What is a microclimate?
First off, there's a difference between a climate zone change and a microclimate. Gardeners living on or near the border of two climate zones deal with adjacent yet different climates--and they sometimes think of these as microclimates. For example, greater Seattle straddles Sunset Zones 4 and 5; tender plants that thrive in marine-influenced Zone 5 will freeze if moved into a Zone 4 neighborhood a few blocks away.
Used in its correct sense, though, "microclimate" refers to local conditions resulting in different climates within a single garden or slightly larger area in a neighborhood--all within one climate zone. For instance, south-facing portions of a garden are typically warmer than those facing north, and areas near walls and fences tend to be drier than those in the middle of a garden bed. Look for the following microclimates:
Unusually warm sections. A courtyard that receives full afternoon sun and is surrounded by high walls on all four sides will be a garden hot spot. Even a single cement or stucco wall may warm part of a garden if it receives several hours of afternoon sun each day--the wall absorbs heat during the day, then gradually releases it during the cooler hours. Avoid creating hot spots if your overall climate is already a warm one and you want to grow more than just heat-loving plants.
Cool pockets. Because cool air travels downward, the bottom of a slope is colder than the top--so the base of a hill isn't the place for heat-loving summer vegetables. A cold air pocket may also develop halfway down a slope if a hedge or wall impedes airflow. If you garden in a northern latitude or a mountainous region with many frosty days and nights each year, be particularly careful not to plant tender plants or vegetables in such cool pockets.
Windy areas. The placement of buildings on your property can generate a local wind tunnel. Even if the enhanced wind isn't especially severe, it can still damage the leaves of sensitive plants and dry out the soil, resulting in a need for extra water. To neutralize a wind tunnel, you can plant fast-growing trees or shrubs facing into the wind. Don't construct a solid wall; it will simply deflect the wind briefly, then drop it back into the garden a few feet away.