At sea level, the climate is tropical—hot and humid with average yearly temperatures of about 27 °C (80.6 °F) and little daily variation. At the interior's higher altitudes, the average yearly temperature is 20 °C (68 °F), and nights are generally cool. Annual rainfall varies from 5,000 mm (196.9 in) on the southwestern slopes to 1,000 mm (39.4 in) in the northern lowlands. The rainy season runs from October to May.