Container rain: when it rains inside the box
A sealed steel box, tens of thousands of litres of humid air, day-night swings of 20 degrees and more: the physics of container rain, explained.
Where the water comes from
Every container travels with moisture on board — even when everything looks bone dry at loading. It sits in the air enclosed at the port of origin, and above all in hygroscopic cargo and packaging: wood, paper, cartons, textiles and agricultural goods continuously release water vapour into the container air.
Warm air holds far more water vapour than cold air. As long as temperatures are stable, nothing happens. But when the container cools below the dew point — at night, in cooler waters, at the destination port — the vapour condenses on the coldest available surface: the steel roof, the walls, or the cargo itself.
Container sweat and cargo sweat
Practitioners distinguish two damage patterns. Container sweat: droplets form on the roof and walls and drip onto the cargo — literal rain inside the box, typical for voyages from warm to cold climates. Cargo sweat: condensation forms directly on cold cargo surfaces when warm, humid air reaches them — typical for voyages from cold to warm, or when doors are opened in tropical ports.
What it does to your goods
Mould and mildew on foodstuffs and agricultural products, corrosion on machinery and steel, collapsed cartons, delaminated furniture, detached labels. The insidious part: the damage usually becomes visible only at destination — weeks after the temperature swing that caused it, and long after the risk passed to the buyer.
Rule of thumb: the dew point
At 30 °C and 80 % relative humidity the dew point sits around 26 °C — a night-time cooling of just a few degrees is enough to start condensation on the container roof.
- shipments suffers condensation damage in transit
- 1 in 5
- of all cargo claims are caused by water damage and contamination
- 11 %
- estimated annual cost of container rain across global supply chains
- $6–8 bn
FAQ
Frequently asked questions about container rain
What exactly is container rain?
Container rain is condensation that forms inside a shipping container when the enclosed humid air cools below its dew point. Water droplets form on the steel roof and walls and drip onto the cargo — literally raining inside the box.
Which cargo is particularly at risk?
Hygroscopic goods and packaging that carry moisture themselves: wood, paper, cartons, textiles, furniture and agricultural products such as coffee, cocoa or rice. But metal goods also suffer — through corrosion caused by condensation.
When does condensation occur during the voyage?
Whenever the container cools below the dew point of the enclosed air: at night, in cooler waters, or at the destination port. At 30 °C and 80 % relative humidity the dew point is around 26 °C — a few degrees of cooling are enough.