Four regional European climatic types can be loosely distinguished. However, each is characterized by much local, topographically related variation. The climate in mountainous regions, for instance, is partly determined by altitude, and a variety of climatic types may be “stacked” vertically upon a mountain. Further, the great cities of Europe—because of the scale and grouping of their buildings, their industrial activities, and the layout of their roads—create distinct local climates, including urban “heat islands” (city centres that are warmer than outlying areas) and pollution problems.