Why cities on opposite sides of a mountain receive different amounts of rain?

The orographic effect is known to be the change in the atmospheric conditions even if there are two cities that are just separated by mountains. The change may be caused by the mountain itself mainly because of the change in elevation.

There are instances when some cities on one side of the mountain will be experiencing wind but the other portion will be experiencing rain. There are times when this would cause clouds to form and also precipitation. Rain is sometimes needed in certain areas especially those who would need water for irrigation.