Buildings also offer protection from wind in many places. Urban areas may be a full Hardiness Zone warmer than rural areas just a few miles away. These warming effects carry over into summer, as well. Urban microclimates can trap heat, creating a scorching environment that can damage plants. Topography has a profound effect on microclimates.
Cold air is heavier than warm air.
On winter nights, some valleys may be 10 degrees or more colder than neighboring slopes. These valleys may also be more prone to frost. Hilltops may not suffer as much from frost or cold temperatures on nights with radiational cooling. But if they are exposed, winter winds can often wreak havoc. Winds dry out plants, and are particularly hard on evergreens, which cannot replace moisture lost through their needles or leaves when the ground is frozen.