Convection currents absolutely do have momentum. If you want a very simple analogy, boiling water is strongly self stirring especially in a cold environment, w

Convection currents absolutely do have momentum. If you want a very simple analogy, boiling water is strongly self stirring especially in a cold environment, whereas cold water in a cold environment does not self stir. This effect is very old popular science filler material, and in the 80s there was possibly a Mr Wizard or similar TV episode where the effect was demonstrated by taking aquarium air tubing and an "empty" mustard squeeze bottle (only worked with one specific brand, plochman's mustard I think) and fill the "empty" mustard bottle such that you've made free yellow food coloring, then stick the aquarium hose in the clear bucket of freezing water and squirt in some mustard food coloring and watch how the convection currents in the previously hot container are huge and the water turns uniformly yellow in seconds whereas the previously cold water the yellow diluted mustard-water just sits there, pretty much. Its easy to use the hose and a stopwatch to try and quantify the currents. Now a days kids would probably use a smartphone camera or ipad and then use a video editing program to quantify the actual water current speeds vs time/location, but its easy to miss the important overall qualitative interpretation by trying too hard to generate too much quantitative information.