In my experience transatlantic flying offers some of the smoothest conditions around. You should try flying in the tropics, even at 40,000 feet !!

However, turbulence is associated with two masses of air moving in different directions which rub against each other. This can happen on a small or a large scale anywhere. One of the largest is the Jet Stream, a mass of air that runs round the earth pretty much at Newfoundland latitude, although it varies up and down. It moves eastwards at about 200 mph at jet altitudes. If you are going east you try and ride with it; if going west you try to avoid it. And the Jet Stream is a very common cause of turbulence with this different airspeed.

The prevailing weather across the Atlantic is also a series of low weather systems, some of which can reach up to jet height too. It's interesting to discuss with those from airline flight operations departments the extent to which they try to predict and avoid possible turbulence, if possible (and sometimes it isn't), or push on through it, as turning to avoid weather costs significant sums in extra fuel and extra hours on the aircraft. It all depends on the relative balance at an airline's executives between the flight operations department and the beancounters - which is not to say that the best of them don't run through it sometimes.