Why is it too hot in car radiator?

The most common reason for overheating of a car radiator is that there is not enough water in the radiator. Radiators can lose water in a variety of ways, and often need to be refilled. The other possibility is that the water is not being circulated properly. I should add that cars do heat up quite a bit in the sunlight on a hot day, which adds significantly to the chances of overheating the radiator.