over the 1981–2010 period. Typically the coldest night of the year at the Botanic Garden will fall to . Such minimum temperatures and frost averages are typical for inland areas across much of southern and central England. Sunshine averages around 1,500 hours a year or around 35% of possible, a level typical of most locations in inland central England. Ecology The city contains three Sites of Special Scientific Interest (SSSIs), at Cherry Hinton East Pit, Cherry Hinton West Pit, and Travellers Pit, and ten Local Nature Reserves (LNRs): Sheep's Green and Coe Fen, Coldham's Common, Stourbridge Common, Nine Wells, Byron's Pool, West Pit, Paradise, Barnwell West, Barnwell East, and Logan's Meadow. Green belt Cambridge is completely enclosed by green belt as a part of a wider environmental and planning policy first defined in 1965 and formalised in 1992. While some small tracts of green belt exist on the fringes of the city's boundary, much of the protection is in the surrounding South Cambridgeshire and nearby East Cambridgeshire districts, helping to maintain local green space, prevent further urban sprawl and unplanned expansion of the city, as well as protecting smaller outlying villages from further convergence with each other as well as the city. Demography At the 2011 census, the population of the Cambridge contiguous built-up area (urban area) was 158,434, while that of the City Council area was 123,867. In the 2001 Census held during University term, 89.44% of Cambridge residents identified themselves as white, compared with a national average of 92.12%. Within the university, 84% of undergraduates and 80% of post-graduates identified as white (including overseas students). Cambridge has a much higher than average proportion of people in the highest paid professional, managerial or administrative jobs (32.6% vs. 23.5%) and a much lower than average proportion of manual workers (27.6% vs. 40.2%). In addition, 41.2% have a higher-level qualification (e.g. degree, Higher National Diploma, Master's or PhD), much higher than the national average proportion (19.7%). Centre for Cities identified Cambridge as the UK's most unequal city in 2017 and 2018. Residents' income was the least evenly distributed of 57 British cities measured, with its top 6% earners accounting for 19% of its total income and the bottom 20% for only 2%, and a Gini coefficient of 0.460 in 2018. Historical population Local census 1749 Census: Regional District 1801–1901 Civil Parish 1911–1961 District 1971–2011 Ethnicity Religion Economy The town's river link