The relationship between clouds and aerosols – or small particles in the air – is complicated to say the least. Water vapor will rarely condense into cloud on its own, and so normally requires some sort of particle for it to condense around. Yet scientists have found that the mass burning of agricultural land, which releases masses of aerosols into the air, rather than creating clouds, actually prevents their formation.
“Fire-emitted particles crippled the atmosphere’s ability to build clouds and thunderstorms, and that ultimately caused a decrease in rainfall during what's already a seasonal drought,” explained Michael Tosca, who coauthored the paper published in Geophysical Research Letters. “Less clouds and rainfall dry out the land and make it easier for farmers to ignite more fires, which data show they probably do.”
Around half of all fires on Earth are in Africa. Normally set during the dry season, the smoke from the fires joins together to form huge plumes that travel far and wide from the original site, transferring nutrients to distant lands and oceans. In order to assess the impact that these plumes have on cloud formation, the scientists at NASA used images taken by the Multi-angle Imaging Spectroradiometer instrument (MISR) on the Terra spacecraft between 2006 and 2010.
Picture of fires burning in sub-Saharan Africa taken by MISR on the Terra spacecraft. NASA.
They focused on fires set south of the Sahara and north of the equator, and combined the images taken with weather data from other satellites that pass over the region a couple of times a day. The researchers then matched images taken by MISR of smoky areas to smoke-free areas that had statistically identical weather conditions. They then watched how the different weather systems evolved in each area over time.
Clouds form when warm air containing lots of water vapor rises. As the air gets higher it starts to cool, and as cool air cannot hold onto as much water vapor as warm air, some of the vapor condenses onto aerosols and forms clouds. It is this upward movement of warm air, or the convection current, that the scientists think the plumes of smoke inhibit.
The aerosols contained in the smoke belched out by the fires in sub-Saharan Africa contain lots of black carbon particles. Their dark color means that the particles absorb lots of sunlight and thus warm up. This has the effect of creating a warm layer of smoky air, which in effect blocks the convection current. This prevents the water-vapor-laden warm air from rising, and thus impedes the formation of rain-producing clouds.
“We are able not only to show that the clouds decrease in the presence of aerosols, but that aerosols inhibit convection,” said Tosca. “This effect is predicted by models, but it's really cool to see it in actual data.”
Header image: Michael Deeble/Flickr CC BY-NC-ND 2.0.