What will happen if cake is expose to cool air before setting?

When cake batter is exposed to cool air before setting, it can cause the cake to sink in the center and develop a crust on top. This is because the cool air causes the batter to cool and set quickly on the outside, while the inside remains liquid and continues to rise. As the inside of the cake rises, it pushes up the crust on top, causing the cake to sink in the center.

To prevent this, it is important to bake cakes in a preheated oven and to avoid opening the oven door during baking. This will help to ensure that the cake cooks evenly and does not sink.