Cloud thinning over North Sea

[Sunshineandshowers84]

Could anybody tell me what causes cloud to thin over North Sea Coasts of Scotland and England.

This happens all the time when the winds are westerly's. A front of heavy and persistent rain can come across Scotland giving rain to all parts. Yet the cloud as soon as it goes over the North Sea thins but continues to rain. Even in showery conditions the cloud thins and it could look like quite a nice day just 10 miles out at sea. This thinning of cloud nearly a daily occurrence over Eastern Scotland and Firth of Forth but would like to know what causes this.