IN SHORT...
The Cook Islands weather is typically tropical. Summer is hotter and more humid with a higher rainfall and the risk of tropical storms and sometimes cyclones (hurricanes). Winter is cooler and drier. Aitutaki and the northern islands are noticeably warmer than the capital island, Rarotonga, which accumulates cloud around its mountain peaks. 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).
SUMMER: NOVEMBER TO APRIL
The summer rainy season begins in December and lasts until April. 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!"
WINTER: MAY TO OCTOBER
This is the "dry season". 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)!!!