Close The science
A simplified, honest model. Constants marked model are teaching values. Independent, educational; not affiliated with any game, film or religious organisation.
1. How a cloud gets charged
Graupel (soft hail) falls through a rising cloud of ice crystals. In a collision the graupel gains charge of one sign and the crystal the opposite sign. Below the reversal temperature (about -15 C, typical value ) the sign flips. Updrafts carry light positive crystals up and heavy negative graupel stays low, giving a tripole : positive on top, a main negative layer, and a small positive pocket near 0 C.
Page model: dQ/dt = k (w/12)^1.3 · f_ice · L with layer thickness L between the -3 and -15 C levels at T(z) = -6.5 (z - H0) C (z in km). This is a statistical cartoon, not the Takahashi laboratory table (that is planned).
2. When the air breaks down
Dry air at sea level breaks down near E_b = 3×10^6 V/m, but measured fields inside storms are only about 1–4×10^5 V/m. So lightning needs help from ice and water particles that sharpen the field (and possibly runaway electrons): still an active research topic. The page lets you set a threshold E_c (0.1–3 MV/m). Field of the charge disc: E = Q / (2 ε0 π R^2) with model radius R = 1.1 km: 20 C gives 0.30 MV/m.
3. Leader and return stroke
A faint stepped leader (steps of about 50 m, about 2×10^5 m/s) descends from the cloud. When it is close to the ground, upward streamers rise from tall objects. At connection the bright return stroke (about 10^8 m/s, median peak current 30 kA, about 5 C per stroke, channel about 30 000 K) races back up. A flash often has 2–4 strokes. On screen the leader is drawn finer and slowed by a factor of roughly 1000 for viewing ; the branching path is a random-walk sketch of a lattice breakdown model, not a full simulation. Energy of a flash: W = ½ Q V; Q = 20 C, V = 10^8 V gives 1.0×10^9 J.
4. Thunder and distance
Light arrives almost instantly (3.3 µs per km). Sound travels at v_s = 331.3 + 0.606 T m/s (343 m/s at 20 C), so d = v_s · Δt: about 3 s per km, 5 s per mile. 6.86 s gives 2.35 km. Thunder is rarely heard beyond about 25 km (refraction and absorption).
5. Why the oak? Rolling sphere
In the electro-geometric model a sphere of radius r_s = 10 I^0.65 (metres, I in kA) rolls over the landscape; the leader attaches where the sphere first touches. I = 30 kA gives 91 m. A tall isolated tree or post touches it first; a lightning rod taller than the tree makes the sphere touch the rod instead. This is a design heuristic for protection, not a prediction for a single strike. Check: the IEC 62305 class radii (20/30/45/60 m) correspond to 3/5/10/16 kA, which the formula reproduces within 6 % (they are rounded design values).
Around the strike point current spreads in the ground: V(r) = ρ I / (2π r) with soil model resistivity ρ = 100 Ω·m. Between two feet a stride s = 0.7 m apart: ΔV = ρ I / (2π) (1/r - 1/(r+s)); r = 10 m, I = 30 kA gives about 3.1 kV (an illustration of why the ground near a strike is dangerous, not a medical statement).
What happens inside the tree: sap and bark water can be heated and boiled by a fraction of the flash energy. Heating 1 kg of water from 20 C to steam takes m(c ΔT + L_v) = 0.335 + 2.26 = 2.6 MJ. The page assumes 5 % of the flash energy goes into the tree (model ).
6. Shelter and the Faraday cage
In a closed metal shell the charge sits on the outside surface and the field inside is about zero (Gauss' law), which is why a hard-top metal car or a building with a metal frame and wiring is a much better place than the open or under a tree. Rain, hail and drop sizes: a 2 mm raindrop falls at about 6.5 m/s (Gunn–Kinzer 1949; about 9 m/s only for ~5 mm drops); a 2 cm hailstone at v_t = √(4 ρ_i g D / (3 C_d ρ_a)) = 19.8 m/s.
Educational, NOT safety or medical advice. In a real storm follow your national weather service (for example NWS or the Met Office). Rule of thumb often quoted: when the flash-to-bang gap is 30 s or less go indoors, and wait 30 minutes after the last thunder. Never shelter under a tree. [verify against the official guidance before publication]
7. What do we actually know about Perun?
Very little, and some of it was reconstructed later. Every claim in the Folklore Lens carries a level:
8. Honest simplifications
Charging is a statistical cartoon; no corona, runaway electrons or lab tables. The leader is a decorative random walk; the real one steps about 50 m in microseconds. The cloud is a shader layer plus a 2D section, not a volumetric simulation. Soil is one resistivity; the oak is a single 24 m point target; terrain and cloud are idealised. Colours are vertex-painted with baked ambient occlusion (no Cycles texture bake in this slice). Independent numerical checks
Each value is recomputed by a different route (closed form, integration or a published table) and compared with the function this page uses.
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