Quote Originally Posted by Dr.Kamiya View Post
Cooling system is sealed so atmospheric pressure shouldn't be affecting internal pressure.
Not fully sealed.

The coolant pressure cap keeps the pressure inside the system at a set pressure above atmospheric pressure. So if you lower the atmospheric pressure, the valve will release the excess until it equalizes again to the set cap pressure above atmospheric pressure. Hence, the cooling system will run at a lower pressure at high elevations.

But the ambient air itself will be less dense and will have a reduced cooling effect.
Agree. The radiator will not be able to shed as much heat due to less air molecules to transfer to.

As a side note, you will see in many equipment like electronics (ie. servers, IT equipment, some consumer electronics, etc.) wherein the specs may have a stipulation about the maximum altitude the said equipment can be safely operated. It's because of the less dense air at altitude, which can interfere with the equipments' own cooling capabilities.

Case in point, most projectors has an on-screen setting wherein you can set it to Normal or High-altitude mode. The main difference is, in Normal mode, the fans operate at various speeds and would likely spin at low speed to reduce noise. But once the projector gets hot enough, the fans ramp up to high speed. But in High-altitude mode, the fans are always at high speed to compensate for the thinner air.