When smoke rises from a chimney, the column soon loses its shape as the smoke wafts skyward. Yet puffy cumulus clouds, which have a similar structure, tower into the sky without breaking apart. Now a researcher claims to have found the answer to this long-standing mystery, and it could have implications that range from weather prediction to pollution control.
A column of smoke is made up of patches of intensely spinning air, or vortices, that suck in surrounding air--a process physicists call entrainment. This outside air tends to disperse the smoke. Previous research showed that how well clouds stick together also depends on entrainment, but the factors that influence this process are poorly understood.
To investigate, Rama Govindarajan, a fluid dynamicist at the Jawaharlal Nehru Center for Advanced Scientific Research in Bangalore, India, created a computer model of cloudlike flow. She found that inside the vortices, ringlike regions sporadically give off heat as the moisture inside them condenses. Giving off heat makes the rings spin faster, which in turn reduces the amount of air they suck into the cloud, Govindarajan reports in the 1 April issue of Physical Review Letters.
"It's been very difficult to reproduce basic cloudlike flows in models so far," says Ganapati Bhat of the Center for Atmospheric and Oceanic Sciences at the Indian Institute of Science in Bangalore. Bhat says such models would be useful for weather prediction. For instance, he says, rain is not as likely from clouds that entrain more air. Govindarajan says the work could also lead to better tracking of smoke from forest fires or other air pollution.
JNCASR Fluid Dynamics Unit