Why must air traffic controllers at high-elevation Bolivian airports (such as El Alto, SLLP at 13,325 ft) pay meticulous attention to cold temperature altimeter corrections during winter?

1. Extreme sub-zero temperatures cause true aircraft altitude to be significantly LOWER than indicated barometric altitude, reducing real obstacle clearance over surrounding mountain peaks if cold temperature corrections are not applied.
2. Cold air makes aircraft altimeters spin backward.
3. Cold temperatures disable control tower radio transmitters.
4. Aircraft groundspeed drops to zero in cold air.