The bottom of most containers is warmed up by the heat produced by the lamps of the lower level. As a consequence water evaporates and condenses on the coldest spot: the lid.

The RH in the container can be controlled by bottom cooling, as the water vapour will condens on the medium surface. The higher the difference between medium temperature and the temperature just below the lid, the lower the relative humidity.

e.g. 20 °C medium temperature, 23°C temperature below the lid --> 85 % RH

Bottom cooling can help to overcome hyperhydricity.

Water droplets indicate that RH in the container is 100%