the climbing rate gauge should have HUGE variations caused by atmosphere

Actually, pressure can change a lot at a particular level, that's why Altimeters are adjustable (barometric setting knob). This setting is most easily found when on the ground, just turn the knob till you have set the airfield elevation on the instrument. In flight, corrections must (practically) be sent by radio from weather or control centers and be relevant for the aircraft's location. Corrections are then, obviously, periodically needed during changing weather conditions. None of this really has any bearing on the vertical speed instrument, as it reacts to level altitude pressure changes (as while sitting on an airfield) much too quickly to register any vertical speed errors. Sit in a parked aircraft while a storm blows through and you won't see the VSI move at all.

[jf1981]

Originally Posted by zipper

In flight, corrections must (practically) be sent by radio from weather or control centers and be relevant for the aircraft's location.

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.

[nadasero]

Because of this I am using altimeters on my bike since about 20 years. They are very precise if it goes for altitude changes. They measure in steps of one Meter and if you hold them over your head, you get usually two meters more than on the ground.

Today I'm using a GPS-System (Garmin Edge 705). It has a barometric altimeter in addition to the GPS because this is more precise for small altitude changes. It gives precise readings for the steepness of a climb and the climbing speed.

The absolute precision is not that good. Over a day, the error can grow up to 100 meters if the weather changes. Typically it is less than 20 meters. A plane, flying from one weather system to the next can see changes of more than 50 mbar which can add up to an error of 500 meters at see level.

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.

[jf1981]

Originally Posted by zipper

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.

Yes, in France depending upon where you are, the transition altitude varies, if not indicated 3000 ft used, but currently eg 5000 ft may be used instead in controlled areas. Apparet from that, it's flat land so in mountainous areas I really don't know what rules are applied there, we have some cities in the alps. Have to have a look at it.