I live in West Virginia, zone 6, not a really hot climate but it gets to 100 degrees sometimes in summer and winters are variable but can go below zero F. We have a root cellar which works pretty well. It works pretty well because: It's in the right place, quite close to the house but dug into a hillside south of the house, so its entrance faces north; there are
trees all around it so it's in shade in summer; I don't use it much in summer but I want to put potatoes in there as early as July. I find they do fine in the cool dark, despite it being about 70 degrees, a good 30 degrees higher than ideal. Come October, when I may have other things--apples, pears, cabbage--I want to store in there, I begin hauling my buckets of potatoes into my attached
greenhouse (the nearest part of my house) overnight on chilly nights so I can leave the door of the root cellar open, which slowly drops the temps in there (if I left the potatoes in there,
mice would go for them).
I made a choice which likely makes it warmer in summer--rather than create a
concrete roof topped with dirt, we did something typical of this area--built a garden shed atop the root cellar. I had plans to put a vent in the door and one in the floor of the garden shed to control ventilation and reduce humidity but never got around to it--and it is extremely humid in there, but root cellars are supposed to be humid. My potatoes keep beautifully in this cellar--I haven't stored apples and pears there much because the squirrels keep stealing them, but that's another problem.