Galapagos Islands climate information
For the newly arrived visitor, perhaps the most striking thing about the Galapagos is their climate. If you are expecting the swaying palms and lush vegetation of other tropical isles, you're in for a surprise. Darwin opens his chapter of The Vogage of the Beagle on the Galapagos with the following remark: "Considering that these islands are place directly under the equator, the climate is far from being excessively hot." He also notes that "Excepting during one short season, very little rain falls, and even then it is irregular; but the clouds generally hang low." (While it is true that the Galapagos are generally not "excessively hot", particularly for the tropics, they are not cool either, as daytime temperatures in the lowlands typically reach 30° C (85° F) or higher.)
In the same paragraph, Darwin identifies the principal reason for the Galapagos' dry and moderate climate: "this seems chiefly caused by the singularly low temperature of the surrounding water, brought here by the great southern Polar current." Today, the "the great southern Polar current" is known as the Peru, or Humbolt, Current. Carrying an enormous volume of cold water northward from the Antarctic region, it keeps the western coast of South America temperate and dry. As it passes northern Peru, the Humbolt current bends to join the Equatorial Current flowing westward across the Pacific, bathing the Galapagos in cool water. The Humbolt current has a mirror image in the northern hemisphere, the southward flowing California Current, which is responsible for California's pleasant climate. Both the Humbolt and California currents are parts of large gyres, called geostrophic currents, separately circulating water in the North and South Pacific. Similar current systems operate in the Atlantic and Indian Oceans (though Indian currents are complicated by the monsoons).
There is another reason for the peculiar climate of the Galapagos, of which Darwin was unaware, and that is ocean upwelling. Upwelling refers to the rise of deep water to the surface; this can occur as a result of both current patterns and winds. Though the actual cause is complex, a simple explanation goes as follows. As water of the Humbolt Current turns westward, it spreads out, or diverges. Since the water is spread out over a greater area, extra water must come from below, or upwell, to make up the difference. A more important reason for upwelling, however, has to do with winds and a phenomenon known as Ekman Transport. The trade winds blow from southeast to northwest in the southern hemisphere and from northeast to southwest in the northern hemisphere. Thus both blow towards the equator. However, the winds push water not straight ahead, but at a 45° angle to the wind direction (45° to the left in the southern hemisphere and 45° to the right in the northern hemisphere). This is Ekman Transport, which, like the Coriolis Force, is a result of the Earth's rotation. Thus although the trade winds are blowing toward the equator, they push water away from it! Once again, the divergence in surface water allows deep water to rise to the surface. The oceans are thermally stratified, so that the water rising from depth is colder than the surface water. In some areas, the water temperature can fall below 20° C (68° F), particularly west of Isabela. For most people, this is too cold for comfortable swimming!
Areas of high productivity occur along the South American coast, as the Humbolt current joints the Equatorial Current, and on the western sides of the larger Galapagos Islands.
The windward sides of the islands, the southeast, receive more moisture than the leeward sides. As elevation increases, the climate changes, in the Galapagos as elsewhere. This is particularly true on the windward sides of the islands. Accompanying this climatic change is progressive change in vegetation. These changes can be divided into a number of climate zones that are perhaps best seen on Santa Cruz. The increase in precipitation that creates these zones occurs as moist ocean air is forced up over the islands. As it rises, it cools and water vapor condenses as fog, mist, or rain. During much of the year, the ocean cools the lower part of atmosphere, creating a temperature inversion at around 300 to 700 meters (1000 to 2000 feet) elevation. The inversion inhibits the rise of moist air over the islands, so that the highest elevations are often above the clouds.