Heat that is coming from inside the car as well as outside of the car can cause the temperature gauge of the car to increase. When temperatures are extremely high within the inside of the car, the temperature gauge will greatly increase and the car could suffer from many different problems.
The most likely cause for a car's temperature to increase on the inside is a lack of coolant or coolant that is not working properly. In order for a vehicle to run, the engine needs to get extremely hot, but it is only able to get so hot before it overheats. The coolant that is added to a car along with a cooling system will ensure that the car does not get too hot, but remains at a steady heated temperature. If there is not enough coolant in the car or if something is preventing the coolant from working properly, the engine will not be able to cool down and will continuously overheat. Cars that are left to sit in the sun will generally not overheat as a direct result from external heat, but cars that have been sitting in cold temperatures for a long period of time may need to warm up before they are able to run optimally.