In Nosy Be (or Nossi-bé) the climate is tropical, hot all year round, with a rainy and hotter season from November to April, and a dry season from May to October.
In the period that could be called winter, from June to September, at night the temperature usually drops below 20 °C (68 °F), and sometimes can even drop to around 15/16 °C (59/61 °F), but during the day it's almost always warm, thanks to the winds that blow from the interior of Madagascar.
Here are the average temperatures.
Average temperatures - Nosy Be

Nosy Be	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Nosy Be	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	23	23	23	22	20	19	18	18	19	20	22	23
Max (°C)	31	31	31	31	31	29	29	29	31	32	32	31
Min (°F)	73	73	73	72	68	66	64	64	66	68	72	73
Max (°F)	88	88	88	88	88	84	84	84	88	90	90	88

The island is located off the northwest coast of Madagascar, in an area that in the austral summer is affected by the hot and humid monsoon coming from the north-west, while in the austral winter the cooler and drier south-east trade winds prevail. The climate of the neighbouring islands, such as Nosy Komba, Nosy Mitsio, Nosy Tanikely and Nosy Sakatia, is similar.

In an average year, about 2,200 millimetres (86 inches) of rain fall. In the rainiest months (December, January and February), the rains are very frequent, occurring almost daily, and can sometimes be torrential. On the contrary, at the height of the dry season, from June to September, there are rare showers, which in May and October are a bit more frequent.
Here is the average precipitation.
Average precipitation - Nosy Be

Nosy Be	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Nosy Be	J	F	M	A	M	J	J	A	S	O	N	D	Y
Prec. (mm)	465	425	285	140	60	50	35	40	40	95	190	360	2185
Prec. (in)	18.3	16.7	11.2	5.5	2.4	2	1.4	1.6	1.6	3.7	7.5	14.2	86
Days	20	16	13	7	3	3	3	3	3	5	10	14	100

The amount of sunshine in Nosy Be is good from May to October, while it is acceptable in the rainiest months, which as we said are December, January and February, because apart from showers and thunderstorms, the sun shines for a few hours a day.
Sunshine - Nosy Be

Nosy Be	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Nosy Be	J	F	M	A	M	J	J	A	S	O	N	D
Sun (hours)	6	6	7	8	9	8	9	9	9	9	8	7

From November to mid-May, Nosy Be may be affected by tropical cyclones of the South-West Indian Ocean, which in any case are more common from January to March.



For swimming, the sea is warm all year round, since the water temperature ranges from 26 °C (79 °F) in the period from July to October, to 29 °C (84 °F) from January to April.
Here are the average sea temperatures.
Sea temperature - Nosy Be

Nosy Be	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Nosy Be	J	F	M	A	M	J	J	A	S	O	N	D
Sea (°C)	29	29	29	29	28	27	26	26	26	26	28	28
Sea (°F)	84	84	84	84	82	81	79	79	79	79	82	82

The best time to go to Nosy Be goes from mid-May to October. In the first half of May, usually the weather is already good, but there is still some risk, though low, of the passage of tropical cyclones, as happened with cyclone Kesiny, which affected this area around the 10th of May 2002.