Storm & Sky formula solvers
Expected Annual Lightning Strikes to a Structure
Storm & SkyIEC 62305's first arithmetic step: multiply the local ground flash density by the structure's collection area and a factor for how exposed its surroundings are, and you have the expected number of direct strikes per year. It is a long-run average over a very lumpy process, and the number it produces is usually far smaller than one.
Flash-to-Bang Distance to a Lightning Strike
Storm & SkyLight arrives effectively instantly and sound does not, so the gap between the flash and the thunder is a distance divided by the speed of sound. Because that speed follows air temperature, the temperature is an input here rather than a constant hidden inside a folk rule — and the answer is a measurement, never a safety margin.
Flat Roof Snow Load (ASCE 7)
Storm & SkyThe ASCE 7 flat-roof snow load: the ground snow load from the code's map, reduced by 0.7 because a roof sheds and drifts differently from open ground, then adjusted for wind exposure, for how warm the roof is underneath, and for what the building is for. A teaching aid, not a design authority — the governing code is.
Hailstone Terminal Velocity
Storm & SkyBalance a hailstone's weight against the drag on a sphere and the fall speed drops out: proportional to the square root of the diameter, so a stone four times as wide falls twice as fast. A two-centimetre stone arrives near 20 m/s, a golf ball near 30, and a softball near 45.
Koschmieder Visual Range
Storm & SkyHow far you can see, from how strongly the air scatters and absorbs light. A black object against the horizon sky loses contrast exponentially with distance, and Koschmieder's 1924 result sets the visual range at the distance where that contrast has fallen to two per cent — which makes the numerator −ln(0.02) = 3.912.
Lightning Channel Length from the Duration of the Rumble
Storm & SkyThunder rumbles because the whole channel goes off at once and you hear it a piece at a time: sound from the near end reaches you first, sound from the far end last. The duration of the roll is therefore the difference in range across the channel divided by the speed of sound — three seconds of rumble is about a kilometre of lightning.
Lightning Collection Area of a Rectangular Structure
Storm & SkyIEC 62305's equivalent collection area: the patch of ground that, if lightning fell on it, would instead have struck the building. It is the footprint, plus a strip three building-heights wide down every side, plus a quarter-circle of the same radius at each corner — one rectangle, two side strips and one full circle when you add the four corners up.
Lightning Striking Distance (Rolling Sphere Radius)
Storm & SkyThe distance at which a descending lightning leader commits to whatever it is going to hit, as a function of the peak current the flash will carry. This is the electrogeometric relation of IEC 62305, and it is the geometry behind the rolling sphere: roll a sphere of this radius over a structure, and every surface it touches is a surface lightning can reach.