Using a clean, empty container, make a 50/50 mix of water and anti-freeze (any auto parts store will carry anti-freeze). Then, when the engine is cold (say, in the morning), remove the radiator cap and pour just enough fluid to sufficiently cover the little "fins" inside the radiator. Replace the cap when done. How did the coolant level get to be low? Has the radiator boiled over once or twice in the past? This would account for it. Alternatively, there may be a leak somewhere in the cooling system. Have you ever noticed a puddle of coolant underneath this vehicle? If you have not, maybe it's an internal leak, like that which results from a blown head gasket. If this is the case, white smoke will come out of the exhaust as you're driving. This is caused by coolant leaking into the engine's combustion chambers and burning off as steam. Whatever the case, please have this vehicle checked out by a mechanic