|Packaging and dunnage may also be sources of relatively large quantities of sweat. If a container carrying 20 metric tons of general cargo contains 2 metric tons of wooden dunnage and case lumber and if this has a water content of 30% instead of the usual 15%, the packaging alone introduces 0.6 metric tons = 600 kg of water into the container. Such large quantities of water occur in the case lumber if the lumber is too green or the cases were stored in the open air and soaked with rain.
These 600 kg of water evaporate (desorption) until the container air is saturated with water vapor (100% relative humidity). During the night, the container cools down and the air can no longer "hold" the water vapor, condensation therefore appearing on the container ceiling and wall (container sweat). The quantities of condensation may be so great that it actually "rains". This process may be repeated day after day. Its intensity depends, among other things, on temperature differences, which may increase when passing from one climatic zone to another.