For many of us, cloud formation was one of the first scientific phenomena we understood. We were taught the sun heats up the water on the Earth's surface until the water evaporates, gently ascends into the sky, and comes together to create clouds. It was a simple concept involving just water, the sun, and the sky. As we grew, we accepted this idea as a basic, easily understood truth. We’d no sooner doubt it than doubt whether the Earth goes around the sun or whether static electricity allows a balloon to stick to a wall.

But as it turns out, we've all been misled. Scientists have known for years that cloud formation is an extremely dirty business that depends on microscopic particles (aerosols) from the surface of the Earth. These particles allow water vapor to condense, and without them cloud droplets, mist, or fog could not form at all.

The presence of the aerosols makes matters much more complicated in that many researchers do not know exactly where all these particles come from. While the consensus is that cloud-forming aerosols mostly originate from natural sources, an increasing number of scientists believe that humans may produce a large quantity through the burning of fossil fuels. By doing so, we are making the clouds above us brighter, altering their lifetime and possibly creating huge changes in our environment.
 

Although clouds appear pristine, aerosols—airborne grime— are required for water droplets to form. Most aerosols are generated by natural sources, but human activities are increasing their concentration in the atmosphere. As a result, the properties of clouds may be changing around the globe. (Photograph by Robert Simmon, NASA Earth Observatory)

Testing theories of man-made cloud formation has been a difficult task. In most urban areas, scientists are unable to discern exactly how pollutants contribute to forming clouds because the atmosphere over the land is too tumultuous. As an alternative, researchers from NASA’s Goddard Space Flight Center and a number of universities around the country have turned to studying "ship tracks" — clouds formed from the aerosols coming from large ships. Not significant sources of pollution themselves, ships release their exhaust into the relatively clean and still marine air, where the scientists have an easier time of measuring the effects of fossil fuel emissions on cloud formation.

What they have found is that the sulfur dioxide released from ships’ smokestacks could be forming sulfate aerosol particles in the atmosphere, which cause the clouds to be more reflective, carry more water and possibly stop precipitating. This is proof that humans have been creating and modifying clouds for generations through the burning of fossil fuels. The study may go a long way towards explaining some of the climactic mysteries in the world, such as why global warming is affecting the Southern Hemisphere much more quickly than the Northern Hemisphere (King et al. 1993).

At the Core of Cloud Formation

The bright cloud arcs in this image are "ship tracks." They are often seen in near infrared satellite imagery of the Eastern Pacific, like this scene from a NOAA weather satellite. (Image courtesy Dr. Michael King, NASA GSFC)