Carved double basses are more sensitive to humidity than plys... And they're a little bit more fragile regarding bumps and falls. From your temperature and humidity ranges I can make these observations:
Just a simple rule of thumb, usually if you are comfortable with the weather environment, your double bass is also going to be comfortable. If you're too hot or too cold or too dry or too moist, it'd probably be best to put your double bass in it's bag or inside a temperature and humidity moderated building.
If you see guitarists or mandolin players putting their valuable instruments away or staying out of the weather, you might want to consider doing that too. Banjos are generally bolted (not glued) together so they aren't as much of a concern regarding temperature or humidity, although no one wants to play their valuable instruments in the rain.
Never -- NEVER -- store your double bass in a car. On a moderately warm day (80°F and up) in the sun the inside of your car can reach 130°F within an hour or two even with the windows cracked an inch or two. Glues that hold your double bass together start to release at that point.
Also, avoid playing your double bass in direct sunlight on a hot day. Dark finishes in particular get very hot, hot enough to release glued seams, joints and/or to do other damage to the finish. There are reasons that people use awnings and easy-ups for jams at bluegrass festivals. And avoid setting your double bass down in direct sunlight, even in the bag.
Along those lines, also be very careful how you store your double bass when you are not holding it in your hands. Instrument stands, especially for a double bass, are not reliable if there are any winds, or if the ground is not solid. Your double bass' neck is its Achilles heel, and severe falls and bumps can easily damage the neck or the area where it is attached to the body.
Also, double basses are always big and in the way, even when you are playing it, so watch out for other people around it, especially if various forms of alcohol or other mental relaxant methods are being used. It is very easy for someone who isn't fully in control to bump your double bass and damage it, especially if they also are carrying an instrument.
Double basses can also be difficult to schlep long distances between jams. You might want to get some sort of wheel device, but be aware that some of the most convenient wheel devices for double bass do not work well in high grass, loose dirt, sand or gravel. Some of them also can cause damage to your double bass if not sized, cushioned or used properly.
A nicely cushioned double bass bag can protect your double bass very well while also providing some insulation from weather, and also ease your mind about its safety.
Oh, also, if you are traveling in an RV or trailer or are going to be jamming in one, be aware that most cabin doors on RVs are barely wide enough to fit a double bass through them. Combined with the steps to get up to the door, they are a special danger for double basses. Be very careful about the whole instrument, but especially the bridge and neck, while carrying your double bass through these doors.
Bluegrass festivals are great places for double basses, but they can also have potential for damage. I never hesitate to bring mine, but I watch it and people around it, like a hawk.
Best of luck with all this!