The climate of the Cook Islands, an archipelago in the Pacific Ocean, located south of the equator and "free association" with New Zealand, is tropical, with a rainy period from December to March, and a relatively dry period from June to September, without, however, a real dry season.
The islands are divided into two groups: the northern islands, closer to the equator, are warmer and have a stable temperature, around 27/28 °C (81/82 °F) as daily average throughout the year.
Here are the average temperatures of the northern island of Manihiki.

In the north, the sea temperature is high throughout the year as well, as can be seen in the following table.

On the contrary, on the southern islands there is a cooler period from June to October, when the daily average drops to around 21/22 °C (70/72 °F), while in the warmest period, from January to April, it doesn't go above 25/26 °C (77/79 °F). In July and August, at the acme of the cool period, maximum temperatures drop to around 25 °C (77 °F) on the southernmost islands of Rarotonga and Mangaia, and around 27 °C (81 °F) in Palmerston and Aitutaki, although night temperatures remain in both cases around 20 °C (68 °F). However, from June to September, sometimes the temperature at night can drop to around 15 °C (59 °F) in Rarotonga and Mangaia.
Here are the average temperatures of the capital, Avarua, on the southern island of Rarotonga.

On the southern islands, the sea temperature also decreases a bit in the winter months, although it remains high enough for swimming, as can be seen in the following table.

Rainfall is abundant throughout the country: on the northern islands it ranges from 1,900 to 2,800 millimeters (75 to 110 inches) per year, becoming progressively more abundant from east to west, while on the southern islands it hovers around 2,000 mm (80 in) per year.
Here is the average precipitation in Manihiki.

The reason why on the northern islands the differences in rainfall are greatest, lies in the fact that the islands located in the east are closer to the arid zone of the Kiribati archipelago. Unlike in Kiribati, on the Cook Islands the rains are still plentiful, but during the years of La Niña, when the cool easterly winds that blow near the Equator from South America become more intense, the dry area becomes wider and reaches the Cook Islands, while in the years of El Niño, when the cool winds slow down, the sea warms up, and the frequency of cyclones increases.
In Avarua, more than 200 mm (8 in) of rain fall per month from December to February, and about 70/100 mm (2.8/3.9 in) from June to September. Here is the average precipitation.

The amount of sunshine on the Cook Islands is fairly uniform throughout the year, which means that even in the rainiest months there is a fair amount of sunshine hours: showers are more intense and more frequent, but do not last long and do not shield too much the sun, even in this period.

Tropical cyclones

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The Cook Islands are situated in the area of the South Pacific tropical cyclones, which may pass over the islands from November to April (although they are more likely from December to March). The northern islands are located right in the area where cyclones form, so they are still in the early stages of their life, and then evolve migrating south, although it is possible that some of them could grow rapidly, becoming intense in the same area where it has just formed. An intense cyclone, formed a little earlier than the normal period, was Martin, which hit the islands at the end of October 1997. Another intense cyclone was Percy, which affected the islands from late February to early March 2005.

Best Time

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The best time to visit the Cook Islands runs from mid-May to mid-October on the northern islands, where the heat is muggy, but tempered by the trade winds; showers can sometimes be intense, but they do not last long, so the sun will shine again soon.
As regards to the southern islands, the best time is more or less the same, from mid-May to mid-October, but you will find lower temperatures, less abundant rains (though still possible), and the trade winds, which may sometimes be a little cool for those who are sensitive to cold. It will be therefore useful to bring a sweater or a sweatshirt for the evening.

What to pack

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In winter (June to August), bring light clothes for the day, a scarf for the breeze, a sweatshirt for the evening, possibly a light jacket for the southern islands; a light raincoat or umbrella.
In summer (December to February), bring light clothes, of natural fibres, a sun hat, a scarf for the breeze, a light sweatshirt for the evening, a light raincoat or umbrella.