In most pages, Solarbeam isn't marked as a STAB move for Fire pokémon, since it usually isn't Fire type. However, in the case of Castform, any time that Castform is Fire, so is Solarbeam. Because of this, I marked Solarbeam as a STAB move for Castform's Fire type form. User142 (Talk) 10:11, 14 June 2006 (UTC)
Your logic is flawed for two reasons: one is quite simply that Solarboam is not powered up during strong sunlight (but rather becomes a one-turn move), the other is that whatever the case, STAB refers only to boosts due to type. This means that even if you were correct about Solarbeam's boost, it would still not be called STAB. If to elaborate, during strong sunlight or rain, not only does Castform type change (hence STAB), but Fire or Water-type moves receive an equivalent boost to that of STAB. It is necessary to distinguish between the two effects, if only for the fact that they can be combined into a 2.25 boost when a Fire or Water-type Pokémon (or Castform) is involved. Treating them as one effectively disregards that advantage - a vital one in Castform's case.
As a rule, the different terminology are best left separate. This also applies to base stats and type effectiveness - factoring Defense and Special Defense into the latter results in meaningless statistics. Simply put, the damage formula is not in direct proportion to the defense stat, unlike your assumption in computing the numbers. I will be glad to explain the formula at length if you are not convinced, as I hope to recruit you to revert your many edits pertaining to type effectiveness. --Unown Lord
Reading your sentence again, it seems that I was confused as to the source of your misconception. I imagined you had thought Solarbeam was powered up under strong sunlight in addition to Fire-type moves, which I could understand, but you actually seem to have thought its type changed. Either way, it is a mistake.
I will say that in Castform's case, weather effects deserve to be taken into account when computing type effectiveness in the same way Ninetales' Flash Fire ability is factorized. However, you considered weather effects as doubling or halving, when it is, in fact, a 1.5 or 0.66 multiplier. --Unown Lord
Castform has a sand form, correct? If so, some should get a pic up.