The sky was filled with altostratus clouds that came in from the South and were very slowly creeping North.
This thick cloud cover tends to indicate the presence of a stable air mass (and light winds). The streaks and the undulating wave formation in the cloud cover is indicative of wind shear through the cloud formation.
While we were under heavy cloud cover, we knew that today would stay pretty calm and the winds would remain light for as long as the clouds covered the sky. If we were sailing today, we would have kept a closer eye on the clouds, knowing that if the clouds cleared out or got thicker: stronger winds would follow.
Clear skies would indicate that a cold front (high pressure) system would blow in and blow rather hard on us. Thicker clouds would indicate that we were moving deeper into the warm front (low pressure) system and stormier conditions would become present.
Either way, this cloud covered sky lets us know that calm winds will be upon us.