There just has to be a whirlwind 'around'. We assume that if it is within the range of the call lightning/lightning storm spell then it is fine.
(n.b. if it meant you had to be inside a whirlwind, it would say 'more damage when within a tornado or an air elements whirlwind'. As it stands, it just requires that the whirlwind is part of the composition of the area).
My druid used to use SNA to get lots of small air elementals and then run them in shifts to give the call lightning boost.
