In the northeastern United States, are most/all/none/some outdoor swimming pools
heated? I am not referring to small outdoor pools in people's
backyards but rather the 25-50 yard/meter swimming pools often owned
by towns or universities. Do these pools have heating systems or are
they just heated by the warm weather and sunlight in the summer time.
I ask specifically about the northeast because it may be different in
warmer area's they may keep pools open year round even when it is a
little cool, which
would require a heating system. I only want to know about the northeast.

I live in the NE and can tell you from my limited experience (have
used about 6-8 different public pools in Pennsylvania), outdoor pools
are not heated and are generally open from Memorial Day to Labor Day. 
Indoor pools are usually heated and are generally open year round.