Ah... it was the wind that got you. You can either build a wind break (two pieces of plywood held together with a hinge) that can be positioned to block the wind, or do what I do... buy a welding blanket and wrap it loosley around the smoker and use a couple of spring clips (big metal clothes pins) to hold it together. The welding blanket is nice because it blocks wind, insulates it for cold weather smoking, and best of all folds up for easy storage when not needed. They are about $45, can buy them online, harbor freight, or a welding supply store.
(note the discoloration is due to the smoke... not the heat. arrange the top so the vent isn't blocked)