Wood is still living.  You'd never leave  your baby or pet animal in the car all day....so the same goes for your dulcimers.  I suggest  you get an exception to bring it into your work place.  they could use a few tunes at lunch, I'd say!! Nothing wrong with asking.   

When hanging out with my dulcimer buddies in Tucson, Arizona, we will often go out to lunch after a rehearsal and we take our dulcimers into the restaurant with us.  There's always some space where they can be safely set and it keeps them out of those extreme temperatures.

leaving any stringed wooden instrument in a parked car for extended times can be disastrous.  as has been mentioned its the glue

that can let go and you have a case full of kindling wood.  i traveled for years with numerous dulcimers in a van over the mountains....through

the desert and every sort of weather.  when i parked the van for more than an hour or so i took the instruments out of the van and put them

in my travel trailer that was insulated and could be kept warm ..... or cool.  my advise is "let the dulcimer live with you" and never in a parked car,

basement or attic

Ken, that's a good suggestion on the padded/insulated case.  

One of my instruments has a Sassafras top.  I took it with me to Cape Cod last year where it got exposed to a little more humidity than normal, which must have revived the oil in the wood somewhat. The Sassafras scent would hit you in the face when you opened the case.  At home, it's a pretty constant 32-35% humidity when the heat or AC is running, so the oils stabilize and don't give off so much scent.

A padded, insulated case, like a Bag Lady dulcimer bag, will definitely help with environmental changes.  These days I keep two dulcimers in my Bag Lady double-bag aboard my liveaboard sailboat, all year 'round, with temps that range from 100+ to 30 and humidity that ranges from under 30% to 100%.  They keep their tuning very well, seldom needing adjusting more than a few cents.  When I lived out West in Colorado, Arizona and Utah, the same bag helped them adjust from home temp/humidity to outdoor temps and humidities year round -- well below freezing to well above 100F.  I've never kept one in a PARKED car for 6 or 8 hours, in any temp, but have had them inside while I've driven for those kinds of times.  Today, in fact I drove diagonally across Florida for 6 hours with temps in the hi 80s.  Yes the AC was on, but the sun was shining bright on the bag in the back end of the SUV and it was plenty warm back there.  No problems.

Thanks for the advice everyone.  All excellent suggestions.  It will be hot during the day.  I hadn't thought about the glue.  I will have to come up with a plan 'b'.  Now I'm going to check out that dulcimer care and maintenance group. 

What I'd always heard was, "If you wouldn't be able to sit there for an hour (or two--or eight--ACK!), then don't expect your dulcimer to "be okay with it".

Kandee, changes in humidity rather than temperature are the challenge.  Wood takes on and gives off moisture continuously and will naturally mimic the ambient air conditions.  The wood will give off and take on moisture until it is in balance with the moisture in the air.  As wood takes on water it expands and as gives off water it contracts.  Dry enough conditions, for a long enough period of time, can eventually cause cracking.  The more stable you can keep the humidity in the instrument, the better.

Wood doesn't expand and contract much due to air temperature.  

The good news for you is that the process is usually slow, so you likely won't hurt your dulcimer by leaving it in your trunk for a day.  I wouldn't leave in the cabin of the car where temps can get really high (well above 120).  That could cause issues with the glue joints if it was made with a traditional hide glue or something like that.  If it was made with a contemporary wood glue (which it most likely was) you probably won't have an issue.  

Kandee,

First, congratulations on getting your dream dulcimer.  My dream dulcimer is always the next one!

Second, you might want to join the Dulcimer Care and Maintenance group here.  I'm sure you will get better advice than I can offer.

Third, I am no expert, and I don't even play one on TV, but my sense is that extreme temperatures, extremes in humidity, and quick changes in both of those are all potentially harmful to any wooden instrument.  I would not leave an instrument in your car all day in the Indiana summer unless you can ensure that it not get too hot.

Now that I've spent all this money on my dream dulcimer I want it to last a very long time.  What are the guidelines for weather conditions?  As in, can I put my dulcimer in it's bag and leave it locked in my vehicle all day to be played after work somewhere?  I work in a government building with metal detectors and taking it inside isn't an option.  I believe I read somewhere that cold (winter) is hard on wood instruments.  I live in northern Indiana and the winters are rough.  What about heat (summer)?   Anyone have a basic high/lo temp gauge that they use?  Any advice would be appreciated.