Hygrometer
The laptop spent the night in the boot. It is minus two outside and the house is warm. You bring the bag in, and the natural thing is to open it up and check everything is fine.
That check is the risk. Not the cold night — the ninety seconds after you open the lid.
The comparison, again
The device is at outdoor temperature, say 28°F. The room is at 68°F with a dew point of 50°F.
Everything below 50°F in that room collects water. The laptop is more than twenty degrees under it, and it will keep collecting until it warms past that threshold.
Not just the case. Air reaches inside through vents and seams, and the coldest parts — heatsinks, metal shielding, the board itself — condense first.
Why powering it on is the specific mistake
Water on electronics is survivable surprisingly often, as long as there is no power.
Water on electronics with current flowing is a different matter. It provides paths between traces that should be isolated, which causes shorts, and it drives electrochemical corrosion that continues long after the visible moisture has gone.
So the sequence that kills a device is: cold in, opened immediately, powered on to check, moisture present, short. The sequence that saves it is: cold in, left alone, warmed, then powered on.
How long to wait
| Device and exposure | Wait |
|---|---|
| Phone, cold pocket | 15–30 minutes |
| Laptop, a few hours in a car | 1–2 hours |
| Laptop, overnight in a freeze | 3+ hours |
| Desktop or large gear from storage | Half a day |
The outside of a case reaching room temperature does not mean the inside has. Metal chassis and heatsinks lag, and they are the parts that matter.
Leave it in the bag
The same trick that works for camera gear and instruments: keep it in its bag or case while it warms.
The bag insulates, so the device warms slowly, and it keeps warm humid room air away from cold surfaces. Condensation forms on the outside of the bag instead of on the electronics.
A sealed bag is better still, because the only air in contact with the device is the cold, dry air that came in with it.
The reverse case, in summer
The same thing happens in the opposite season and catches people out because it feels wrong.
A device from an air-conditioned room, carried into humid summer air, is the cold object again. Outdoor dew points above 70°F mean anything from a cool building will fog immediately — laptops, cameras, phone cameras, and the lens of anything with glass in it.
If it already happened
Power it down straight away and do not restart it to see whether it works. Every attempt to check runs current through a wet board.
Leave it unpowered, opened up if you can do so safely, somewhere dry and warm, for at least a day — longer if you saw actual water. The instinct to test it immediately is what converts a scare into a repair.