Straight lines appear to curve on a rounded surface. Practice seeing and drawing these lines will greatly improve your ability to draw what you see.
Print out the latitude and longitude lines in the figure at the right. Trace over these lines about ten times then draw them free hand. As you trace, do not just make the line, but visualize the shape as a three-dimensional form. This process will kinesthetically add these curves to your drawing repertoire. You are training your internal pattern recognition system to see these curves . Use these lines to block in the patterns and shapes you observe and to help you in drawing curved lines you see in nature.