Refer to figure.
Anabatic winds are winds that flow up sloping terrain during the day, caused by the heating of mountain slopes. As the sun rises, slopes facing the sun quickly warm up. The air in contact with these slopes heats up through conduction. This warm, less dense air then rises and flows up the sides of the slope.

Katabatic winds are winds that flow down sloping terrain. At night, the surfaces of mountain slopes cool down, causing the air in contact with them to become colder and denser. This cold, dense air sinks towards the valley floor, accumulating and pooling in the center of the valley. If the air temperature in the valley drops below the dew point, valley fog forms, which is a common occurrence in autumn in valleys with rivers and streams.