Why do southerners in England pronounce Ask like ohsk?

It might not be spelt great, but people know what I mean.

Southerners in England, although not all....the Cornish and in many areas in the South (mainly farmers) actually pronounce the word "ask" similar to Americans. But mainly people pronounce it which seems to me like "ohsk" whereas the rest of the country pronounce it a simple "ask".

Why is that? It's the same with "dance, chance, flask, task" etc etc. People in north pronounce the a different to those in the south in general.

Does anyone know?

All those words were historically pronounced with the [æ] in the sound "cat" and "map" in British English but around the 18th century different pronunciations for certain words (generally those before fricatives like [f] and [s], and some before [nt]..but with exceptions of course) began to arise in Southern Britain. It's kind of a complicated issue to explain fully because it varies a lot by region and the extent that the [A] (as in "father") replaced [æ]. Many northern varieties of UK English and the vast majority of North Americans retain the historical [æ] in these words (in fact, besides an occasional [Ant] for "aunt", I've never heard Americans use [A] in these words, altho they apparently do exist in some smaller East Coast varieties), and Australians vary in their degree to which [æ] is used in those words.

For more details wikipedia has a good article on it.

http://en.wikipedia.org/wiki/Broad_A

cheers for the link.

No prob...hope that clears some things up...it does a good job explaining it.