The spark — storm triggers

A sky can be loaded with fuel, shear, and moisture and still produce nothing. Storms don't start themselves — something must give the first push that launches a bubble of warm air through the lid. Forecasters call it a trigger (or convective initiation); the Tornado Outlook calls it the Spark.

>The usual suspects

• Daytime heating — the simplest spark. The sun cooks the ground until a patch of air gets hot enough to punch through on its own. This is why so many storms fire between 14:00 and 18:00.
• Fronts — where cold air pushes under warm air, the warm air has nowhere to go but up. Squall lines form this way, often in a neat row along the front.
• Hills and mountains — wind forced up a slope is lift for free. Storm chasers in Europe watch the foothills first.
• Outflow from other storms — a dying storm's cold exhaust spreads along the ground like a mini cold front and can launch the next storm. Storms breed storms.
• Converging winds — where surface winds meet (sea breezes, valley winds), the collision squeezes air upward.

>How the outlook sees it

The Spark meter reads the model's own storminess signals: is rain or thunder already showing? Is the lid (CIN) thin enough for the day's heating to break? A loaded sky with a strong lid and no trigger reads as "capped" — everything on the table, spark missing. That's the meter to watch on humid summer days: when the spark line flips, the rest of the recipe is already waiting.