overheating?

[rhughes@domain.elided]

Harvesters,

Yesterday, after a mostly-freeway 15 minute commute home, in 30 degree
weather, I noticed a thin cloud of steam escaping from my grill.  Popped
the hood, and saw that the small amount of steam was emanating from the top
of the radiator.  No specific place -- it was not coming from the cap -- it
was just wafting out from under the flat steel bar that sits on top of the
radiator.

I checked the overflow tank, and it had plenty of coolant.  It didn't seem
like a big deal, and I was stumped, so I let it go.  But this morning,
after reversing the same commute, I noticed the same cloud of steam --
bigger this time.

Any ideas?  It doesn't seem like it could be overheating (my temp gauge is
long gone), given freeway speeds and sub-freezing temps.  The fan belts
seem fine.

I'm thinking maybe the radiator has a minor rupture?  Can I fix this with
JB Weld or somesuch?  Any other thoughts?

rob