Altitude parameter in TV transmitters...

The specification for altitude is to make sure that all blowers and fans installed in the equipment will provide the minimum air flow and pressure to cool down hot devices, this is, the air flow and pressure supplied by blowers and fans is a function of the environment pressure that is also a function of the altitude.
This parameter is fundamental in high power transmitters using a vacuum tube in the final stage cooled down trough forced air. The tube's temperature and its life time are a function of the air volume and pressure is passing through it and the flow and pressure provided by a blower will be reduced as the altitude increases.

NandoPG

Tnx NandoPG,

But this parameter exists also in the liquid cooled solid states series.
How it can be explained

There is also the problem of voltage breakdown of air decreasing with altitude.

Dear E. Madanian,
Even if you have a water cooling system, you need a minimum pressure at the input of the pump is used to force the water circulation to guarantee a minimum hydrodynamic flow.
I have seen some old troposcatter transmitter's infrastructure in high altitude sites using positive pressure room to compensate the low atmosphere pressure so that the water cooled klystrons used in the transmitters were kept at the nominal temperature.

NandoPG