This was a remarkable sight from the beach on Anglesey looking back towards the Lleyn Peninsula and Snowdonia. What I think was likely to be happening was evaporation of the sea due to strong sunlight. The wind was blowing the air laden with vapour towards the coast line. On reaching the mountains, the air was forced upwards and cooled. This would make the water vapour condense into clouds. Cumulus clouds are formed by evaporation on days like this. The line of mountains probably explains the line of cumulus.
