You can put winter tyres on a road bike. They'll give you better grip in wet or icy conditions. However, if it's snowing then you're options are limited really and it may have to be the trainer. If that's the case an hour is plenty - just make it a worthwhile hour. Rather than an hour at a steady pace try and use a turbo session that will build or maintain some fitness..... or you can do as I did last winter and head to San Diego for a couple of weeks.... 75 degrees in February - bliss. I went with CCSD.com - saved me from a winter of miserable riding in the cold!