In contrast is the cirrus uncinus, a type of ice cloud that resembles a comma. These clouds appear diffuse near their bases, where the number of ice crystals decreases as they fall and evaporate into the dry air below. The cloudy air mixes with the clear air, giving the diffuse appearance. Thus, how sharp a cloud boundary appears is a function of how much the cloud air mixes with the clear air environment. If the environmental air has a high relative humidity, just a little mixing will cause the air to reach saturation, producing a relatively diffuse boundary.
Proximity also has a big effect on how well defined a cloud appears.
Up close the edges of the line may look wavy and interrupted. Step away, and the line looks sharp and crisp. The next time you have an opportunity to fly, observe the clouds as you pass through them. You will notice that the edges of a cloud are often diffuse. Just try to stay out of the cumulonimbus cloud, whose distinctive boundaries could give your airplane a sharp bounce.