What causes turbulence on airplanes?

The weather conditions....

Changes in air patterns

An air temperature difference.

When the plane is on a certain height it travels by passing the air backwards some time due to weather or air pocket there is less air to push back and continue flying that makes the plane jiggle.

In warm areas of the world air pockets are common and in cold areas heavy clouds are common, now that means when there is a heavy cloud the engine is not pushing air its pushing water (clouds) so there is turbulence and in hot areas when an air pocket arrives this means the engine is not pushing any air in fact there is no rich air to push so it again causes turbulence and this is very common and its not that dangerous as it feels like it is.

The atmosphere is not a steady-state medium. It is always moving. Winds aloft, winds blowing around obstructions (e.g. mountains), atmospheric and terrestrial cooling?all these things combine to keep the atmosphere in a state of steady change.

The dynamics of an airplane in flight can be compared to a ship at sea whereas the air is invisible but the water isn?t. The ocean is not a mill pond and neither is the sky.

If we heat water on a stove, we can see bubbles (turbulence) formed by the heat rising. Same thing in the air?heat from the Earth?s surface rises and churns the atmosphere. This causes thermal or convection turbulence. (This is why there is generally less turbulence in the morning and on overcast days?there is less convectional cooling.)

Or imagine a river rapid. The flowing water is blocked and disrupted by rocks, the river narrows, etc. Air currents (winds) are subject to the same thing when they blow over mountains and other obstructions.

Here?s a good link with a more detailed explanation:

http://www.aopa.org/asf/wx/articles/2040.html?PF