Working for birds in Africa


Fri, 25/01/2013 - 11:41 -- abc_admin

Jamestown and Ladder Hill, St Helena

Image Credit: 
Beau Rowlands

St Helena (15° 56’S, 5° 42’W) is one of the most isolated land-masses in the world. It lies some 1,900 km west of Angola and 3,300 km east of Brazil. The nearest land is Ascension Island 1,100 km to the north-west. The island covers an area of only 122 km2 and has maximum dimensions of 16 km by 10 km.

St Helena is of volcanic origin and is part of the Mid-Atlantic Ridge system, though it lies to the east of the main ridge. The island rises from a depth of 4,224 m, having first emerged around 20 million years ago. The major axis is aligned north-east to south-west and is surmounted by a ridge rising to 823 m at Diana’s Peak. The island comprises the remains of two shield volcanoes, one centred in the north-east of the island around Flagstaff Hill and the other in the Sandy Bay area. Volcanic activity ceased around seven million years ago. St.Helena’s topography is dominated by a radial drainage system centred on the central ridge. This has produced a series of steep-sided valleys, some up to 300 m deep, separated by generally narrow ridges. Between the valley mouths the coastline consists of cliffs ranging from 80 m to 570 m in height. Some of the bays have pebble or boulder beaches, which are generally narrow. Jamestown (the capital), Half-Tree Hollow and Longwood are the only settlements of significant size (Ashmole & Ashhmole 2000).

Although St Helena lies well within the Tropic of Capricorn its climate is best described as sub-tropical (Mathieson 1990). The most striking feature of the island’s weather patterns is their variability, both between years and within the island. Temperatures are subject to the ameliorating influence of the South-East Trade Winds blowing over the cold Benguela current. Within the island, temperatures vary markedly with altitude. At Jamestown (sea level) the annual mean is 22oC but at Hutt’s Gate (627 m) only 16oC. The coolest months are normally August and September and the warmest March and April. The seasonal pattern and quantity of rainfall varies greatly between years. Precipitation increases with altitude: Jamestown receives, on average, less than 300 mm of rain annually but at an altitude of 800 m rainfall rises to over 900 mm. In most years rainfall is heaviest between March and September. Above 400 m, low cloud can cause prolonged foggy conditions between March and November. Wind direction is dominated by the South-East Trades, which prevail on 70%-80% of days, usually at a strength of Force 4-5. Gales can be frequent between September and November.

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