The Firth of Clyde encloses the largest and deepest coastal waters in the British Isles, sheltered from the Atlantic Ocean by the Kintyre peninsula which encloses the outer firth in Argyll and Ayrshire, Scotland.

The Kilbrannan Sound is a large arm of the Firth of Clyde, separating the Kintyre Peninsula from the Isle of Arran.

The Firth's climate enjoys the benefit of the Gulf Stream from America.