"Smoking" rain clouds over the Amazon

Forest fires increase weather turbulence and have global implications

Forest fire in the southern part of the Amazon basin © M. Welling, Max Planck Institute for Chemistry
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Large-scale slash-and-burns cover the Amazon basin each year during the dry season, especially in the months of September and October, with thick smoke. Initial results of an international study now show that the effects of the smoke of these fires on weather and climate are much greater than previously known. The smoke affects cloud formation and precipitation and leads to thunderstorms and hail. The altered cloud properties also lead to warming of higher layers of the atmosphere, which could have global effects on the climate.

Aerosols as climatic factors

Small particles in the air, called aerosols, are of great importance for cloud formation. Condensation of water vapor on these particles causes droplets of cloud, and no aerosols in the atmosphere can form clouds. Aerosol particles are formed by both natural and human-influenced processes. Since the Industrial Revolution, the number of human-made particles in the atmosphere has increased dramatically.

Therefore, the influence of aerosols on the atmosphere and the climate has been the focus of scientific interest for decades. A direct effect is that these particles reflect the sunlight and thus reduce the heating of the earth's surface by the sun. This means a cooling of the atmosphere. Another effect is that the particles can alter the properties of the clouds and precipitation. This eventually leads to a cooling effect. Overall, these cooling effects over the past century may have offset a significant proportion of greenhouse gas warming.

Fire clearings as aerosol spin

The number of particles in the atmosphere is quite small under natural conditions. Therefore, the concentration of cloud droplets is low when clouds form in clean air. In polluted air, on the other hand, the number of aerosol particles and thus also the cloud droplets is much larger. Clouds with a higher number of droplets reflect more sunlight and therefore - just like with the direct aerosol effect - have a cooling effect on the earth.

HPV vaccination in a young woman © CDC

However, since the amount of condensable water vapor remains the same, their size must decrease with increasing number of droplets. Now smaller droplets do not collide so easily, which is a prerequisite for the formation of precipitation. Thousands of cloud droplets need to merge to form a droplet large and heavy enough to fall to the ground as raindrops. This aspect forms the focus of the scientific project SMOCC (Smoke, Clouds, and Climate), an international research project led by Prof. Meinrat O. Andreae from the Max Planck Institute for Chemistry. "We want to better understand how the huge amount of smoke particles produced by slash and burn in the Amazon affects clouds, weather and climate, " says Prof. Andreae. display

Smaller droplets due to smoke

The Max Planck researchers found that the anthropogenically produced smoke actually dramatically reduces the cloud size in the clouds. As a result, the formation of precipitate was suppressed; when it did, the formation of precipitation shifted from about 1, 500 meters above the cloud base in unpolluted clouds to more than 7, 000 meters in pyro-clouds. Also so-called "clouds of smoke", which grew out of the polluted smoke vapor, showed a considerable suppression and delay of precipitation formation.

The delayed formation of precipitates leads to heat transport to higher layers of the atmosphere. Heat is released when water vapor condenses on the aerosol particles and when the liquid water freezes to ice. At higher altitudes, it is cold enough to freeze the water. The heat released increases the upswing and leads to greater turbulence. This can cause thunderstorms, lightning, heavy showers and hail. Up to two centimeters of hailstalks were observed on the ground in this type of cloud.

Another effect of the suppressed and delayed formation of precipitation is that the large amounts of heat released lead to considerable changes in the regional and global air circulation can. Aerosol particles, water vapor and gaseous pollutants can be transported through these clouds to great heights of the atmosphere and thus over huge areas, possibly even over distribute the whole globe. Therefore, these effects could have global implications, which will be the subject of future investigations.

(MPG, 01.03.2004 - NPO)