St Helena enjoys a mild sub-tropical climate due to the South Atlantic Anticyclone (SAA) which controls weather and climate over the central South Atlantic. The resulting wind-stress causes the anticyclonic gyre circulation in the Ocean, the South East Trade Winds, where the Benguela Current conveys relatively cold water from high to low latitudes along Namibian and South African west coasts. The latter continues westward to form the South Equatorial Current (Feistel, Hagen, & Grant, 2003).

St Helena’s sub-tropical latitude affords it fairly constant temperatures; the average annual temperature range at sea level is 21-28^oC, this reduces by 1.3 degrees every 100 m rise in elevation. Rainfall is often extremely localised, creating desert-like conditions of the outer part of the island where 175mm of rain falls per year, compared to the Peaks where cloud and mist accumulate bringing 290 days of overcast conditions and 1050mm of rain each year (Kew, Trust, OTEP, & Lambdon, 2017). Mean monthly temperatures since 1893 have rarely been below 15 °C or above 22 °C (iMC Worldwide, 2014).

The island has a single manned weather station at Bottom Woods, which is operated by the UK Meteorological Office using local staff. The Bottom Woods Station is the only weather station on St Helena which reports to international data sets. Data from the weather station is commonly listed as St Helena Island, with records from 1892 to the present. The weather station at Bottoms Woods started operations in 1976, replacing the weather station at Hutts Gate. Other than one missing month in 1892, the islands long term climate record appears complete. Table 1 below and Figure 1 (top left) summarise the long-term average rainfall and temperature record for the island.

Table 1: Saint Helena Long Term Average Rainfall and Temperature (1892 to 2014)

Based on the long-term climate data presented in Figure 1, the island has two distinct wet seasons centring on March and July. November is the driest month of the year. August and September are the coolest months with February and March the warmest. In general, humidity varies little, being in the mid-sixties (percent) in the warmer months and in the low to mid-seventies (percent) for the rest of the year (SAERI et al., 2016). Mean annual rainfall has been shown to increase with elevation across the island, with rainfall at altitudes greater than 656mASL being 61% greater than rainfall between sea level and 328mASL (Sir William Halcrow and Partners, 1969).