– especially along its east coast, which experiences a distinctive sea fog known as fret – although the Clean Air Act 1956 and decline of heavy industry have seen sunshine duration increase in urban areas in recent years. Language and dialect English Dialect The English spoken today in the North has been shaped by the area's history, and some dialects retain features inherited from Old Norse and the local Celtic languages. They are a dialectal continuum, middle areas that have a crossover between varieties spoken, around the North. Traditional dialectal areas are defined by their historic county or combined historic counties; including Cumbrian (Cumberland and Westmorland), Lancastrian (Lancashire), Northumbrian (Northumberland and Durham) and Tyke (Yorkshire). During the Industrial Revolution urban areas gained some or further distinction from traditional dialects; such as areas Mackem (Wearside), Mancunian (Manchester), Pitmatic (Great Northern Coalfield), Geordie (Tyneside), Smoggie (Teesside), Scouse (Liverpool) and around Hull. Linguists have attempted to define a Northern dialect area, some correspond the area north of a line that begins at the Humber estuary and runs up the River Wharfe and across to the River Lune in north Lancashire. This area corresponds roughly to the sprachraum of the Old English Northumbrian dialect, although the linguistic elements that defined this area in the past, such as the use of doon instead of down and substitution of an ang sound in words that end -ong (lang instead of long), are now prevalent only in the more northern parts of the region. As speech has changed, there is little consensus on what defines a "Northern" accent or dialect. Northern English accents have not undergone the TRAP–BATH split, and a common shibboleth to distinguish them from Southern ones is the Northern use of the short a (the near-open front unrounded vowel) in words such as bath and castle. On the opposite border, most Northern English accents can be distinguished from Scottish accents because they are non-rhotic, although some Lancashire and Northumberland accents remain rhotic. Other features common to many Northern English accents are the absence of the FOOT–STRUT split (so put and putt are homophones), the reduction of the definite article the to a glottal stop (usually represented in writing as t or occasionally th, although it is often not pronounced as a /t/ sound) or its total elision, and the T-to-R rule that leads to the pronunciation of t as a rhotic consonant in phrases