How is a sky full of clouds represented in a weather map?

The central circle represents the cloud cover.

Explanation:

A black circle means the sky is completely overcast and cloudy.

White or empty circle means that the sky is completely clear of clouds.

The amount of black in the circle is the fraction of the sky that is covered.

Sky Cover. The total amount of clouds in tenth. Plotted in the station circle according the ... The wind speed is plotted as feathers and half-feathers representing 10 and 5 knots, ... For values above 10 miles, the values are omitted from the map.