Galapagos: Geography and Climate
The Galapagos Islands are volcanic in origin and are situated in the Pacific Ocean, straddling the equator, 612 miles west of Ecuador in mainland South America.  There are no sedimentary rock on the Galapagos, instead all rock formations are the result of lava flows stemming from volcanic eruptions.
The climate of the Galapagos is unusually dry for the tropics, although there are considerable differences between the islands and much annual variation.  There are two distinct seasons: the warm/wet season which spans from December to May.  During this season the skies are usually clear and the temperature can exceed 86 degrees fahrenheit, it is also humid at times with heavy rainfall.  The Garua season spans from June to November, bringing a subtropical climate with an average temperature of 77 degrees fahrenheit.  Rainfall is concentrated on the souther and eastern slopes of the higher islands, which usually have thick cloud cover.  The norther and western slopes are generally drier.  
During El Nino years (an average of every 7 years), the relative flow of warm currents is much greater than usual, the surface temperature of the sea is much higher and there is greater rainfall.  This results in the rapid growth of vegetation and an increase in the populations of many of the landbirds.  El Nino years also suppress the upwelling of cold waters, reducing the amount of food available for seabirds and marine mammals, which results in a significant decline in the population of these species.