Geography
[edit]
Terraced Portland Stone houses in Fortuneswell, Underhill
Isle of Portland from the airIsle of Portland from Chesil Beach
The Isle of Portland lies in the English Channel, 3 kilometres (2 mi) south of Wyke Regis, and 200 km (120 mi) west-southwest of London, at 50°33′0″N 2°26′24″W / 50.55000°N 2.44000°W / 50.55000; -2.44000 (50.55, −2.44). Portland is situated approximately halfway along the UNESCO Jurassic Coast World Heritage Site; the site includes 153 kilometres (95 mi) of the Dorset and east Devon coast that is important for its geology and landforms. The South West Coast Path runs around the coast; it is the United Kingdom's longest national trail at 1,014 kilometres (630 mi). Portland is unusual as it is connected to the mainland at Abbotsbury by Chesil Beach, a tombolo which runs 29 kilometres (18 mi) north-west to West Bay. Portland is sometimes defined incorrectly as a tombolo—in fact Portland is a tied island, and Chesil Beach is the tombolo (a spit joined to land at both ends).
There are eight settlements on Portland, the largest being Fortuneswell in Underhill and Easton on Tophill. Castletown and Chiswell are the other villages in Underhill, and Weston, Southwell, Wakeham and the Grove are on the Tophill plateau. Many old buildings are built out of Portland Stone; several parts have been designated Conservation Areas to preserve the unique character the older settlements which date back hundreds of years.
The Isle of Portland has been designated by Natural England as National Character Area 137. It is adjoined by the Weymouth Lowlands to the north.
The Isle of Portland lies on Upper Oolite limestone.
Geology[edit]
Geologically, Portland is separated into two areas; the steeply sloping land at its north end called Underhill, and the larger, gently sloping land to the south, called Tophill. Portland stone lies under Tophill; the strata decline at a shallow angle of around 1.5 degrees, from a height of 151 metres (495 ft) near the Verne in the north, to just above sea level at Portland Bill. The geology of Underhill is different to Tophill; Underhill lies on a steep escarpment composed of Portland Sand, lying above a thicker layer of Kimmeridge Clay, which extends to Chesil Beach and Portland Harbour. This Kimmeridge Clay has resulted in a series of landslides, forming West Weares and East Weares.
2.4 kilometres (1.5 mi) underneath south Dorset lies a layer of Triassic rock salt, and Portland is one of four locations in the United Kingdom where the salt is thick enough to create stable cavities. Portland Gas applied to excavate 14 caverns to store 1,000,000,000 cubic metres (3.5×1010 cu ft) of natural gas, which is one per cent of the UK's total annual demand. It was proposed that the caverns should be connected to the National gas grid at Mappowder via a 37-kilometre (23 mi) pipeline. Plans had it that the surface facilities should be complete to store the first gas in 2011, and the entire cavern space available for storage in winter 2013. As part of the £350 million scheme, the Grade II listed former Old Engine Shed would be converted into a £1.5 million educational centre with a café and an exhibition space about the geology of Portland.
Portland Bill[edit]
Portland Bill Lighthouse and visitors' centre
Main article: Portland Bill
Portland Bill is the southern tip of the island of Portland. The Bill has three lighthouse towers. The Higher Lighthouse is now a dwelling and holiday apartments whilst the Lower Lighthouse is now a bird observatory and field centre providing records of bird migration and accommodation for visitors, which opened in 1961. The white and red lighthouse on Bill Point replaced the Higher and Lower Lighthouses in 1906. It is a prominent and much photographed feature; an important landmark for ships passing the headland and its tidal race. The current lighthouse was refurbished in 1996 and became remotely controlled. It now contains a visitors' centre giving information and guided tours of the lighthouse.
Portland Ledge and Portland Race[edit]
Portland Ledge is an underwater extension of Portland Stone into the English Channel at a place where the depth of Channel is 20 to 40 metres (about 10 to 20 fathoms). Tidal flow is disrupted by the feature; at 10 metres (about 5 fathoms) deep and 2.4 kilometres (1.3 nmi) long, it causes a tidal race to the south of Portland Bill, the so-called Portland Race. The current only stops for brief periods during the 12+1⁄2-hour tidal cycle and can reach 4 metres per second (9 mph) at the spring tide of 2 metres (6 ft 7 in).
Ecology[edit]
Portland's cliffs and quarries have extensive specialised flora and fauna.
Due to its isolated coastal location, the Isle of Portland has an extensive range of flora and fauna; the coastline and disused quarries are designated as Sites of Special Scientific Interest. The Isle of Portland SSSI encompasses 352 hectares (870 acres), and includes 17 monitored features ranging from Jurassic fossils, calcareous grassland, rock sea-lavender and nationally scarce butterflies. Sea and migratory birds occupy the cliffs in different seasons, sometimes these include rare species which draw ornithologists from around the country. Rare visitors to the surrounding seas include dolphins, seals and basking sharks. Chesil Beach is one of only two sites in Britain where the scaly cricket can be found; unlike any other cricket it is wingless and does not sing or hop. Ten British Primitive goats were introduced to the East Weares part of the island to control scrub in 2007.
The comparatively warm and sunny climate allows species of plants to thrive which do not on the mainland. The limestone soil has low nutrient levels; hence smaller species of wild flowers and grasses are able to grow in the absence of larger species. Portland sea lavender can be found on the higher sea cliffs; unique to Portland, it is one of the United Kingdom's rarest plants. The wild flowers and plants make an excellent habitat for butterflies; over half of the British Isles' 57 butterfly species can be seen on Portland, including varieties that migrate from mainland Europe. Species live on Portland that are rare in the United Kingdom, including the limestone race of the silver-studded blue.
Climate[edit]
Climatic ergograph for the nearby Wyke Regis weather station
The mild seas which almost surround the tied island produce a temperate climate (Köppen climate classification Cfb) with a small variation in daily and annual temperatures. The average annual mean temperature from 1991 to 2020 was 11.5 °C (52.7 °F). The warmest month is August, which has an average temperature range of 14.8 to 19.5 °C (58.6 to 67.1 °F), and the coolest is February, which has a range of 4.7 to 8.4 °C (40.5 to 47.1 °F). Mean winter temperatures are amongst the highest in the British Isles, and by far warmer than the United Kingdom average. However, due to the island's proximity to the sea, summers are cooler than the national average, with temperatures rarely climbing to the extremes seen in in-land areas further north. As a result of its coastal extremity and mild winter minimum temperatures, Portland is suitable for plants with the Royal Horticultural Society's hardiness rating H2.[B] Mean sea surface temperatures range from 7.0 °C (44.6 °F) in February to 17.2 °C (63.0 °F) in August; the annual mean is 11.8 °C (53.2 °F).
Climate data for Isle of Portland (1991-2020 normals, extremes 1967-)
Month
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Year
Record high °C (°F)
13.4(56.1)
13.1(55.6)
19.7(67.5)
22.9(73.2)
24.0(75.2)
30.8(87.4)
27.8(82.0)
30.2(86.4)
25.6(78.1)
21.1(70.0)
18.2(64.8)
14.5(58.1)
30.8(87.4)
Mean daily maximum °C (°F)
8.7(47.7)
8.4(47.1)
9.7(49.5)
11.9(53.4)
14.5(58.1)
17.0(62.6)
19.0(66.2)
19.5(67.1)
18.1(64.6)
15.0(59.0)
11.9(53.4)
9.6(49.3)
13.6(56.5)
Daily mean °C (°F)
6.9(44.4)
6.6(43.9)
7.7(45.9)
9.4(48.9)
12.0(53.6)
14.6(58.3)
16.6(61.9)
17.1(62.8)
15.7(60.3)
13.1(55.6)
10.1(50.2)
7.8(46.0)
11.5(52.7)
Mean daily minimum °C (°F)
5.2(41.4)
4.7(40.5)
5.6(42.1)
7.0(44.6)
9.5(49.1)
12.2(54.0)
14.3(57.7)
14.8(58.6)
13.4(56.1)
11.1(52.0)
8.2(46.8)
6.0(42.8)
9.4(48.9)
Record low °C (°F)
−10.4(13.3)
−8.4(16.9)
−4.5(23.9)
−1.6(29.1)
0.7(33.3)
3.0(37.4)
8.0(46.4)
8.4(47.1)
4.7(40.5)
1.3(34.3)
−2.8(27.0)
−6.5(20.3)
−10.4(13.3)
Average precipitation mm (inches)
74.1(2.92)
52.7(2.07)
46.3(1.82)
47.6(1.87)
42.7(1.68)
41.1(1.62)
36.9(1.45)
47.2(1.86)
46.2(1.82)
76.4(3.01)
82.6(3.25)
78.6(3.09)
672.3(26.47)
Average precipitation days (≥ 1.0 mm)
12.3
10.5
9.1
8.6
8.0
7.2
6.4
8.0
8.0
12.0
13.4
12.6
116.1
Mean monthly sunshine hours
66.7
90.5
134.0
192.2
228.5
230.3
237.4
212.4
172.5
116.1
80.1
62.2
1,822.6
Source 1: Met Office
Source 2: Starlings Roost Weather
The mild seas that surround Portland act to keep night-time temperatures above freezing, making air frost rare: on average 6.3 days per year — this is far below the United Kingdom's average annual total of 55.6 days of frost. Days with snow lying are equally rare: on average zero to five days per year; almost all winters have no more than one day with snow lying. It may snow or sleet in winter, yet it almost never settles on the ground—coastal areas in South West England such as Portland generally experience the mildest winters in the UK. Portland is less affected by the Atlantic storms that Devon and Cornwall experience. The growing season lasts for more than 310 days per year,[D] and the borough is in Hardiness zone 9.[E]
Weymouth and Portland, and the rest of the south coast, has the sunniest climate in the United Kingdom. Portland averaged 1822.6 hours of sunshine annually between 1991 and 2020, which is 42% of the maximum possible,[C] and 36% above the United Kingdom average of 1402.7 hours. December is the cloudiest month (62.2 hours of sunshine), November the wettest (82.6 millimetres (3.3 in) of rain) and July is the sunniest and driest month (237.4 hours of sunshine, 36.9 millimetres (1.5 in) of rain). Sunshine totals in all months are well above the United Kingdom average, and monthly rainfall totals throughout the year are less than the UK average, particularly in summer; this summer minimum of rainfall is not experienced away from the south coast of England. The average annual rainfall of 672.3 millimetres (26.5 in) is well below the UK average of 1,163.0 millimetres (45.8 in).