OK, people, stay with me here. As per previous post, I had a highly
succesful bbq run in my WSM this weekend. Thanks again.

But here's the thing: I was all set to use sand in the tray, instead
of water, but when I set up the uninsulated WSM at 8:00 am, the
ambient temp was only 32 deg, and weather.com said it wasn't going to
get over 45-50 deg, all day. So I thought that maybe a big heat sink
wasn't the way to go. Instead, I just stuck a cast iron utility box
cover (the kind that covers the water valves and similar stuff in the
streets; actually, it was a round monitoring wellbox cover, 12.5"
diameter, 5/8" thick) on the bottom grate.

It worked perfectly. By adjusting the air-flow controls (per this
groups advice), I was able to maintain the temp at 225-250 deg
(measured with a permanent thermometer installed in the crown of the
WSM) from 8:00 am to 6:30 pm with ONE LOAD of Kingsford "Hardwood
Charwood" and hickory chunks. Long enough to bbq the pork shoulder to
195 deg, THEN the 10 lb spatchcocked turkey to perfection.

BUT HERE's THE THING: I forgot to cover the cast iron heat deflector,
and it got soaked and puddled in drippings, which baked on after I
took the turkey off. So, I just got out my propane torch and burned
off the worst of it, then stuck it in my Weber gas grill and burned
off the rest, it came out nearly spick-and-span after about 2 hrs. So
then, I stuck it inside my WSM to keep it clean and dry. That was an
hour ago, and right now, my WSM is running at about 150-125 deg (53
deg ambient) with NO FIRE, just the cast iron heat sink.

So, the question beckons: what is thew mechanism/purpose working
with the heat sink? Is the purpose to have a BIG thermal mass, get it
hot, and let it re-radiate to cook the food? Or to just deflect heat
so there's no radiative heat transfer? Or just to absorb excess heat
to regulate the temp? Or all three?

The reason I ask is this: it could make a big difference when I bbq in
summer (ambient temp at 105-110 deg, 10 % humidity), vs winter
(ambient temp at 35-45 deg).

Agian, thanks a heap,
-Zz