the climbing rate gauge should have HUGE variations caused by atmosphere

Originally Posted by jf1981

Above 3000 ft agl we use standard reference 1013,25 (29.92), there would no point keeping accurate true altitude (some areas of the world, even impossible to know local pressure), only accurate pressure altitude is needed.

Interesting. Here in the US we use pressure altitude (QNE) only for all aircraft above 18000 ft MSL. Below that the combination of not wanting aircraft flying into the ground (unintentionally) during airport operations with reduced visibility and having those aircraft (below 18000 ft), both terminal and en-route, synchronized vertically requires that they all set their altimeters to their local pressure settings (QNH). Any air traffic controller or Flight Service Station can provide local pressure information, and will give it (and winds, etc) spontaneously after querying intentions.