This section of the book examines local or topoclimates. As noted in the Introduction (Chapter 1), these have a horizontal scale of hundreds of meters up to about 10 km. They are determined by the influence of topographic elements such as hill slopes, valley bottoms, and ridge tops as well as by water bodies and the built landscapes of towns and cities. They are best developed during calm, clear weather when radiational contrasts are maximized, but also during strong air flow conditions when dynamical effects are most apparent. Their vertical dimension is the planetary boundary layer that varies diurnally between about 500 m at night and 1500 m by day over level terrain.