A through line is marked as a black line on low level significant weather charts and surface pressure charts because: 15 out of 35 answered. A) it signifies an area of abnormally high wind speeds B) it is a useful area for navigation thanks to the relatively calm, clear skies it produces C) it signifies a line of convergence/convective weather which might otherwise not be apparent from the isobar pattern D) it isnt a sufficiently significant feature to warrant being drawn with isobars