The Cook Islands weather is typically tropical to sub-tropical with just two seasons...dry and wet.  There's actually not much difference between the two.  In the Northern Cooks Islands,  temperatures are fairly constant throughout the year, while in
the Southern Cooks (including Rarotonga and Aitutaki) there is a difference of around 4°C between the warmer and cooler months.  Mangaia in the south is the coolest island; Penrhyn and Rakahanga in the North are the hottest as they're closer to the equator (typically 5-7 degrees celcius warmer than Rarotonga).

WET SEASON: NOVEMBER TO APRIL/MAY

The wet season begins in late November and lasts until April or May, and sometimes longer in the Southern Cooks..   November to March is also the cyclone (hurricane) season.  It can become hot and humid (29 degrees celcius/84 fahrenheit by day) with bright sunny mornings and late afternoon downpours around Rarotonga.  As the heat accumulates over the Pacific Ocean during this season, depressions can form bringing with them thunderstorms, strong winds and the occasional tropical cyclone.   According to the national newspaper, The Cook Islands News: "on Rarotonga we judge how hot the weather is by checking how much the tar seal on the roads has melted!"

DRY SEASON: JUNE TO OCTOBER
July is "midwinter" and  the average daily temperature is 25 degrees celcius/77 fahrenheit.  This drops to around 19 degrees celcius/66 fahrenheit at night.   And on really cold nights, it can fall as low as 14 celcius/57 fahrenheit!  The lowest winter temperature on record was in 1965 when the thermometer fell to just over 9 degrees C (48F)!!!

Warming trends have been evident evident in both annual and seasonal maximum and minimum temperatures, across the Cook Islands since 1950.  The sea-level rise measured by satellite altimeters since 1993, is about 4 mm  a year.   Source: A Nagari, Director, Cook Islands Meterological Service in a presentation to the World Meteorological Assocaition regional seminar in 2011