What causes variations in altimeter settings between weather reporting points?

• A. Unequal heating of Earth's surface
• B. Uniform atmospheric pressure
• C. Consistent elevation of all reporting points
• D. Instrument calibration differences

Answer: (A)

Variations in altimeter settings between weather reporting points come from differences in the atmospheric pressure at the surface caused by uneven heating of the Earth's surface. When the sun heats some areas more than others, it creates temperature gradients that drive developing weather patterns and moving high- and low-pressure systems. These pressure differences mean that the mean sea-level pressure (the value you dial into the altimeter as the local setting) varies from place to place. So, each reporting point has a different local QNH to reflect its own pressure field, and that's why altimeter settings differ.

If pressure were uniform everywhere, or if all reporting points shared the same elevation, the settings would be the same. Instrument calibration issues can cause readings to be off, but they don’t explain the regular, location-to-location variation caused by actual weather-driven pressure differences.