Stratocumulus undulatus

Today I was on the river the Vecht, a 44 km long river from the city of Utrecht to the IJsselmeer.
At the end of the day, when it was cooling down after a very sunny day, these clouds formed. They have a wavelike arrangement of the cloud parts, in parallel rollers, which are separated by cloud strips. Looking through the clouds you can spot a clear blue sky.
So these clouds are called Stratocumulus undulatus.