Incoming solar energy that reaches the land surface is divided mainly in infrared, visible and UV. UV radiation has wave lengths from 100 to 400 nm. The World Health Organization recommends studies and monitoring of UV radiation because of its biological effects in living organisms. The use of automatic Davis weather stations, Vantage Pro Plus/Vantage Pro2 Plus type, with a sensor model 6490, allowed UV radiation measurements at Isla del Coco, Costa Rica, located in the Eastern Tropical Pacific. The stations were set near Chatham Bay (5°32’51” N - 87°02’43” W, 142 m.a.s.l) and Wafer Bay (5°32’24” N - 87°03’26” W, 132 m.a.s.l). Measurements were recorded: Apr. 5-11, 2008, Mar. 1-6, 2009, Apr. 22-27, 2010, Jul. 2-8, 2011 and Mar. 15-21, 2012. Data were described for the cloudiest and sunniest days according to the UV index (UVI) as a standardized value that allows easy data interpretation in terms of the hazard exposure for living beings. This is especially important for coral reefs (the island has the largest Pacific reefs in Costa Rica). The maximum UVI average range in Chatham decrease from 14 for sunny days to 3 for cloudy days, and from 16 to 4 for the sunniest and cloudiest days, respectively. In Wafer, these values were 12 to 4 and 14.2 to 3.0, respectively. The average exposure to UV radiation was considered high for sunny days from 9:30 to 14:30hr in Chatham and from 10:00 to 14:30 in Wafer, but these values for the sunniest days increase from 9:00 to 15:00 in Chatham and from 9:00 to 14:30 in Wafer. The equipment used was able to identify events with extremely high UVI values during the expeditions. The generation and use of this information is useful for understanding and modeling the ecosystem dynamics, and it is also valuable for the rangers’ work and for the tourism sector.

Keywords: Ultraviolet index, Isla del Coco, Cocos Island, Costa Rica, Eastern Tropical Pacific.