Do you need to know, when is the best time to go to Cook Islands? Then you should take a look here, what weather you can expect there during the year.
The climate of Cook Islands is tropical oceanic. The weather is moderated by trade winds. Dry season is from April to November, wet season from December to March. You can expect typhoons from November to March.
When speaking about the usual weather in this state, the information is relevant for example for these popular places: Aitutaki, Atiu, Mangaia, Mauke, Mitiaro, Northern Cook Islands, Palmerston Island, Rarotonga, Southern Cook Islands.
Let's look now what weather you can expect here. (Warning: As we don't have reliable data for this country, we use data from nearby locations for these statistics.) The warmest month here is March, the average maximum temperature is 31.7 ℃ (89.06 ℉). The average minimum temperature in this month is 26.8 ℃ (80.24 ℉). The coldest month here is July, the average maximum temperature is 28.2 ℃ (82.76 ℉). The average minimum temperature in this month is 24.7 ℃ (76.46 ℉). 
When travelling to some places in Cook Islands, always consider the altitude of the the place as well. The highest point here is Te Manga (652 m) and the lowest point is Pacific Ocean (0 m). So there are not big differences in altitudes in this country. But the altitude affects the weather even so. Keep in mind, that the temperature usually decreases by 1.2 - 1.9 ℃ (2.2 - 3.5 ℉) per 200 m (656 ft), depending on the other conditions. (And we should add, that in the very special condition named thermal inversion even the temperature may be higher in higher altitudes.)
Typical (usual) weather in Cook Islands in January is this: The average highest temperature in Cook Islands in January is 30.3 ℃ (86.54 ℉). The average lowest temperature is 25.1 ℃ (77.18 ℉). At the beginning of January you can expect higher temperatures, the average highest temperature is around 30.5 ℃ (86.9 ℉). At the end of January you can expect higher temperatures, the average highest temperature is around 30.75 ℃ (87.35 ℉). The average rainfall is 266.8 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 13:05 (hours and minutes), in the middle of the month 12:59 and at the end of the month 12:50.This data is approximate because we do not have exact data for this country. It is important to say, that the weather may differ significantly in different altitudes.Typical (usual) weather in Cook Islands in February is this: The average highest temperature in Cook Islands in February is 31.2 ℃ (88.16 ℉). The average lowest temperature is 25.8 ℃ (78.44 ℉). At the beginning of February you can expect lower temperatures, the average highest temperature is around 30.75 ℃ (87.35 ℉). At the end of February you can expect higher temperatures, the average highest temperature is around 31.45 ℃ (88.61 ℉). The average rainfall is 229.2 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:50 (hours and minutes), in the middle of the month 12:38 and at the end of the month 12:24.This data is approximate because we do not have exact data for this country. It is important to say, that the weather may differ significantly in different altitudes.Typical (usual) weather in Cook Islands in March is this: The average highest temperature in Cook Islands in March is 31.7 ℃ (89.06 ℉). The average lowest temperature is 26.8 ℃ (80.24 ℉). At the beginning of March you can expect lower temperatures, the average highest temperature is around 31.45 ℃ (88.61 ℉). At the end of March you can expect lower temperatures, the average highest temperature is around 31.05 ℃ (87.89 ℉). The average rainfall is 178.8 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:24 (hours and minutes), in the middle of the month 12:10 and at the end of the month 11:56.This data is approximate because we do not have exact data for this country. It is important to say, that the weather may differ significantly in different altitudes.Typical (usual) weather in Cook Islands in April is this: The average highest temperature in Cook Islands in April is 30.4 ℃ (86.72 ℉). The average lowest temperature is 26.1 ℃ (78.98 ℉). At the beginning of April you can expect higher temperatures, the average highest temperature is around 31.05 ℃ (87.89 ℉). At the end of April you can expect lower temperatures, the average highest temperature is around 30.1 ℃ (86.18 ℉). The average rainfall is 179.3 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:56 (hours and minutes), in the middle of the month 11:43 and at the end of the month 11:31.This data is approximate because we do not have exact data for this country. It is important to say, that the weather may differ significantly in different altitudes.Typical (usual) weather in Cook Islands in May is this: The average highest temperature in Cook Islands in May is 29.8 ℃ (85.64 ℉). The average lowest temperature is 25.6 ℃ (78.08 ℉). At the beginning of May you can expect higher temperatures, the average highest temperature is around 30.1 ℃ (86.18 ℉). At the end of May you can expect lower temperatures, the average highest temperature is around 29.8 ℃ (85.64 ℉). The average rainfall is 130.1 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:31 (hours and minutes), in the middle of the month 11:20 and at the end of the month 11:13.This data is approximate because we do not have exact data for this country. It is important to say, that the weather may differ significantly in different altitudes.Typical (usual) weather in Cook Islands in June is this: The average highest temperature in Cook Islands in June is 29.8 ℃ (85.64 ℉). The average lowest temperature is 25.1 ℃ (77.18 ℉). At the beginning of June you can expect lower temperatures, the average highest temperature is around 29.8 ℃ (85.64 ℉). At the end of June you can expect lower temperatures, the average highest temperature is around 29 ℃ (84.2 ℉). The average rainfall is 96.6 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:13 (hours and minutes), in the middle of the month 11:09 and at the end of the month 11:09.This data is approximate because we do not have exact data for this country. It is important to say, that the weather may differ significantly in different altitudes.Typical (usual) weather in Cook Islands in July is this: The average highest temperature in Cook Islands in July is 28.2 ℃ (82.76 ℉). The average lowest temperature is 24.7 ℃ (76.46 ℉). At the beginning of July you can expect higher temperatures, the average highest temperature is around 29 ℃ (84.2 ℉). At the end of July you can expect higher temperatures, the average highest temperature is around 28.45 ℃ (83.21 ℉). The average rainfall is 81.6 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:09 (hours and minutes), in the middle of the month 11:13 and at the end of the month 11:21.This data is approximate because we do not have exact data for this country. It is important to say, that the weather may differ significantly in different altitudes.Typical (usual) weather in Cook Islands in August is this: The average highest temperature in Cook Islands in August is 28.7 ℃ (83.66 ℉). The average lowest temperature is 24.4 ℃ (75.92 ℉). At the beginning of August you can expect lower temperatures, the average highest temperature is around 28.45 ℃ (83.21 ℉). At the end of August you can expect higher temperatures, the average highest temperature is around 29.05 ℃ (84.29 ℉). The average rainfall is 58.5 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:21 (hours and minutes), in the middle of the month 11:31 and at the end of the month 11:44.This data is approximate because we do not have exact data for this country. It is important to say, that the weather may differ significantly in different altitudes.Typical (usual) weather in Cook Islands in September is this: The average highest temperature in Cook Islands in September is 29.4 ℃ (84.92 ℉). The average lowest temperature is 24.6 ℃ (76.28 ℉). At the beginning of September you can expect lower temperatures, the average highest temperature is around 29.05 ℃ (84.29 ℉). At the end of September you can expect higher temperatures, the average highest temperature is around 29.6 ℃ (85.28 ℉). The average rainfall is 64.5 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:44 (hours and minutes), in the middle of the month 11:57 and at the end of the month 12:10.This data is approximate because we do not have exact data for this country. It is important to say, that the weather may differ significantly in different altitudes.Typical (usual) weather in Cook Islands in October is this: The average highest temperature in Cook Islands in October is 29.8 ℃ (85.64 ℉). The average lowest temperature is 24.6 ℃ (76.28 ℉). At the beginning of October you can expect lower temperatures, the average highest temperature is around 29.6 ℃ (85.28 ℉). At the end of October you can expect higher temperatures, the average highest temperature is around 29.85 ℃ (85.73 ℉). The average rainfall is 99.8 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:10 (hours and minutes), in the middle of the month 12:24 and at the end of the month 12:38.This data is approximate because we do not have exact data for this country. It is important to say, that the weather may differ significantly in different altitudes.Typical (usual) weather in Cook Islands in November is this: The average highest temperature in Cook Islands in November is 29.9 ℃ (85.82 ℉). The average lowest temperature is 24.7 ℃ (76.46 ℉). At the beginning of November you can expect lower temperatures, the average highest temperature is around 29.85 ℃ (85.73 ℉). At the end of November you can expect higher temperatures, the average highest temperature is around 30.3 ℃ (86.54 ℉). The average rainfall is 198.7 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:38 (hours and minutes), in the middle of the month 12:49 and at the end of the month 12:59.This data is approximate because we do not have exact data for this country. It is important to say, that the weather may differ significantly in different altitudes.Typical (usual) weather in Cook Islands in December is this: The average highest temperature in Cook Islands in December is 30.7 ℃ (87.26 ℉). The average lowest temperature is 25.3 ℃ (77.54 ℉). At the beginning of December you can expect lower temperatures, the average highest temperature is around 30.3 ℃ (86.54 ℉). At the end of December you can expect lower temperatures, the average highest temperature is around 30.5 ℃ (86.9 ℉). The average rainfall is 278.4 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:59 (hours and minutes), in the middle of the month 13:05 and at the end of the month 13:06.This data is approximate because we do not have exact data for this country. It is important to say, that the weather may differ significantly in different altitudes.