You'd expect a ride this bumpy if you were driving an off-road vehicle over rocky, uneven terrain, but a bump bump BUMP as you glide 30,000 feet above that terrain in a modern jet liner, might surprise you, not to mention scare the bajeebers out of you. After all, you never notice hard lumps and bumps as you breathe air. What makes an airplane go bump?
Airplane bumps are caused by regions of air moving at different speeds, for example, a layer of fast-moving windy air rubbing against a layer of relatively still air. Where the two masses of air rub against each other, you get turbulence, a chaotic and unpredictable mixing of wind. This happens frequently near storm clouds, where a plume of warm air and clouds rises into cooler, upper layers. It can also happen above mountain ranges, when mountains deflect the surface wind upward, pushing dense air high into the thinner atmosphere.
As a plane flies through the boundary layer of turbulence, it will encounter sudden, random changes in wind speed. A strong tailwind might turn into a strong headwind, or an updraft might suddenly turn downward. Because the airplane is immersed in this air, you experience these wind changes as bumps. Imagine driving a car along a perfectly smooth and flat highway, except the highway itself is moving unpredictably, lurching forward and backward, up and down.
While mild turbulence is to be expected on many flights, severe turbulence, though rare, can cause injury or even death.