Topography: Slope

At night, air is chilled by direct contact with a land surface which is rapidly losing heat by radiation. The chilled air, being denser, flows down slopes to the flat land or valleys below, and is replaced by warmer air from above the land surface. The turbulent surface air over slopes at moderate elevations is therefore usually warmer at night, and in the early morning, than that settled over the adjacent flats and valley floors. This band on a hill slope is known as its ‘thermal zone’, and especially in cool climates is valued for viticulture because of its enhanced ripening potential and length of frost-free period. The steeper the slope, the more pronounced is its thermal zone. See also hillside vineyards.