Moisture damage is caused by temperature differences

The ultimate cause of all moisture damage in cargo shipments is a difference in temperature. It may be a change in temperature, eg when a container cools down after a hot day in the sun. Or it may be a difference in temperature between different parts of a cargo, eg when a cargo loaded at a cool temperature and then shipped across the equator.

There is a maximum amount of moisture that air at a particular temperature can hold. Any excess moisture beyond this amount will be condensed as a fog of small droplets floating in the air or as dew on a nearby cold surface. 
Warm air can hold more moisture than cold air. When warm air cool down it becomes more humid, even though there is no change in the amount of moisture the air contains. If the temperature drops enough condensation will occur.

How humid the air is depends on its Relative Humidity (RH). This is quite simply the percentage of moisture held in the air out of the maximum that air of that temperature could hold. Entirely dry air has RH 0 . Maximally humid air has an RH of 100.
There is rarely any moisture damage if RH is less than 60-70%.

As a rough rule of thumb the amount of moisture the air can hold doubles for every 10 C over “normal” temperatures. If for instance air of 20 C and RH 50% is cooled to 10 C, the RH will reach 100%. Any further cooling would cause immediate condensation. If the air was then heated, the RH would drop below 100% and any condensation that had happened would over time reevaporate back into the air.