Try this. Make a circle and color it black. Make sure there is no stroke. Change it's transparency to 0% then copy it, paste it in front (ctrl+F) and change the transparency of the copy to about 40% then shrink it to about half its size. Now select both circles and with the blend tool click on the same node in both circles (first click on the left node of the 40% circle then the left node of the 0% circle for example). Now double click on the blend tool and choose specified steps from the pull down. Set the steps between 20-30 steps (the more steps, the smoother the blend). Now grab your blended shape and make copies of it overlapping each other at different amounts. You can resize your copies too for the smaller billows.
If the smoke is too dark or light, with the open selector (white arrow) click on one of the 40% circles then in the select menu choose same opacity so you can change all the copies at once.
"Great spirits have always encountered violent opposition from mediocre minds." -Einstein