Atmospheric electricity: The variation of atmospheric potential is caused by which factors?

Introduction:
Atmospheric potential (the electric potential in the atmosphere) varies due to multiple meteorological and physical factors. Understanding its drivers is part of atmospheric physics and helps explain phenomena like fair-weather electric fields and storm electrification.

Given Data / Assumptions:

• Temperature contrasts exist over Earth's surface.
• Air density changes with altitude, humidity, and temperature.
• We consider general, fair-weather contexts absent specific storms.

Concept / Approach:
Temperature gradients influence convection and charge separation mechanisms, while air density impacts conductivity and ion mobility. Together these alter the vertical potential profile and local electric field intensity observed at the surface and aloft.

Step-by-Step Solution:
Relate temperature differences to convection cells that transport ions and charge.Link air density changes to electrical conductivity variations that shift potential gradients.Conclude that both factors contribute to atmospheric potential variations.

Verification / Alternative check:
Balloon and aircraft measurements show diurnal and synoptic-scale variations correlating with temperature structure and density stratification in the troposphere.

Why Other Options Are Wrong:
a or b alone: Each explains part of the effect but not the full picture.Neither: Contradicts established observations.Ocean waves only: Too narrow; waves can influence local electrification but are not the sole cause.

Common Pitfalls:
Attributing potential changes to a single factor; the atmosphere is a coupled thermo-electrodynamic system.

Final Answer:
Both (a) and (b).