The synoptic weather systems described in Chapter 5 provide the backdrop for the local and regional variation of weather and climate that is a marked feature of the New Zealand environment. The nature of surface topography creates real problems for weather and climate prediction, because of the interaction of synoptic circulation systems with smaller scale dynamic and thermal effects. Surface-atmosphere interaction therefore provides the observed regionality of weather and climate. The term mesoscale is often used to describe atmospheric phenomena of a particular time and space scale lying between the synoptic and microscales (Figure 6.1), and is assumed here to include local and regional effects frequently observed around New Zealand. This chapter therefore examines atmospheric phenomena that have dimensions of the order of a few hundred metres to around 200km, which generally result from processes that operate over time periods of an hour to a day or so. However, climate features that are observed at this scale are the result of the integration of such mesoscale meteorological processes over much longer periods of time. Local and regional atmospheric phenomena are of particular significance to human activity, as their impact can be serious.