Map from Google - Comoros

The climate of the Comoros is tropical, with a hot and rainy season from December to April, when the north-west monsoon prevails, and a relatively cool and dry season from May to November, when the south-east trade winds blow. The average daily temperature goes from 27 °C (81 °F) in the warmest period (January to April), to 23 °C (73 °F) in the coolest months (July, August and September).
Here are the average temperatures of Moroni.
Average temperatures - Moroni
MoroniJanFebMarAprMayJunJulAugSepOctNovDec
MoroniJFMAMJJASOND
Min (°C)232323232120191819202223
Max (°C)303031303028282828293031
Min (°F)737373737068666466687273
Max (°F)868688868682828282848688

Of the three main islands that make up the archipelago, the two smallest ones (Mohéli and Anjouan) are covered by a hillock in the middle, while the largest, Grande Comore or Ngazidja, is home to a real mountain, the volcano Karthala, 2,360 metres (7,743 feet) high. The slopes enhance the cloud formation, associated with showers and thunderstorms, especially on the main island, which in fact is rainiest. On Karthala, 5,000 millimetres (195 inches) of rain per year fall; in the capital Moroni, which is located on the windward side in the rainy season (that is, it's exposed to the north-west monsoon), 2,700 mm (106 in) of rain per year fall.
Here is the average precipitation in Moroni.
Average precipitation - Moroni
MoroniJanFebMarAprMayJunJulAugSepOctNovDecYear
MoroniJFMAMJJASONDY
Prec. (mm)365295280315255265245150110951102202705
Prec.(in)14.411.61112.41010.49.65.94.33.74.38.7106.5
Days181618181212121011121216167

In Fomboni, in the island of Mohéli, the total annual rainfall is 2,100 mm (83 in). Ouani, in the Anjouan Island (also known as Nzwani), receives 1,800 mm (70 in) of rain per year. January is the rainiest month throughout the archipelago, with more than 300 mm (12 in) of rain along the coast, while the driest month is October, when in Moroni 95 mm (3.7 in) of rain fall, which shows that there's no shortage of showers and thunderstorms even in the dry season, at least in the main island. However, these rains though intense are short, so that they soon make room for the return of the sun. In the smaller islands, during the dry season the showers are less frequent and less intense.
Here is the average precipitation in Fomboni.
Average precipitation - Fomboni
FomboniJanFebMarAprMayJunJulAugSepOctNovDecYear
FomboniJFMAMJJASONDY
Prec. (mm)3302802452401801451551056090951952115
Prec.(in)13119.69.47.15.76.14.12.43.53.77.783.3
Days17151613121010746713130

Despite the rains, due to the absence of rivers, the Comoros suffer from a shortage of fresh water.
The Comoros are located in the path of tropical cyclones, which can pass over the islands from November to mid-May, with a greater chance from late December to mid-April.

Comoros

The sea in the Comoros is warm enough for swimming all year round: the temperature ranges from 25 °C (77 °F) in August, to 29 °C (84 °F) in the period from February to April.
Sea temperature - Moroni
MoroniJanFebMarAprMayJunJulAugSepOctNovDec
MoroniJFMAMJJASOND
Sea (°C)292929292827262526262728
Sea (°F)848484848281797779798182

When to go

The best time to visit the Comoros and enjoy their yet uncrowded beaches, runs from June to October: it's the least sultry, the driest and sunniest, although some showers must still be put into account, especially in the main island. In the latter, since the rains are still abundant in June and July, you can choose the period from August to October.

What to pack

In winter (June to August): light clothes, a sweatshirt for the evening, a light scarf for the breeze; for the reef, equipment for snorkeling, water shoes or rubber soled shoes. For women it is best to avoid shorts and miniskirts outside of tourist resorts. To climb Karthala, hiking boots, fleece, raincoat.

In summer (December to February): tropics-friendly, loose fitting clothing, light shirts and pants of natural fibres (cotton, linen) or synthetics that breathe, light sweatshirt for the evening, light raincoat or umbrella.