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Moisture Processes in the Container 

The inside of a container is closed off from the outside world. This creates a “closed system” inside the container, so the temperature and humidity inside the container will be different than in the warehouse. The temperatures inside can vary greatly during a voyage, and can go from below freezing to over 60 degrees Celsius. When the air is warm, it can hold more moisture than when it is cold. When warm air of a high moisture content begins to cool, it becomes humid, and when it cools enough, the moisture it was holding is released as condensation.

 This can occur even with a small decrease in temperature of 5-10 degrees Celsius. When condensation occurs, the water is released onto the coldest surface available – in a container, this is usually the walls or ceiling. Once it has condensed onto the ceiling, it drips down as “container rain”, and onto the cargo. Condensation can also occur inside the cargo itself, which is referred to as “cargo sweat”. 

These are often very damaging to the cargo. In addition to this, humidity can also cause issues. At around 60% relative humidity, metals begin to corrode and become discoloured, and at higher levels of 80% or above, mould starts to grow. Relative humidity refers to how much moisture is in the surrounding air as a percentage of how much the air can hold in total. As such, it can never be more than 100%. Beyond this, the moisture in the air is released as condensation. Typically, a relative humidity of below 50% is safe.