Forest fires produce large amounts of smoke. And researchers have long known that the suspended particles they carry can help condense water droplets in clouds. However, droplets are so small that they surprisingly prevent the formation of rain.
When flames ravage forests, the smoke carries with them a quantity of tiny particles. Their impact on, the clouds or the has not yet been well studied. But (United States) took advantage of the in the United States in 2018 to conduct seven research flights on this subject.
Remember that theform when water vapor condenses into droplets. Thanks to tiny particles suspended in the . When the air is dry, the little vapor present clings to only a few particles, forming large droplets. Which will end up in rain. But when forest fires have dispersed smoke into the atmosphere, there is no shortage of particles that trap water vapor.
More droplets, less rain
As a result, the instruments carried by the researchers have shown that cumulus clouds formed in the presence of smoke contain, on average, five times morethan the others. However, contrary to what one might imagine, it would appear that this large amount of droplets does not make clouds more likely to produce rain. Quite the contrary. Because these droplets are also half the size of the others. The probability that these collide and merge until they become heavy enough to produce rain would be like this “Practically zero”.
The researchers also note that the smoke particles tend to absorb theof and heat the air around them. Clouds are denser and therefore brighter. They reflect sunlight and prevent the soil from heating up. The temperature difference between the ground and the air at altitude is thus less. And with it, the convective updrafts that form thunderstorms.
The combination of these effects could trigger a feedback loop in which more forest fires lead to morewhich would lead to more fires. In western United States, the number of rainy days during the rainy season has declined by 4% per decade since 1979.