A hill or mountain top will be radiatively warmed by the Sun which in turn heats the air just above it.
Air at a similar altitude over an adjacent valley or plain does not get warmed so much because of the greater distance to the ground below it.
This creates a lower pressure region into which the air at the bottom of the slope flows, causing the wind.
It is common for the air rising from the tops of large mountains to reach a height where it cools adiabatically to below its dew point and forms cumulus clouds.
Conversely, katabatic winds are down-slope winds, frequently produced at night by the opposite effect, the air near to the ground losing heat to it faster than air at a similar altitude over adjacent low-lying land.