Hygrometer
Two hours in a car park in February. You bring the case in, put it down, and reach for the catches because you want to see that it is fine.
Do not open it. That is the whole of the advice, and the reason is worth understanding because it runs against every instinct.
What is about to happen
The instrument is at car-park temperature — say 35°F. The room you have carried it into is at 68°F with a dew point of maybe 50°F.
Anything below 50°F in that room collects water. Your guitar is fifteen degrees under it.
Open the case and warm, humid air floods onto cold wood, colder metal and a cold finish. Water condenses immediately — on the strings and frets first, because metal is the most conductive and therefore the coldest thing there, then across the top, then anywhere air can reach inside the body.
Why the closed case is the answer
The case is an insulator, and that is exactly what you want.
Left shut, the instrument warms slowly by conduction through the case, and the small volume of air sealed in with it warms at the same rate. Nothing inside ever sits far below the dew point of the air touching it, because the air touching it is the cold air that came in with it.
By the time you open it an hour later, everything is close to room temperature and there is nothing to condense.
Opening it immediately is the only way to get the outcome you were worried about.
How long
| Situation | Wait |
|---|---|
| Half an hour in a mild car | 30 minutes |
| A couple of hours, cold day | 1–2 hours |
| Overnight in a freezing car | 3+ hours, or leave it until morning |
| Any hard freeze | As long as you can bear |
If you must check, feel the outside of the case first. Cold case, cold instrument.
The other damage
Condensation is not the only problem, and possibly not the largest.
Rapid temperature change makes wood, finish and glue expand at different rates. Nitrocellulose lacquer in particular can craze — a network of fine cracks — when a cold instrument is warmed quickly. Seams and glue joints are stressed by the same differential.
Slow is the answer to that as well, which is convenient: the closed case solves both problems with the same action.
The humidity problem underneath
Winter transport also means moving between air of very different water content. Cold outdoor air carries almost nothing; heated indoor air is the same air with its percentage collapsed, often down to 20% or less.
Instruments generally want around 45–55%, and prolonged exposure to heated winter air dries wood out — leading to sharp fret ends, dropping action, and in bad cases cracks.
That is a case-interior measurement and needs a hygrometer in the case. This site reads outdoor air and cannot tell you what the inside of a case is doing — but it can tell you how dry the air arriving in your house is about to be.
The general rule
Any cold object entering warm humid air will collect water until it warms past the dew point. Same mechanism for a laptop from the boot, a camera lens, and a pair of glasses.
The fix is always the same: let it warm slowly, in something that keeps the humid air off it.