This can happen for a variety of reasons. Aeroplanes are most affected by:

• MECHANICAL TURBULENCE

As the movement of the air is impossible to observe with the naked eye without a contrast agent, it is easier to imagine the movement of water, which is similar to air movement. If we put our hand into a stream flowing steadily, we see how the water starts to swirl around the hand. The same thing happens with air. A uniformly moving air mass that flows into a mountain or a high building does not move evenly after collision, but will be in turbulent motion. When flying through this kind of air mass, we experience the aeroplane jolting. Mechanical turbulence develops near the ground and is often felt before landing.

• CLEAR AIR TURBULENCE

Turbulence 10 km above the ground is caused by air mass movements (jet streams) that collide with each other or change direction, just as the hand in the water does. Flight crews that enter areas with high turbulence will generally inform others, which is why the captain will usually be able to alert the passengers before entering turbulent air. Clear air turbulence can also cause the aeroplane to suddenly drop a few meters in places where the air flows down. In such cases when the aeroplane drops 3-10 meters, passengers might sometimes feel as if the plane has dropped a few kilometres, which is of course not true. 

• THERMAL TURBULENCE

As the name implies, this is turbulence caused by a change in temperature. We experience thermal turbulence when intense sunshine has heated up the ground (especially forest masses) and we feel the aeroplane shake as it passes through an air mass flowing up from the ground. It is especially strong under and in the clouds. Glider pilots use these air masses skillfully to fly in the air without any engine powers for hours.