Due to two sea drifts surrounding Galapagos, there are only two seasons existing on the isles warm ( wet ) and cold (dry).
Surprisingly, contrary to common expectations the wet season is the best time for holiday on Galapagos.Here is why…
Wet ( warm) season Jan-May
The wet season starts with appearance of warm drift just around Christmas, and so the drift got the nice name El Nino (the boy) , but is definitely not welcomed around Ecuador, as it brings the rainstorms and the floods damaging the country. However on Galapagos the rain is not as heavy, the average rainfall reaches 100 mm ( 4 in ) in March, and for most of the season the tourist enjoy the sunny weather very seldom broken by an occasional tropical rain. The average water temperature floats around 23 C, the air is clear, all above makes the wet season the best time to visit Galapagos.

Average temperature on Galapagos Islands in Fahrenheit
Dry ( cold) season Jun-Dec
The first Humboldt at the beginning of May cools the air and is responsible for garua - misty clouds covering persistently the mountains peaks and sometimes go down towards the sea. The temperature is lower than one can expect judging from equatorial position of the isles, but it is still warm with the average water temperature around 18 C,
clean and good for snorkeling, but wet suit may be handy. This is a good time for hiking in cool but comfortable air temperature, with some windy days.

Average temperature on Galapagos Islands in Celsius
There are visitors coming to Galapagos Isles all year round, but December around Christmas vitness the significant number of tourists, and the tourist season peaks in January lowering slowly down in following months till the end of warm season in May or beginning of June.
If you plan your vacations on Galapagos not having in mind the behaviour of particular species to see , but the weather, consider the warm season, however as the isles are located on Equator even during the cold season is reasonably warm, finally every time is perfect to visit the ‘Enchanted Islands’.

Average Rainfall on Galapagos in mm