What is cold weather?
The answer to the question above depends on several factors that include where you live, your personal tolerance level for cooler temperatures, your goals for the workout session and your long-term goals.
I have coached athletes living in various warm climates and warm coastal regions. Most of these athletes consider the weather to be too cold for cycling when temperatures drop much below 40 degrees Fahrenheit. At the other extreme, I worked with a cyclist living in New Hampshire that mounted cleats on his hiking boots, put studs on his mountain bike tires, wore ski gloves rather than cycling gloves and would go outdoors for a cycling session in temperatures below zero with the ground completely covered in snow and ice.
Even when athletes live in the same location, there is variation in what temperature individuals consider to be "too cold for outdoor cycling". In my home state of Colorado, I've worked with athletes that refuse to ride (or run) outdoors when the thermometer dips below 40 degrees F. Still others are hard-core commuters and ride to and from work in the worst conditions.
What is best? Should you just "toughen up cream puff" and head outside for your workout or are you better off doing your workout indoors?
Workout Intensity Goals
If you are commuting to and from work, the main goal of those bike sessions is to arrive safely at your destination. Any workout-specific intensity goals, such as lactate threshold intervals, become secondary.
Pro cold-weather ride: If you enjoy being outside and your workout session is related to commuting or is primarily aerobic, bundle up and head outdoors.
Con cold-weather ride: If you have specific workout goals that cannot be accomplished in poor weather conditions, and these workout sessions have an effect on your long-term goals, consider moving those workout sessions indoors. Of course, missing one key workout session several months before a big event will likely not affect your race goals; however, missing multiple key workouts will have negative consequences on your race performance.