In general, it rains a lot more in the Green Season than in the High Season. This season is called the green season because the rain makes everything much greener. It is a great time to visit Manuel Antonio or drier areas because of this. This is also the peak surfer season as the waves are at their best.
Weather: It doesn’t rain all day during the green season; you’ll wake to sunny mornings with blue skies and showers typically roll in between 2 and 4 p.m., sometimes later. September and October are usually the rainiest months, especially in the Central Valley and Pacific. Temperatures are comfortable, averaging in the 70’s in the Central Valley and 80’s on the coast. – See more at: http://costarica.com/
Sometimes the Niña o ‘El Niño’ affect Costa Rica and make for a drier green season – this has happened several times since the 90s.
Sea turtles come to nest during this period of time so some have come to call it ‘Turtle Season’. The sight of leatherbacks and other species coming on land and laying their eggs is awe inspiring and is a must-see.