(1) Less than 500 feet from the base of any cloud;
(2) More than 500 feet above the surface of the earth;
(3) From an area where the ground visibility is less than three miles; or
(4) Within five miles of the boundary of any airport.
(b) Paragraph (a) of this section does not apply to the operation of a balloon or kite below the top of any structure and within 250 feet of it, if that shielded operation does not obscure any lighting on the structure.