With isothermal humidification, water is heated to boiling point to produce steam. The steam is generated either electrically or using steam boilers powered by combustion and then introduced directly into the room through blowers or into an air handling unit (AHU).
Adiabatic humidification is when water is atomised through friction with the air. Water is reduced to tiny particles (aerosols) which go from the solid state to the gaseous state using ambient heat. Evaporation speed is inversely proportional to the diameter of the droplet produced and directly proportional to the speed it is introduced into the air.