Just returned from a 4800km road trip that took me through the Grand Tetons and Yellowstone.  I'd driven around the side of Yellowstone several times going back and forth to Colorado, so this time while on the return leg I thought "I'm in no hurry -- I should really go check it out."  I passed the Tetons and entered the park from the south side right at dawn, saw Old Faithful erupt, plus Grand Prismatic and countless other thermal pools and geysers.  Just spectacular and totally worth it.

Watsisname wrote:

Yellowstone

Was the weather hot? 

It was chilly when I arrived in the morning (about 7C), and by mid-day it rose to around 25.  But much more comfortable than at lower elevations where it was often above 35C.

The steam clouds can be quite pleasantly warm -- or scalding, if you get too close.  Then there are the unfortunate souls who went into the pools...