Tropical areas are the areas of the world that are near the equator where the weather is very warm. Temperature determines season in the tropics.
Transpiration is the passage of water or vapor from a living body through a membrane or pores
Rainfall is the amount of rain that falls on a particular area, since the tropic is predominantly temperate, rainfall has no impact in determining seasons there
Evaporation is about converting liquid to vapor. It is just a process and a determinant factor in season determination