A container is a closed system with its own unique ”climatic system” inside. It differs from the warehouse in that the variation in temperature is much greater. It is not unusual to have containers where temperatures range from freezing to over 50° Celsius (122° Fahrenheit) during the course of a single voyage.
The central fact is that warm air can hold more moisture than cold air. That means that if warm air is cooled, it becomes more humid. And if it is cooled enough, some of the moisture may rain out and condense. That is exactly the same phenomenon that causes dew in the grass or fog on a cool day.
The Relative Humidity (RH) is a percentage measure of how much moisture the air holds as compared to the maximum amount of moisture air at that temperature can hold. That means that completely dry air has a RH of 0%. The RH can never be more than 100%, or any excess moisture will rain out. There is little risk of damage to any cargo if the RH is below 50%.
The Humidity Changes when the Temperature does.
In a container a fast temperature change of 5°to 10° Celsius is often enough to cause problems. Water will condense on the coolest available surface, which is often on the container ceiling or walls. From there it may drip down onto the cargo and cause damage, also known as “container rain”. At other times it condenses on the cargo, say on the inside of the pallet wrap, also known as “cargo sweat”-, which is usually even more damaging
Even without any condensation, elevated humidity over a period of time is sufficient to cause damage. Many metals will corrode or discolor at a rather modest level of humidity of 60% to 70%. At higher levels of humidity, 80%to 90%, moulds will grow, labels will peel and corrugated boxes will start to soften.
It is important to realize that the humidity of the air changes only as a result of the change in temperature. When air cools it becomes more humid, even though the moisture content in the air remains the same.
The humidity in a container will go up and down throughout the voyage, as a result of changing temperature. If the temperature changes rapidly enough there are sure to be moisture risks, even if the container may be fairly dry.
In a container, moisture evaporates into the air during periods when the container is warm. The warm dry air can accept a lot of moisture. Or warm moisture containing air enters from the outside through “Container Breathing”. When the container cools down, that air becomes very humid. This is when the risk for moisture damage increases.
But the temperature does not only have to change over time to make a difference. It is also risky when different parts of a container are at different temperatures. When warm air moves into a colder part it becomes humid and perhaps even condenses moisture. Tons of moisture can be redistributed within a container during a voyage through such processes. Strange patterns of damage may arise, such as mold only in certain parts of the cargo