The weather in Cornwall can change really fast, it's quite common for there to be cloud and mist on one coast, but the other is bright and sunny.  Down in the west of Cornwall you can get between coasts in 30 mins or so, so it's well worth checking out these webcams for a live view of the weather. Below the map are individual links.

View webcams in a larger map The Minack Theatre, on the south west coast, near the beautiful Porthcurno beach. Live webcams
Mounthaven Hotel Webcam View from Mounthaven hotel, Marazion,  towards St Michaels mount,  in south west cornwall.
Falmouth Harbour Heligan Wild Webcams, showing Owl nesting boxes ( Spring 2011)