What is it, can it be defined, and is it important?

The use of the name mesoscale describing a class or a scale of motions has become common over the last decades. This gives the impression that the mesoscale is a well defined scale, however, when attempting to define it unambiguously, this turns out to be almost impossible. As a consequence mesoscale by itself is close to useless to describe something, unless more information is given. Still, motions on an intermediate scale exist, and often have a profound impact on local climate and on regional dispersion of air pollutants. For environmental modeling, motions on these intermediate scales must be properly resolved, since hardly any parameterizations exist. In fact, the very reason for this lack of parameterizations is the lack of a unique property that can be assigned to mesoscale. In modeling it is important to keep this ambiguity in mind, since the success may lie on designing simulations so that the important mesoscale circulations are resolved, and this may have to be done differently for different situations.