Monsoon season on a domestic lane: humidity outside, condensation inside

At 06:00 on the second week of the monsoon, the yard reads 28 °C and 94% relative humidity. The pack-out room next to it is air-conditioned to 20 °C and the doors stay shut, so on paper nothing is wrong. Then a pallet of 400 gel packs comes in from the blast freezer at minus 18, and inside ninety seconds every pack wears a film of water. A fitter wipes the first row with a rag, the rag is soaked, and the row behind it is already beaded again.

That room was not doing anything unusual. It was doing exactly what physics says it should, and the damage showed up four days later at a distribution centre 900 km away, where eleven of twenty master cartons were refused for wet packaging. The product inside was fine. The cartons were not, and a buyer who cannot stack a wet carton does not care that the gel held its temperature.

Dew point, not humidity, tells you what will condense

Relative humidity is a ratio and it is a poor guide on its own. What matters is the dew point, the temperature at which the air in the room has to be cooled before it starts giving water up. At 28 °C and 94%, the dew point is roughly 27 °C. Anything in that room below 27 °C is a condensing surface, and it will stay one until the moisture in the air is used up or removed.

A pack at minus 18 is forty-five degrees below the dew point. You cannot wipe fast enough. The water does not arrive in drops from a leak; it is manufactured on the surface, continuously, out of the air you are standing in. This is why the rag approach fails and why the problem gets worse the harder the room’s air conditioning works. A colder room holds less absolute moisture, but it also makes every surface colder than the dew point for longer.

Where the water actually ends up

Almost none of it lands on the payload. It lands on the outside of the pack, then transfers by contact to whatever is touching the pack: the carton wall, the divider, the paper void fill, the pallet slip sheet. Corrugated board is a sponge with opinions. Suppliers’ own curves show stacking strength falling steeply as moisture content climbs, and a carton that gains five or six points of moisture can lose a large share of its compression strength without looking obviously damaged.

The second destination is the gap between packs. Water sits in the flutes of the divider, runs to the lowest corner of the carton, and softens the base. When the pallet is stretch-wrapped and stacked two high in a trailer, the bottom carton carries the load with a base that has been quietly weakened for three days. The failure is rarely dramatic in the pack-out room. It is a sagging pallet at the far end of the lane.

Three changes that cost less than one rejected pallet

The first is an equalising step. Bring frozen packs into a closed anteroom at 12 to 15 °C, still bagged, and let them sit for twenty to thirty minutes before they meet the packing bench. Surface temperature comes up, the dew point gap narrows, and the amount of water that forms drops sharply. You are not warming the core; you are taking the edge off the skin.

The second is bagging at the right moment. A plain polyethylene sleeve over each pack set, sealed before the pack leaves the freezer, keeps the water on the outside of the bag where it can be shaken off, instead of on the film and the carton. The third is timing the carton. Close master cartons in the packing room rather than leaving them open on the dock, and open them again only at the vehicle, not at the receiving dock. Every minute a carton spends open in humid air is a minute of free water.

What to check when the lane has already run

Ask the receiving site for photographs of the pallet before it was unwrapped, not after. Look at the bottom carton of each stack, at the flap edges and the base corners, because that is where water accumulates first. Weigh a sample of twenty cartons against a dry reference from the same batch; a gain of 250 to 400 grams per carton is water, not product variation. Run a finger along the inside of a carton wall. If it feels cool and slightly slick, the pallet was packed in air that was above the dew point of the room.

Then measure once, cheaply. A £60 thermo-hygrometer on the packing bench, logged every fifteen minutes for a week, will tell you which hours of your shift are dangerous. In most monsoon plants the answer is the early morning and the moment the roller door opens for a trailer, and both are manageable once you can see them.

If you are rebuilding a wet-season lane this year, start with the placement and bagging rules in our logistics ice pack user guide, then log your own dew point for seven days before you change a single fill weight. The numbers from your own bench will beat any general recipe, including ours.