If the car is overheating, there is something seriously wrong. You could be out of coolant, oil, or the head by be blowing. In all cases you should stop immediately (pull off the road safely) and allow the car to cool before investigating the cause. If it is water or oil then fill the car up before restarting. If it is the head then you need a tow to a garage. You MUST NOT continue driving as an over hot engine will cease and you will need a new engine!
Each car overheats at a different temperature you need to look up and find out on the internet
check your fluids it is probably low on antifreeze and if it still does it change your thermostat that should fix the problem
The cooling system is probably getting old ... but to leave a car idling for 15 min is not a good practice for the car or for your gas milage.
It makes the car go slower, uses more gas, and overheats the brakes. I don't recommend it.
YES
It can cause the head gaskets to blow. This must be repaired before driving again. If really overheated, you can blow your engine. This would need replacing.
it sounds like you have a leaking heater core
Shorts out and overheats or has too much resistance for some other reason and overheats.
Car overheats, your heat in your car stops working
Poor coolant circulation in the cooling system.
it can..rain has NOTHING to do with your radiator or the thermostat
Your question is not clear...