[Frost]

Guys, you are all not entirely correct. Let's look at the facts that I DO know:

Fact #1:
The boiling point of a liquid will change based on the pressure it is exposed to. Eg: Water at standard atmospheric pressure boils at 100 deg C or 212 deg F. If the water was taken up the top of say, Mount Everest, it would boil at a significantly lower temperature.

Fact #2:
Liquids DO expand and contract due to temperature. I work in this industry and make a good living out of it so I know this as fact. You'd be surprised at how many 'engineers' conveniently forget thermal expansion for fluids happen. Liquids can also compress (contrary to popular belief that they don't).

Okay, now extrapolating to a car (key word: extrapolate - these are not 'confirmed' facts by me), the cooling loop that you can visually see on the car engine bay is the overflow / reservoir which is usually there to allow for thermal expansion and contraction much like in my industry how we have thermal expansion tanks that can take in additional fluid or squeeze it back into a system. This means that depending on when you check you car, the level will most likely vary.

Also keep in mind that ALL fluids even when exposed to ambient temperatures CAN evaporate (even if the temperature never actually hits the boiling point). There is an equilibrium between the air and the fluid and the fluid vapour pressure allowable in the air. If you are in a really dry room, you don't have to heat the water much to actually get it to evaporate (albeit it may take a while). The coolant loop in your car is not truly "sealed" and thus can evaporate to atmosphere quite readily (again, it may take forever but it IS possible). When I say truly sealed, I mean something like your refridgerant in your AC or fridge. THAT is truly sealed with no way of it getting out short of a physical leak.

The moral of the story? If you are that worried - look out for blue patches in your engine bay and under your car. If that stuff is truly leaking and/or evaporating off a surface, you will see signs of blue crust. Keep a log of the level, air temperature, coolant temperature (if you can) in order to truly diagnose whether you are leaking or simply in transition between expansion and contraction.

A leak is a leak and you will physically see something (puddle, crust, etc.).

Let's use our heads and get facts ladies and gents. Not some heresay.