Why is it bumpier below where the clouds form and once you get above the bases the air tends to be smoothers?
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1$\begingroup$ Welcome to Av.SE - nice first question! $\endgroup$– Ralph J ♦Jul 10, 2021 at 21:42
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1$\begingroup$ Because thermals only reach up into the clouds. $\endgroup$– Peter KämpfJul 10, 2021 at 22:21
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$\begingroup$ The question contains a misconception. Actually the air can be very bumpy inside clouds. It is more accurate to observe that the air tends to be smooth above the cloud tops than the cloud bases. Of course if you are above cloud base and are not inside a cloud, then you are not in an updraft, but you might conceivably be in a downdraft. Anyway to a first approximation the cloud tops (not the cloud bases) mark the top of the "mixed layer" where it is turbulent. $\endgroup$– quiet flyerJul 12, 2021 at 14:58
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In some sense the question is backwards. But, that really leads to the answer. Clouds form when there is air movement. For example of how clouds can form: when there is local heating, such as due to the sun heating a large black parking lot, the air over that will rise. Flying through that air is not as smooth as in other places. As that air rises up it expands and cools leading to an increase in the relative humidity. If it does that enough it reaches 100% RH and a cloud forms. But, that also leads to a decrease in the energy available to move that air up, so it doesn't keep rising up. So, the air above clouds tend to not have as much energy and movement as below.
So, clouds form where there is air movement, and they "mark" that movement.
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$\begingroup$ A suggestion to improve this answer-- actually, the latent heat released when the water vapor condenses into a cloud releases more energy into the column of rising air, decreasing its rate of cooling and thus further stoking the thermal updraft. (Google "wet adiabatic lapse rate" and "dry adiabatic lapse rate".) This can be an important factor in the formation of thunderstorms or towering cumulus clouds. Still it remains the case that the air is smooth above the cloud tops because the cloud tops are also the tops of the thermal updrafts. $\endgroup$ Jul 11, 2021 at 1:42
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$\begingroup$ Re "But, that also leads to a decrease in the energy available to move that air up, so it doesn't keep rising up. " -- see prev. comment-- actually, to understand why the air eventually stops rising, you have to look at the density (temperature) vs altitude profile of the surrounding atmosphere. $\endgroup$ Jul 11, 2021 at 1:42
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$\begingroup$ The fundamental problem with this answer is that it suggests that cloud formation tends to stifle updrafts, which is not true at all (except of course for effects relating to shading of the ground.) $\endgroup$ Jul 12, 2021 at 14:51