Weather conditions greatly affect the pressure of the atmosphere (the barometric reading). Altimeters report altitude as a function of atmospheric pressure. Typically pilots will obtain a local barometric reading from the nearest airport. They will then set the Kollsman window to the setting that they received. This action will adjust the altimeter reading, eliminating error due to local weather conditions.
Reading the Kollsman Window (Barometric Reading)
The Kollsman window is located at the 3 o’clock position on the altimeter dial. This window allows access to read a sub-dial, which contains the barometric readings. The arrowhead indice located precisely at the 3 o’clock position on the altimeter’s main dial is used as the reference point for reading the barometric sub-dial. Most altimeters will have a sub-dial, which reads out in milli-bars or InHg.
The Kollsman window in above Altimeter reads 1013.2 mb.
The Barber Pole
The “Barber Pole” on the face of the altimeter is visible only when the altitude is above sea level. When the altitude is below sea level the barber pole is no longer visible. This is provided to avoid the error of reading –1,000 Ft. as being +10,000 Ft.
Happy landings. Be safe.
