Climate

    "Considering that these islands are placed directly under the equator, the climate is far from being excessively hot."

    "Excepting during that one short season, very little rain falls, and even then it is irregular, but the clouds general hang low."

                       
                                                        --Charles Darwin, The Voyage of the Beagle




    The reason why the Galapagos Islands were not as hot as Darwin expected is because of the ocean current known as the Humbolt (Peru) Current.  As it passes Peru, the Humbolt current bends to join the Equatorial Current flowing westward across the Pacific Ocean, hitting the Galapagos with cool water.  The Humbolt itself maintains a mean temperature of 15°C.  It is this cool water that keeps the Galapagos climate from being as warm as other tropical islands.  Upwelling also brings cool water along with nutrients from the ocean depths.  The Cromwell Current (Subequatorial Countercurrent) is the principle reason why the marine environment surrounding the Galapagos is so interesting.  The Cromwell Current is a deep flow of ocean waters originating in the Western Pacific.  It runs beneath the equator in the opposite direction to the bulk westward movement of surface waters (Southern Equatorial Current).  At its core, the Cromwell has a temperature of 13°C.  When the current encounters the submarine Galapagos platform, it upwells to the west of Fernandina and Isabela and dissipates toward the center of the Galapagos.  It is the resultant nutrient-rich water that promotes bioproductivity around the Galapagos.  


The windward sides of the islands (toward the southeast) generally receive more moisture than other sides.  An increase in precipitation creates climatic zones that occur as moist ocean air is forced up over the islands.  Ocean cools the lower part of the atmosphere, creating a temperature inversion at around 200-300 m.  


The Galapagos Islands see only two major seasons a year.  The dry, or garua, season lasts from July to December.  The hot, wet season occurs between January and June with March and April being the wettest months of the season.  Around December, several changes occur in the atmospheric and oceanic currents.  The trade winds slacken and the "International Convergence Zone" (the climatic equator located just north of the geographic equator) shifts southward toward the Galapagos.  The slacking trade winds cause the Southern Equatorial Current, a westward current, to slow.  This reduces upwelling, and warm water invades the islands.  Ambient air, in turn, warms, and the inversion layer breaks down.  Warm air then rises to the point where rain clouds form and daily afternoon showers occur.  Even with this added precipitation, however, low elevations on the islands still receive only limited rain.  Interestingly enough, the highland areas of the islands receive more moisture from the garua (the mist that develops at upper elevations) than from rain.  



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