From AMS Glossary
That portion of the microclimate that is directly attributable to the small- scale variations of ground level.
A great many microclimatic variations are caused by small differences in elevation, steepness of slope, and direction of slope exposure and direction of curvature. These influence the total amount of heat absorbed from solar radiation, the heat loss by outgoing radiation, the small- scale (gravity) airflow in the boundary layer near the soil, and the runoff of precipitation. Such microclimatic differences can be observed even on as small a scale as the ridges and furrows of plowed fields or mounded flower beds.