In short: Cracked shells come from three causes: unstable meringue that was over- or under-whipped, insufficient drying before baking, and an oven that is too hot. Less often, the culprit is air that was never knocked out of the batter.
1. The meringue
Macarons are an exacting French confection, and the most common failure is a cracked shell. It usually starts with the meringue: over-whipped, it is dry and brittle and collapses in the oven; under-whipped, it never builds enough structure to hold the rise.
Aim for firm, glossy peaks that hold their shape on the whisk without weeping.
2. Drying
Piped shells have to form a skin before they go in. Touch one lightly: it should be matte and leave nothing on your finger. Without that skin the shell splits as the steam pushes upwards instead of sideways.
Drying takes anywhere from twenty minutes to an hour and a half depending on humidity. In damp weather, allow twice as long.
3. Oven temperature
Bake at 140–150 °C with the fan on. Above 160 °C the shell sets before the inside has finished rising, and the pressure has nowhere to go but through the top.
Knocking out the air
Rap the tray firmly on the worktop after piping. Large air bubbles left in the batter will break through during baking and leave a crack behind.
Frequently asked questions
What temperature should macarons bake at
140–150 °C with the fan. Above 160 °C the shell sets too early and tears under the steam.
Do I need to tap the tray before baking
Yes, it releases large air bubbles. Left in, they break through in the oven and crack the shell.
Does humidity affect macarons
Considerably. In damp weather the skin takes far longer to form, so drying time has to roughly double.