As an air parcel rises it cools, the water vapour condensing when it reaches the dew point. Sunlight is scattered from the resultant droplets so that the emergent clouds can be seen by human eyes by their white, diffuse light. The best example of this is when air parcels rise from the warm ground in summer; those with only a small buoyancy compared to the surrounding atmosphere produce ‘fine weather’ clouds, those with stronger buoyancy climb higher and form thick clouds and heavy rainfall.
In low pressure systems with their cyclonic (anti-clockwise, in the northern hemisphere) air flow, warm, humid, subtropical air is transported to the north-east. Due to its lower density it moves up over the colder northern air, cooling as it rises. This is how clouds form on the warm fronts of such pressure systems in the mid-latitudes. Conversely, on the western side of the low the cold air moves southward, pushing under the warmer subtropical air and raising it up above its condensation level to form clouds on the cold front. Clouds can also be formed from the cooling and condensation that occurs when air flows over physical obstructions like mountain ranges.