An area of warm air meets and area of cold air.
The warm air is forced over the cold air
Where the air meets the warm air is cooled and water vapour condenses.
Clouds form and precipitation occurs
the warm air mass slides up over the cold one, since it is lighter
as the warm air rises, it cools
as it cools, the water vapour in it condenses to mak tiny water droplets
the droplets form a gently sloping bank of clouds
droplets in the clouds join to form larger drops, which fall as rain
When warm and cold air meet, the warm air (being less dense than cold air) rises over the cold air and as it does so it cools, condenses into clouds and precipitates over a wide area.
Frontal rainfall is when the two different wind systems meet; that is cold and warm wind systems. Since the warm is less denser, it rises up over the cooler and in the process condensation takes place leading to small water droplets that falls as rainfall.