s still pretty warm on the bike if you're working with any intensity. Like you, I spend winters skiing, so 30* is mentally quite comfortable while exercising. When skiing, a forecast of 30* and sunny means I wear a tight baselayer (LS top), training shorts, and a windstopper vest. I personally find dressing for "mid-activity" works best for me and take that approach with skiing and biking. I prefer to be chilled rather than overheated, so I "soldier through" early morning and target 1-2 hours in for comfort.
Wind and moisture control are critical for me - sunny and no breeze at 35* is brisk but comfortable. I've started two longer rides (100miles and 65 miles) in the past 2 weeks at 34*-38* with mid-ride temps of 50 and sunny predicted. Started both of those rides with shorts, a compression sleeveless base layer with ss jersey over and was fine. Also had windstopper arm warmers in my pocket just in case, but didn't need them. Fingers and toes are generally a little cold to tingly for an hour or so, but as the temp climbs above 40 and I'm cranking on the pedals all is good. If mid-ride conditions are expected to be in the low-mid 40s with little or no sun, then I wear an LS baselayer (ski top) under my ss jersey. In the 30*s and no sun or damp/windy conditions I have a sleeveless insulated jersey that I switch to over the LS base layer. Low 30*s and below I wear a full length compression baselayer (from skiing) under my bike shorts and start protecting my feet from cold with windstopper socks or winter cycling shoes. I purchased a Craft compression sleeveless top with a windstopper front panel for use below 30, but I have yet to wear it when I didn't soak my midsection with sweat. Similarly I have a Gore Oxygen jacket for below 30* riding and don't stay dry on the inside with that ether - too warm. Vests - windstopper and/or insulated - work best for me to keep my core warm and allow excess heat/moisture to escape. I have a couple of next-2-skin Mountain Hardwear Transition long sleeve zip tops from skiing that have windstopper panels in the sleeves and chest. They fit snug. Those with an insulated vest are what I have worn to ride in the upper teens to 20*s. Again, protecting large areas of skin from contact with cold wind or drizzle is all I really need if riding at a relatively high intensity level. My challenge has been to block wind/drizzle and not condense perspiration from the inside. I'm a weenie and try not to ride in outright rain. Snow's cool, but haven't done that since I was a kid.
By the time ski season is rolling (around mid-Nov) my biking ends. Too dark in the eves and skiing on the w/e, so I haven't biked below mid-teens. Not sure what changes I would have to make below that temp, but up to there my ski base layers have worked well. YMMV, but hope that gives you something to work with.
12" of snow at Killington Friday ... woohoo!
