In all but the flattest terrain, differences in relief produce recurrent and consistent patterns of cloudiness during turbulent or convective weather. The basic patterns consist of fixed patches or lines of low clouds that persist in an otherwise clear sky. But their outlines may even appear in the higher layers of cloud that cross the region during the passage of larger weather systems. Consequently, the areas beneath them experience predictably higher frequencies of cloudiness and rainfall than the terrain outside their borders. Thus their outlines can be used to draw boundaries that separate areas with different climates.

The information above can be used to develop a system of direct observations of changes in local weather that can be applied in several ways during studies of natural populations. Visual methods of observing the weather are always simpler than methods that involve instruments. Where population phenomena are concerned, they also may be more meaningful; and they leave more time free for biological observations. Instruments are essential, however, in any investigation of the properties or requirements of individuals; e.g., in physiological or behavioral studies.