Why does a plane shake and sway (encounter turbulence) when flying through clouds.?

Clouds are, by definition, zones where a lot is happening. Raising moist air sees the water condensing (that is why this is a cloud: because water condensed and formed droplets in suspension). But in order to stay aloft, there has to be an upward current pushing the droplets up, while that air has to eventually come down at some other location. The largest clouds, cumulus nimbus, are associated with very violet ascending and descending currents. Those are storm clouds, and usually airplane try to avoid them entirely.

At any rate, all this vertical motion in the air masses is what cause turbulence on airplane.

Cloud forms from the rising of warm air and with the presence of sufficient moisture. Therefore in a cloud formation you get uprising and falling air. So it is not all smooth air when you see cloud. Depending on how massive the cloud formation is, the turbulence rate will increase with size. So pilots will avoid massive cloud formations as the up and down drafts can be strong enough to even break the wings. So if the plane is passing through clouds, there will be turbulents. That is why passengers are advised to be seated and belted when this need to pass through cloud which could not be circum-navigated.

Air density varies in and around clouds. This causes lift to be erratic. Usually the plane is flying fast enough to evenly distribute turbulence over the wings to minimize uncomfortable motion.

It's a combination of temperature and moisture, all contributing to air density. Temp changes and density changes cause air turbulence.

for the reason that a aircraft encounter different densities in clouds (water droplets increases density substantially in the various temperatures). when flying through clouds, the humidity contacts the aircraft's surfaces and due differences in density is causes a uneven lift on wings.

Keeping it simple, the first answer was pretty close. You'll also find that once you climb above the clouds, whatever that altitude is, the air will smooth out as a general rule.

Wow, thanks! Exactly what I was looking for. I looked for the answer on the internet but I couldn't find them.