Progressive coalescence or fusion of smaller ones to yield larger and heavier particles which drift down under the influence of gravity and may get deposited on various objects or settle back in the soil or water underneath is the normal way of clearance of these particles from the atmosphere Humidity plays a very important part in settlement of fine particles.
Water vapours condensed on aerosols and make them heavier while they coalesce easily with each other on coming closet Meteorological conditions under which there is no dew, rain or snow formation, these particles settle down simply under the influence of gravity and the process is called Dry precipitation.
Under conditions where enough moisture is available for the rains, dew or snow to occur the clearance of particulate material is very quick. The entire bulk of aerosols may be brought down will dew, rains or snow. Such a process of clearance of aerosols from the atmosphere is known as Wet precipitation.