If we could see them, what would thermals look like?
A) They would look like smoke rings. A small region of air becomes heated by the adjacent surface of the earth warmed by solar radiation. Internal radiative heating (by infrared radiation reemitted from the earth’s surface) often occurs faster than does convective heating, and a ‘blob’ of warm air builds up over the space of several minutes. Eventually, its temperature rises enough for it to break free of the ground as cooler air moves radially inward to replace it, and a rotational ‘doughnut’ shape (known as a toroid) moves upward, expanding somewhat as it rises. As each one leaves the ground, the process repeats itself over each heat generating region, and a series of them ascends from each spot. So if we could see it, what we would see would be a wide area of individual smoke rings, from the surface upward.
B) They look like widely spaced columns. If we could see them, wherever there was thermal activity we would see a forest of individual columns of rising air. They would be separated from each other by distances between 10 to 100 times their individual diameter, and bent sideways at an angle reflecting whatever prevailing wind existed at the time (the stronger the wind, the less vertical would be the columns of rising air). The columns would appear to widen (and weaken) with altitude.
C) Their appearance wouldn’t be very exciting. Whenever and wherever they occur, they are simply rising regions of air, their width depending on the extent and severity of surface heating and atmospheric instability.
D) We still don’t know.
Answer: There has not yet been evidence of either the bubble or vortex ring theory. So far, the only thermals that have been spotted were in the shape of columns. The correct answer is choice B. The statement in choice A regarding convective versus radiative heating and initial near-ground build-up of heated air prior to its breaking free and ascending is correct. There is actually rotation within the rising columns, however it is helical and not toroidal (i.e., there is a slight spinning of the columns), and in some thermals, the core can actually be a downdraft (which does admittedly seem somewhat like what would exist in the toroid scenario) but more often contains a region of stronger lift. Also, the rising column expands, widening and dissipating into a condensation plume (or into a cloud) as the air reaches its dew point at the top. The lift in the last few hundred feet below a cloud can also be more pronounced, so much so that sailplane pilots have sometimes been unable to escape being sucked up into the cloud base. Their actual slope from the vertical can often be much greater than you might think: about four horizontal to one vertical, in a 15 knot prevailing wind. In certain conditions, thermals can be arranged somewhat along ‘rows’ just as the clouds above, in which they culminate, may be arranged in so-called ‘cloud streets’. Finally, as lift dissipates, the columns of rising air will ‘pull away’ from the ground and vanish from the bottom up.