Atmospheric Electricity
The planet as a leaky capacitor charged by storms.
Lesson at a glanceShow the key points
- Ground ≈ −1 nC/m²; field ≈ 100 V/m downward; ≈ 400 kV up to ~50 km.
- Ions from cosmic rays carry ≈ 3.5 × 10⁻¹² A/m², ≈ 1800 A over the Earth.
- Thunderstorm bases sit 20–100 MV below ground potential; lightning brings ≈ 20 C of negative charge per stroke.
- Storms charge the global battery; fair-weather air discharges it.
The Earth and its atmosphere form a giant leaky capacitor, charged by thunderstorms and discharged by fair weather everywhere else.
The fair-weather field
The ground carries a negative charge, a little under 1 nC per square metre, spread fairly evenly. Above it there is a downward electric field of about 100 V/m: over a flat desert, the potential rises by roughly 100 V for every metre you climb. Higher up the field weakens, becoming negligible around 50 km, where the potential is about 400 kV above the ground.
A current in the air
Cosmic rays ionise air molecules, and dust picks up charge by friction, so the air holds both positive and negative ions. The field pushes positive ions down and negative ions up: a vertical current of about 3.5 × 10⁻¹² A through every square metre. The density of ions grows with height and thinner air lets them travel further between collisions, so the air conducts better higher up; by about 50 km it is highly conducting. Picture a conducting shell at +400 kV, 50 km up, leaking current down to the Earth.
Taking 3.5 × 10⁻¹² A/m² over the Earth’s surface (area 5.1 × 10¹⁴ m²), find the total fair-weather current.
The ground’s charge density is about 0.9 nC/m². Roughly how long would 1800 A take to neutralise the whole Earth’s charge?
Thunderstorms recharge the battery
Rising and sinking air carries water vapour into the cold upper atmosphere (about −10 °C at 3–4 km, −20 °C at 6–7 km), where it forms droplets and ice. A mature thunderstorm is roughly 7–8 km across, with its base 3–4 km up and its top at 6–7 km. Its top holds excess positive charge and its base a dense negative charge, putting the cloud base 20–100 MV below the ground’s potential: far larger than, and opposite to, the 400 kV fair-weather difference.
The field below the cloud breaks down the air, and negative charge leaps to the ground: lightning. One stroke delivers about 20 C, and a cloud can recharge for the next in about 5 s. Meanwhile the storm’s positive top leaks up to the conducting layer. Thousands of storms a day thus pump negative charge into the Earth and positive charge upward, acting as the emf; the fair-weather air is the load that discharges it.
- 1Ground: ≈ −1 nC/m²; fair-weather field ≈ 100 V/m downward; ≈ 400 kV up to 50 km.
- 2Ions carry ≈ 3.5 × 10⁻¹² A/m², ≈ 1800 A in total, which would neutralise the Earth in minutes.
- 3Thunderstorms (cloud bases at −20 to −100 MV) and lightning (≈ 20 C a stroke) keep the system charged.
Check yourself
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