When you picture a cloud, you probably see the white puffy part hovering high in the sky. The base is clearly visible above the horizon and the top of the cloud is also in view. These are happy clouds that grace you with shade on a hot ocean day!
Monster clouds are the ones that rise up over the horizon with no visible base. These clouds are so massive that they are located somewhere beyond the curvature of the Earth yet they still take up almost half of the visible sky! These are pressure systems, so massive that they have a different air pressure than their surroundings.
If you are in a high pressure with clear blue skies, these low pressure monsters will look like massive white hazes in the distance. If they are far enough they will look like a white dome, if they are closer, just a hazy white horizon. The winds in these creatures can be quite powerful, so it is best to avoid them.
To do this, you choose your course based on where they are going and stay out of their way. You are a literal ant in a room full of elephants when you are sailing the ocean blue. Don't get stepped on!