Adverse pressure gradient

Okay kiddo, imagine you are dribbling a ball down a street. You start at the top of a hill where the ball rolls nicely and you are able to control it easily. But as you start going down the hill, the ball begins to pick up speed and it becomes harder and harder to keep it under control.

This is similar to what happens to air as it passes over a surface like an airplane wing. When the air is flowing smoothly over the wing, we call this laminar flow. But as the wing starts to get thicker or the shape changes, the air flow becomes disrupted and creates what we call a turbulent boundary layer.

An adverse pressure gradient occurs when the air flow over the wing slows down too much, causing the air pressure on top of the wing to decrease and the pressure on the bottom to increase. This creates drag and makes it harder for the plane to fly efficiently.

So if we go back to our ball example, imagine you are walking down the hill with the ball and suddenly the pavement becomes rough and bumpy. This makes it hard for you to control the ball and it starts to slow down, similar to how the air flow slows down over the wing. And just like how it's harder to control the ball when it gets bumpy, it's harder for the plane to fly efficiently when the air flow becomes turbulent due to an adverse pressure gradient.