Weather-wise, no time of year on the Galapagos has perfect weather, so deciding when to visit the islands based on weather is very much a personal decision.
December to May is the warmer, wetter season. The warm Panama currenty usually dominates over this time bringing sunny weather with occasional tropical showers that turn the islands green. Seas are generally calm, the average temperature is 26C and underwater visibility is good, all ideal conditions for snorkelling and swimming. The daytime air temperature is generally 26-30C, but rather humid.
Between June and November the weather is cooler and drier under the influence of the Humboldt current. Temeratures are typically 23-26C, reduced by wind-chill that can make it a bit nippy to be on deck without the fleece. Skies are often cloudless and rain is scarse, but a fine mist called garua can cloak the highlands.
For wildlife, the natural year on the Galapgos is always changing and there is plenty to see at any time, so there is never a 'wrong' time to visit. Here is a summary of what you can expect to find month-by-month.
December The islands green over as the rainier season starts. Still great weather: mostly sunny days, hardly any wind, waters warmer. Giant tortoise eggs hatch from now until April. Pacific green turtles breed on Santa Cruz. Waved albatrosses fledge. First red pouches of great frigatebirds at Genovesa. Northern migrant birds and somewhales and dolphins rest and feed on their journey south.
January Land birds nest. Marine iguanas turn green, red and black on Espanola. Green sea turtles arrive to lay eggs until March. Land iguanas breed on Isabela. Nazca boobies nest on Espanola. Air and seas are warm until June. Ideal for snorkelling.
February Greater flamingo start nesting on Floreana. Bahama pintail ducks start breeding. Marine iguanas nest on Santa Cruz. Galapagos doves' peak nesting season. Highest sea temperatures, 25C.
March Marine iguanas nest on Fernandina. Waved albatross start to arrive on Espanola on about 21 March. Snorkel with penguins and tropical fish off Isabela. Wettest month in rainy El Nino years. Hot and humid. Air temperatures up to 30C. Swells on north shores.
April Rains end, yet islands remain green. Waved albatross mass arrival and courtship, Espanola. Green sea turtle eggs begin to hatch. Land iguana eggs hatch on Isabela. Giant tortoise hatching season ends. Good visibility for snorkellers. Together with May one of the best months to visit.
May Blue-footed boobies begin courtship on North Seymour. Galapagos penguins very active on Bartolome. Last of the sea turtle and land iguana eggs hatch. Palo santo trees begin to shed their foliage. Waved albatross start to lay eggs on Espanola. Band-rumped storm petrels start nesting. Sea temperature starts to drop towards the end of the month.
June Start of the drier season. Southeast trade winds return, currents become stronger, seas pick up. Giant tortoises on Santa Cruz migrate from highlands to lowlands in search of nesting places. Magnificent frigatebird males on North Seymour develop their red pouches. Southern migrant birds rest on their way north. Humpback whales seen. Sea purslane near the shore turns red until December.
July Sea birds actively breeding. Flightless cormorant courting, Fernandina. American oyster-catchers nest, Santiago. Lava lizards start mating rituals until November. Whales and dolphins most likely to be seen off Isabela. Seas cool, 21C.
August Galapagos hawks court on Espanola and Santiago. Nazca boobies and swallow-tailed gulls nest on Genovesa. Migrant shore birds start to arrive and stay until March. Giant tortoises return to the highlands of Santa Cruz. Sea lion pupping starts in west and central islands. Seas at their coolest 18C and a bit choppy.
September Peak of the garua season. Air temperatures at their coolest: 19C. Galapagos penguins very active on Bartolome. Sea lions very active on west and central islands' harem-gathering males barking and fighting. Sea birds remain active at their nesting sites.
October Garua mist in most locations, but less prolonged in the western islands. Lava herons nesting until March. Galapagos fur seals start their mating period. Blue-footed boobies raise chicks (Espanola and Isabela). Sea lion pupping continues.
November Nice transitional weather. Sea lions harem-gathering on the eastern islands. Brown noddies breeding. Sea lion pups display aquagymnastics to snorkellers. Trade winds have dropped, seas calm and sea temperature slowly rising.