, east of Calne Two-thirds of Wiltshire, a mostly
rural county, lies on
chalk, a kind of soft, white, porous limestone that is resistant to erosion, giving it a high
chalk downland landscape. This chalk is part of a system of chalk downlands throughout eastern and southern England formed by the rocks of the
Chalk Group and stretching from the
Dorset Downs in the west to
Dover in the east. The largest area of chalk in Wiltshire is
Salisbury Plain, which is used mainly for
arable agriculture and by the British Army as training ranges. The highest point in the county is the
Tan Hill–
Milk Hill ridge in the
Pewsey Vale, just to the north of Salisbury Plain, at above sea level. The chalk uplands run north-east into
West Berkshire in the Marlborough Downs ridge, and south-west into Dorset as
Cranborne Chase. Cranborne Chase, which straddles the border, has, like Salisbury Plain, yielded much Stone Age and Bronze Age
archaeology. The Marlborough Downs are part of the
North Wessex Downs AONB (Area of Outstanding Natural Beauty), a conservation area. In the north-west of the county, on the border with
South Gloucestershire and
Bath and North East Somerset, the underlying rock is the resistant
oolite limestone of the
Cotswolds. Part of the Cotswolds AONB is also in Wiltshire, in the county's north-western corner. Between the areas of chalk and limestone downland are
clay valleys and
vales. The largest of these vales is the
Avon Vale. The Avon cuts diagonally through the north of the county, flowing through
Bradford-on-Avon and into
Bath and Bristol. The Vale of Pewsey has been cut through the chalk into
Greensand and
Oxford Clay in the centre of the county. In the south west of the county is the
Vale of Wardour. The south-east of the county lies on the sandy soils of the northernmost area of the
New Forest. Chalk is a
porous rock, so the chalk hills have little surface water, and the main settlements in the county were therefore established at wet points. Notably, Salisbury lies between the chalk of Salisbury Plain and the marshy flood plains.
Climate Along with the rest of South West England, Wiltshire has a temperate climate which is generally wetter and milder than counties further east. The annual mean temperature is approximately . Although there is a marked maritime influence, this is generally rather less pronounced than it is for other south-western counties, which are closer to the sea. July and August are the warmest months with mean daily maxima of approximately . In winter mean minimum temperatures of or are usual and
air frost is frequent. In the summer, the
Azores high pressure affects south-west England; however,
convective cloud sometimes forms inland, reducing the number of hours of sunshine. Annual sunshine rates are slightly less than the regional average of 1,600 hours. In December 1998, there were 20 days without sun recorded at Yeovilton (Somerset). Most of the rainfall in the south-west is caused by
Atlantic depressions or by
convection, though a proportion is caused orographically (uplift over hills). Autumn and winter are the rainiest, caused by Atlantic depressions, which are then most active. Even so, any month can be the wettest or driest in a given year, but the wettest is much more likely to be Oct-Mar, and the driest Apr-Sept. In summer, a greater proportion of the rainfall is caused by the sun heating the ground, leading to convection and showers and thunderstorms. It is often the northern half of the county that sees most of the showers with south-westerly winds in summer, whereas in the south of the county, the proximity of a relatively cold
English Channel often inhibits showers. In autumn and winter, however, the sea is often relatively warm, compared with the air passing over it, and can often lead to a higher rainfall in the south of the county (e.g., Salisbury recorded over 200mm of rain in November 2009 and January 2014). Average rainfall for the county is around , drier parts averaging 700mm (28ins)and the wettest 900mm (around 35ins). About 8–15 days of snowfall is typical. November to March have the highest mean wind speeds, and June to August have the lightest winds. The predominant wind direction is from the south-west.
Green belt The county has a
green belt mainly along its western fringes as a part of the extensive
Avon Green Belt. It reaches as far as the outskirts of Rudloe/
Corsham and Trowbridge, preventing
urban sprawl particularly from the latter in the direction of
Bradford-on-Avon, and affording further protection to surrounding villages and towns from Bath in Somerset. ==Demographics==