 In areas where the heavy rainfall is associated with a wind shift , the wet season is known as the monsoon . Rainfall in the wet season is mainly due to daytime heating which leads to diurnal thunderstorm activity within a pre @-@ existing moist airmass , so the rain mainly falls in late afternoon and early evening in savannah and monsoon regions . Further , much of the total rainfall each day occurs in the first minutes of the downpour , before the storms mature into their stratiform stage . Most places have only one wet season , but areas of the tropics can have two wet seasons , because the monsoon trough , or Intertropical Convergence Zone , can pass over locations in the tropics twice per year . However , since rain forests have rainfall spread evenly through the year , they do not have a wet season . 
