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A thunderstorm forms when warm, moist air near the ground rises quickly and meets much colder air higher up. That motion (convection) builds a storm cloud (cumulonimbus) — sometimes over ten kilometres tall. Inside it, water droplets and ice crystals collide and build up an electrical charge, which eventually discharges as lightning.

A storm needs three things at once: moisture in the lower atmosphere, instability (a large temperature difference between the ground and higher up), and some trigger for the air to start rising — a weather front, ground heated hard on a hot day, or mountains forcing air upward.

How to read the storm-signal strength

Weather models (like ICON-D2, used on this site) do not "see" lightning directly — they calculate a Lightning Potential Index (LPI) from the physics of the cloud: how much ice and supercooled water is rising, how strong the updraft is. Rainfall and convective energy (CAPE) are added on top — together they give one index from 0–100%.

• 0–20% — calm, no real storm signal.
• 20–30% — weak signal, a passing shower is possible, a storm is unlikely.
• 30–45% — a real chance of a storm, worth tracking as it develops.
• 45%+ — a strong signal, a storm with a high chance of lightning.

This is a model forecast, not a radar reading — it shows what the model expects a few hours ahead, not what is happening right now. For the current situation, always check radar too (e.g. in this site's storm tool) and official warnings.