The climate of the Northern Mariana Islands, a U.S. territory in the Pacific Ocean, is tropical, hot and humid all year round, with a relatively cool and dry season from December to June, and a warmer and rainy season from July to November.
Temperatures are stable throughout the year, especially on the southern islands, which are closer to the equator, and are also the inhabited ones, where the daytime temperature ranges from 28/29 °C (82/84 °F) in the coolest period to 30/31 °C (86/88 °F) in the warmest. The heat is sultry, but tempered by the trade winds.
Here are the average temperatures of Saipan, the capital.
Saipan - Average temperatures
Min (°C)242324252526252525252525
Max (°C)282829303031303030303029
Min (°F)757375777779777777777777
Max (°F)828284868688868686868684

Rainfall is abundant, since it reaches 2,400 millimeters (94.5 inches) per year on the southernmost island (Rota) and 2,000 mm (79 in) in Saipan and Tinian. The wettest months are August, September and October, with more than 250 mm (10 in) of rain per month, while in the least rainy months, from February to May, from 70 to 100 mm (1.6 to 4 in) of rain fall per month.
Here is the average precipitation in Saipan.
Saipan - Average precipitation

The other islands, located north of the three permanently inhabited ones, are gradually cooler in winter and generally less rainy throughout the year, though along the slopes exposed to the north-east trade winds (they are volcanic islands, with some elevations in the interior), the rains may be locally more abundant.
In the Northern Mariana Islands, for many months some cloudiness can always form, which can develop and lead to thunderstorms. However, there's a moderate amount of sunshine because between one shower and another the sun comes out. The sunniest period is the least rainy one.
Saipan - Sunshine


The temperature of the sea is warm enough for swimming all year round: it drops to 27 °C (81 °F) in February and March.
Saipan - Sea temperature
Temp (°C)282727282829292929292928
Temp (°F)828181828284848484848482

Tropical cyclones

The Northern Mariana Islands are located in the path of typhoons, the tropical cyclones of south-east Asia, towards which they normally move after passing over the islands. The period when they typically occur is from May to December, though they are more frequent between August and November. However, sometimes they have affected this area at other periods, in April (typhoon Isa in 1997, and Kong-rey in 2007) and January (Roy in 1988).

Best Time

The best time to visit the Northern Marianas runs from January to April because it's the least hot and the least rainy, and (normally) outside the typhoon season. February and March are the best months of all.

What to pack

All year round, bring light clothes of natural fibers, a sun hat, a scarf for the breeze, a light sweatshirt for the evening, a light raincoat or umbrella.
When going to the reef, you can bring snorkeling equipment, including water shoes or rubber-soled shoes.