The climate of the Northern Mariana Islands, 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 in the southern islands, which are closer to the equator, and are also the inhabited ones, where the daytime temperature goes 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 the capital Saipan.
Average temperatures - Saipan

Saipan	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Saipan	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	24	23	24	25	25	26	25	25	25	25	25	25
Max (°C)	28	28	29	30	30	31	30	30	30	30	30	29
Min (°F)	75	73	75	77	77	79	77	77	77	77	77	77
Max (°F)	82	82	84	86	86	88	86	86	86	86	86	84

Rainfall is abundant, since it reaches 2,400 millimetres (94.5 inches) per year in 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 per month fall.
Here is the average precipitation in Saipan.
Average precipitation - Saipan

Saipan	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Saipan	J	F	M	A	M	J	J	A	S	O	N	D	Y
Prec. (mm)	110	75	80	100	95	125	230	285	325	285	180	120	2010
Prec.(in)	4.3	3	3.1	3.9	3.7	4.9	9.1	11.2	12.8	11.2	7.1	4.7	79.1
Days	22	19	19	18	19	19	21	23	21	22	21	22	246

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 become locally more abundant.
The amount of sunshine is never excellent, because there's always the chance to see some cloudiness, which then can develop and lead to thunderstorms, but it's never bad either, because even in the rainiest months, between a shower and the other, the sun comes out. However, the sunniest period is the least rainy.
Sunshine - Saipan

Saipan	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Saipan	J	F	M	A	M	J	J	A	S	O	N	D
Sun (hours)	6	6	7	8	8	7	6	6	6	5	5	5

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

Saipan	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Saipan	J	F	M	A	M	J	J	A	S	O	N	D
Sea (°C)	28	27	27	28	28	29	29	29	29	29	29	28
Sea (°F)	82	81	81	82	82	84	84	84	84	84	84	82

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 in which they typically occur is from May to December, though they are more frequent between August and November; however, sometimes they have passed at other periods, in April (typhoon Isa in 1997, and Kong-rey in 2007), and January (Roy in 1988).


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


All year round, light clothes, of natural fibres, sun hat, a scarf for the breeze, a light sweatshirt for the evening, light raincoat or umbrella.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.