[jlgreco]

It seems if you keep the payload under 6 pounds, and abide by a few other limitations, free balloons are a-okay as far as the FAA is concerned.

http://www.chem.hawaii.edu/uham/part101.html

[plantain]

Read a little closer...

(a) No person may operate an unmanned free balloon unless-

     (1) It is equipped with at least two payload cut-down systems or
     devices that operate independently of each other;
     
     (2) At least two methods, systems, devices, or combinations
     thereof, that function independently of each other, are employed
     for terminating the flight of the balloon envelope; and
     
     (3) The balloon envelope is equipped with a radar reflective
     device(s) or material that will present an echo to surface radar
     operating in the 200 MHz to 2700 MHz frequency range.

<snip>

      (a) Prelaunch notice : Except as provided in paragraph (b) of this
     section, no person may operate an unmanned free balloon unless,
     within 6 to 24 hours before beginning the operation, he gives the
     following information to the FAA ATC facility that is nearest to
     the place of intended operation:
     (a) Each person operating an unmanned free balloon shall:

     (1) Unless ATC requires otherwise, monitor the course of the
     balloon and record its position at least every two hours; and
     
     (2) Forward any balloon position reports requested by ATC.

[elemeno]

That only applies if the criteria in 101.1.a.4 are met:

  (4) Except as provided for in 101.7, any unmanned free balloon
 that-
     
     (i) Carries a payload package that weighs more than four pounds and
     has a weight/size ratio of more than three ounces per square inch
     on any surface of the package, determined by dividing the total
     weight in ounces of the payload package by the area in square
     inches of its smallest surface;
     
     (ii) Carries a payload package that weighs more than six pounds;
     
     (iii) Carries a payload, of two or more packages, that weighs more
     than 12 pounds; or
     
     (iv) Uses a rope or other device for suspension of the payload that
     requires an impact force of more than 50 pounds to separate the
     suspended payload from the balloon.
     

As long as those don't apply to you, you're pretty much good to go. I'd imagine that they're criteria where, should your ballon hit anything, it wouldn't cause any damage.

[tomkinstinch]

If you're near an airport, it's still probably a good idea to notify the FAA that you'll be doing a launch.

[jlgreco]

Indeed. It certainly cannot hurt to ask them.