I have a sales job, and am on the road most days. I often find myself with free time between appointments and would like to take advantage of that free time by practicing guitar. I have a 3/4 size acoustic that I think would be easy enough to play in the car. My question is, what effects will leaving the guitar in the hot (or cold) car have on it?
My advice is to treat your guitar like a human passenger. If you wouldn't leave a person in your car with the windows rolled up on a particular day, then don't leave your guitar there either. Pick it up and carry it wherever you go.
Avoid leaving your guitar in your car in hot weather. Don't expose your guitar to prolonged high temperature or changes in humidity that your physical body can't stand. Remember that in many climates in the summer if you leave your car parked outside with the windows rolled up, the car, with no air circulation, becomes like an oven. The air in the car will reach temperatures high enough to kill people and animals--higher than 125 degrees Farenheit (52 degrees Celsius) when it is 100 degrees Farenheit outside in direct sunlight. Needless to say this will severely fatigue the wood in your guitar but will also cause expansion and contraction of the metal in the frets and the strings. It will also weaken the glue holding the wooden plates and braces together, causing the guitar to slowly fall apart.
If you want your guitar to play in tune with proper string height (action) you will also find yourself needing frequent adjustments to the truss rod as the neck and the metal truss rod bows or warps positively or negatively every time it is exposed to a large swing in temperature.
When I took my classical guitar anywhere in a car in the summer time in Atlanta, Georgia, I frequently had to deal with nylon strings that snapped from the fatigue of all the expanding and contracting due to the changes in heat.
I do not live in a cold climate, but I have read that in the northern US and in Canada, in the winter time, guitars suffer cumulative damage (particularly with cracked and crazed finishes) when they are taken out of a heated home, carried to a car in freezing weather to be transported, and then brought back into a heated home and taken right out of the case and played. I'm told you need to permit the cold guitar to remain unplayed in the unopened case until it slowly warms up and reaches room temperature. You should also de-tune the strings of the guitar before you take it out in cold weather because the metal strings contract in the cold.
With repeated exposure to extreme heat or cold, the tension on the neck and body from the strings will increase and decrease since metal expands and contracts with temperature changes much more than wood. The bridge, top, body and neck are already under considerable controlled tension from the strings; wide swings in temperature over time will weaken everything in the wood and the glue. In extreme cases with prolonged exposure, the bridge plate could come unglued or simply rip loose from the top, or the headstock could snap, or the neck block, where the neck is glued into the body, could pull away, changing the angle of the neck and rendering the guitar unplayable.
I occasionally have to do this, with electrics and acoustics. You have a few problems which can occur:
In cold weather, condensation is your biggest enemy - you will get moisture on metal elements and areas of dense wood. For an electric guitar, this will eventually harm your electrics, but on both kinds it will rust your strings faster.
In a car, temperatures can vary over a wide range - this can warp your guitar, and split the wood, as different woods expand and contract at different rates, and it can cause glues to fail.
Tuning will suffer - every temperature change will change tensions etc., but this can be fine.
An acoustic will suffer more than an electric, in almost every case. As @luser said in his comment, a cheap electric will probably be a better bet.
Keeping it in a case with proper humidification will help somewhat but it will be eventually wear out. Expect warped necks, split tops and lots of tuning. Any extreme temperatures and rapid temperature changes are murder on the wood. I recommend getting a really cheap guitar with heavy and solid construction and accept that it will wear out over a year or two. Avoid frost and parking in direct sunlight. If it's hot don't leave it in the trunk but on the back seat floor out of the sun with one window a little open.
As Luser Droog commented, the wood may experience some problems. Cars tend to experience quite vast fluctuations in temperature, and humidity, meaning that parts, particularly necks and bodies, will expand or contract, playing havoc with tuning.
Moisture and humidity may also cause problems, warping wooden parts or causing splits, as well as accelerating the rusting of strings.
As well as this, be aware that leaving a guitar out of a case in a car exposes it to sunlight, causing woods and varnishes to fade over time if prolonged.
I would advice against placing a wood guitar you like in your car. Rainsong builds graphite guitars that can handle a beating and temperature/humidity changes. http://rainsong.com/
I think all the risks stated above are correct. I have personally seen a lacquer finish cracked by cold temperatures in a car, and a soundboard cracked by dryness in a home.
But you might still want to take those risks depending on how valuable the guitar is, because the benefit of playing might outweigh the cost of a travel guitar or an inexpensive beater. I have a Martin Backpacker that I've taken camping in cold and wet weather, then played next to a hot campfire. I also loaned it to a friend who dropped it and cracked the soundboard. It still serves its purpose. If I were in your situation I would just take the basic precautions of leaving it in the trunk to avoid the most extreme heat and taking it inside overnight during any extreme weather.
The more conservative advice on this page definitely applies to any guitar of significant value. You should also consider the risk of theft, as a musical instrument left in view is a good invitation for a break-in.
Treat it like your child , take it with you and love it as it were your own cause it is ! So it sits next to you at dinner, so what! A few minutes in the car is one thing but an hour or two is another! Would you leave your child in the car for an hour or two?definately get a humidefier!there like 10 bucks online! Oaisis makes a good one!
Some tricks that may help alleviate risks when leaving an instrument in your vehicle are to park in the shade on a warm day, and cover the instrument with a blanket in an attempt to moderate the speed of the temperature changes. A 3/4 size guitar wrapped in a blanket inside a full size guitar case has a much better chance of avoiding serious damage from heat and cold, but there are also limits to the protection provided and prolonged exposure to temperature extremes is certainly risky.