This statement and others I have read like it raise questions. Does it assume 70* surroundings? Is it the yeast or the surrounding air temp that lets it 'rise naturally' into the low or mid 70's?
I live in Southern California, and the ambient temp in my fermentation area (in which sits the fermentation chest freezer) varies between 65 and 75 degrees depending on the time of year. In my case, it is a combination of the surrounding air temp and the heat generated by the yeast during fermentation that lets the temp rise into the 70s.
I ferment in a kegerator with a 2 stage controller (controls heating and cooling), and it sits in my unheated / uncooled garage. During the winter it stays about 45*, during the Summer it stays about 80*. So I cannot rely on ambient temperature to provide a happy medium.
I have only read insufficient explanations in that there are so many variables not dealt with, one could end up way off base. So I guess my confusion lies in the statement "letting it rise naturally." I don't see that happening in my situation, so how do I replicate it through temperature control?
If you have a two stage controller and are fermenting in a kegerator, can't you hook up your controller to both the kegerator and a heating source to keep fermentation temperatures exactly where you want them?
Although I rarely have to use it, I do have one of these heaters that I can tape to the inside of my chest freezer if I need to bring the temperature up a bit. It works pretty well.http://www.northernbrewer.com/shop/electric-fermentation-heater.html