A term sometimes used to describe weather blisters. These are small surface blisters, which can be seen in large numbers over the entire roof area, more predominant during warm weather where roofs are exposed directly to the sun, and which are a result of natural weathering of the surface bitumen. Volatiles and water vapor in the bitumen tend to be driven off by heat, and when the gases are trapped they form small blisters. This type of blistering usually does not cause any failure during the normal life of the roof. Also sometimes called pimpling, pin blistering and bitumen bubbling.