When is it too cold to weld?

[Rutterbush]

I like to read the things people have to say about certain welding strategies and experiences. Most times people that do this stuff on a regular basis are pretty knowledgeable.
I have to follow several welding codes pretty close so that I can train students for area employers. Sometimes I have to re-certify experienced weldors.

The code books that I have to be familiar with have parameters and guidelines. The guideline for welding in the cold is when the ambient temperature gets to 0º F mild steel should be warmed up to ~72ºF and held there till the welding is completed. This is a base line and also my interpretation and not a quote. I'm presently home for holiday vacation and don't have access to the code book.
Here again, I have a welding shop/smithy in my back yard and I do as I need to get the job done. Of course, I'm kinda anal retentive about quality, customer satisfaction and liability.