There are five versions of Weather Ball, Castform's most famous technique, and five associated forms of weather known to impact pokemon battles: normal, hail, sunlight, rain, and sand. (A few classify heavy fog as a sixth, despite its rarity, Weather Ball's unresponsiveness to this condition, and the fact that no known pokemon can create it.) There are, however, only four forms which Castform can take, for they respond no differently than most pokemon to sandstorms.
It is currently thought that Castform's forms are a function not of the weather in itself, but of the associated temperature change; when past their melting point, as in intense sunlight, they turn to fire, when below freezing, as in snow or hail, they become ice. This theory requires an alternate explanation for their water form, which is still poorly understood, but currently states that Castform are subject to the same atmospheric conditions as rainclouds. Yet this theory is unsatisfying to many at the Weather Institute, for rainclouds do not change their form so dramatically.
These scientists believe instead that Castform are truly impacted by the weather, and have shipped in Hippowdon and Tyranitar from far and wide in order to run countless tests in the hopes of determining why Castform do not change with sand. Trainers have a similar interest in these studies, for Castform's vulnerability in these conditions have greatly weakened the versatility of this remarkable pokemon. So far, they have discovered only that Castform would much rather change the weather to rain than try to transform and battle in the sand. Perhaps this is because Castform, created by scientists, seek to please humanity. And while sun, rain, or snow all have their fans, even in the desert rain is far more beloved than sandstorms.