Common methods of humidification in refrigeration systems include steam injection, atomization, evaporative cooling, and ultrasonic humidification. Steam injection involves injecting steam into the air stream to increase humidity. Atomization involves breaking down water into tiny droplets that evaporate quickly, increasing humidity. Evaporative cooling involves passing air over a wet surface, allowing it to absorb moisture. Ultrasonic humidification uses high-frequency sound waves to create a fine mist that increases humidity. The choice of method depends on the specific application, humidity requirements, and system design.