Wind. Glider pilots call them cloud streets and they are our highways in the sky. All the thermals are lined up and all the sink is inbetween, so you can just fly straight, underneath the clouds, at high speed with no circling. for miles and miles and miles. the level of moisture determines if it is a sold cloud or individual clouds in a line.
http://www.faa.gov/library/manuals/aircraft/glider_handbook/media/faa-h-8083-13.pdf
page 9-11 is where the discussion on cloud streets start.
Power pilots - you can use cloud streets to increase your speed as well. It may be a little bumpy but you will be in continuously rising air so you will have to push the nose down further to hold altitude, therefore you will go faster at a given power setting. Use the atmosphere!