St Martin and Maarten Geography & Climate

The island covers about 87 square kilometres in the northern Lesser Antilles, roughly 53 of them French and 34 Dutch — the smallest landmass in the world divided between two sovereign nations.

Terrain

It is hilly rather than mountainous and volcanic in origin but long extinct, rising to Pic Paradis at 424 metres in the French centre. There are no rivers, the soil is thin, and the vegetation is dry tropical scrub rather than rainforest — this is one of the drier islands in the Antilles.

The Simpson Bay Lagoon

A large shallow saltwater lagoon in the south-west, one of the biggest enclosed lagoons in the Caribbean, straddling the border. It is a major yachting anchorage, with lifting bridges on both sides, and Princess Juliana airport’s runway runs along its seaward edge — which is why aircraft approach so low over Maho Beach.

Coast

Deeply indented, with around forty beaches and bays. Reef fringes parts of the coast, and the small satellite islands of Tintamarre and Pinel lie off the north-east.

Neighbours in sight

Anguilla is a few kilometres north across the channel, St Barts to the south-east and Saba and St Eustatius to the south-west — all visible on a clear day and all reachable by ferry or short flight.

The border, in practice

The line runs about ten kilometres across the middle and is marked by a single monument on the main road and almost nothing else — no barrier, no checkpoint, no formality. Visitors regularly cross it several times a day without noticing, which is only a problem where it matters: mobile networks, electrical sockets, currencies and speed limits all change at a line you cannot see.

Exposure

Low, small and in the hurricane belt, the island has no sheltered interior to retreat to.

The island has a tropical maritime climate that is notably dry for the Caribbean — around 1,000 to 1,200 millimetres of rain a year, less than half what the mountainous islands to the south receive, because there is no high ground to wring moisture out of the trade winds.

Temperature

Remarkably steady. Daytime highs run 28 to 31°C year round and nights 22 to 25°C, with the trade winds making it feel considerably cooler than the numbers suggest. There is no cold season and no real variation to plan around.

Wet and dry

The driest months are February to April. The wetter spell runs August to November, when humidity rises and showers are heavier, though they usually pass in minutes rather than settling in.

Hurricanes

This is the serious part. The season runs June to November and peaks in September. The island sits squarely in the hurricane belt and is low and small with nowhere sheltered. Hurricane Irma in September 2017 crossed it as a category five storm with sustained winds around 285 km/h, destroying or damaging the large majority of buildings on both sides and wrecking the airport. Recovery has taken years. Travel in September and October carries real disruption risk, and many businesses simply close.

What the dryness means

The low rainfall explains a good deal about how the island looks and works. There are no rivers and no natural springs of any use, so the water supply is entirely desalinated and expensive; the vegetation is scrub, cactus and manchineel rather than rainforest; and the beaches stay usable in the wet season when islands further south are running with mud. It also means the light is harder and the sun stronger than the sea breeze suggests.

The sea

Between about 26 and 29°C all year, and the UV index is high in every month.