Air holds more water vapour as its temperature rises. Relative humidity is how "full" the air is at its current temperature: 100% humidity is the point where the air can take no more water and condensation begins. The same amount of water vapour that gives 50% humidity at 20 °C reaches 100% when cooled to 5 °C; this is why cold room air saturates easily and water or ice collects on the evaporator surface.
Every food wants a different humidity. Leafy greens, carrots and cabbage want 95–100%; if humidity drops they wilt and lose weight. Onions, garlic and squash want 65–70%; in high humidity they sprout and rot. Meat wants 85–90%, most fruit 90–95%. The food storage conditions guide gives temperature and humidity values for 66 products.
In a cold room humidity is managed through the evaporator: if the temperature difference (TD) between evaporator and room is small, the surface removes less moisture and the room stays humid; if the TD is large, the air dries out. That is why a large-surface, low-TD evaporator is chosen for a fruit and vegetable room, and the opposite for rooms that should stay dry. An ultrasonic humidifier is an add-on used when humidity is still insufficient.