Which factor causes variations in altimeter settings between weather reporting points?

• A. Unequal heating of Earth's surface
• B. Ocean tidal forces
• C. Uniform sea level pressure
• D. Random instrument error

Answer: (A)

Altimeter setting reflects the local sea-level pressure (QNH). Because pressure at the surface varies from place to place, the differences come from how the surface is heated unevenly. Areas that receive more heating tend to develop different pressure patterns than cooler areas, creating pressure gradients that keep changing with weather systems. That’s why weather reporting points have different altimeter settings. Other options don’t explain these systematic, regional pressure differences: tidal forces are negligible for this purpose, uniform sea-level pressure would mean no variation, and random instrument error wouldn’t produce consistent differences between locations.