|Studying Ship Tracks|
Throughout most of the twentieth century, the scientific community was only certain about a few basic facts regarding cloud formation. They knew that in order to form, clouds require water vapor as well as tiny microscopic particles (aerosols) from the surface of the Earth. When water evaporates into the atmosphere, it spreads evenly throughout the surrounding air often to the point where the relative humidity is greater than 100 percent. Aerosols that dissolve easily in water, such as ammonium sulfate and sea salt, give the excess water molecules something to cling to. These aerosols act as the nuclei, or "seeds," around which cloud droplets take shape, and together these droplets form clouds. Were it not for particles in our atmosphere, the sky would almost always be clear and the air around us thick and humid (King et al., 1995).
Many researchers suspected that as the concentration of these particles increases, the properties of the cloud could change. However, they remained unclear on exactly what these changes would be, the effects aerosols could have on rainfall, and most importantly where aerosol particles come from. Many believed that humans produce a large number through the burning of fossil fuels and plants (biomass).
In the late 1980s and early 1990s, a series of investigations on the exhaust from ship’s smokestacks answered some of these questions. Not significant sources of pollution themselves, ships burn fossil fuels and release their exhaust in the form of sulfur dioxide, a gas that leads to the formation of sulfate aerosols in the atmosphere. The exhaust produces clouds that are relatively low in elevation and resemble larger versions of airplane contrails. Unlike contrails, these"ship tracks" are ideal for study since they remain in the air for many hours and are surrounded by relatively pristine marine air (King et al., 1993).
Initially, the ship tracks were used to see if an increasing number of aerosols from pollutants would make the clouds brighter. The scientists believed the additional aerosols from the ships would give the water vapor more nuclei to cling to, so that a greater number of smaller drops would form in the cloud. These smaller drops, in turn, would make the cloud more reflective to sunlight. The same phenomenon can be seen when ice cubes are crushed. As the ice is broken up, the once smooth surface is shattered into many tiny surfaces at varying angles. These tiny surfaces reflect incoming light in all directions and cause the crushed ice to appear white and opaque. Water droplets do not contain all these ridged surfaces, but the researchers were fairly sure their fragmentation would have a similar effect.
Through satellite observation andin situ aircraft measurements, scientists not only showed that their hypothesis was correct, but they also came across an effect that no one had foreseen. In addition to making the clouds more reflective, the aerosols were causing them to retain water and to stop drizzling. The cloud seeding by the ships’ exhaust made the droplets so small that they could no longer easily merge together to reach the size needed for gravity to pull them to the ground. Since no drizzle came out of the seeded clouds, the cloud water just kept building up (King et al. 1995).
The formation of water droplets in clouds is typically governed by the number of aerosol particles present to act as nuclei. If no aerosols are present, water molecules are unlikely to combine and form droplets, resulting in very humid but cloudless air.
Water-soluble aerosols make it easier for individual water molecules to clump together and form droplets. If relatively few aerosols are present, the number of droplets formed are limited by the number of nuclei. The droplets that do form continue to absorb more water molecules, creating low numbers of large droplets.
In aerosol-rich clouds the large number of nuclei leads to many small droplets. The aerosols can inhibit rainfall by preventing the water droplets from becoming large enough to precipitate. (Images by Robert Simmon)
But to take these findings to the next level and prove that man-made sources of pollution make clouds brighter and less likely to precipitate would require testing a wide range of factories and power plants on land. Many deemed it too difficult to measure the effects of aerosols over land given the current level of technology. The wind currents and convection over land are tumultuous, and the clouds are much thicker. In nearly any given industrialized area there are so many things that cause pollution that it’s hard to isolate just one source. In the eastern United States for instance, the number of aerosols spewed out by power plants and factories merge together in the sky to create one big, thick soup of pollution.
|The puffy streaks of clouds cutting through the center of this image are ship tracks—aerosols emitted in the ship’s exhaust cause many small water droplets to form, creating exceptionally bright white clouds. (Image by Mark Gray, MODIS Atmosphere Science Team)|