Smoke from fires prevents rain from falling

Smoke from fires prevents rain from falling
Smoke from fires prevents rain from falling

During the extinguishing of forest fires, everyone looks with hope in the sky for rain clouds. But they still don't appear. American scientists in the course of their experiment made sure that the appearance of rain clouds is hindered by … smoke from fires.

A team of researchers from universities in the western United States has experimentally tested the known fact that tiny particulate ash in the smoke of wildfires affects the formation of raindrops in clouds. This potentially results in less rainfall and increased drought, which further contributes to the spread of fires.

An article about this study was published in the scientific journal Geophysical Research Letters.

When forest fires release smoke into the atmosphere, tiny solid ash particles from burnt trees and grass are thrown into the atmosphere along with combustion gases. Scientists have named the suspension of these particulate matter in air and combustion products smoke aerosol. Water droplets can condense on aerosol smoke particles in cumulus clouds. And they condense.

The condensation process depends on the height of the cloud in the atmosphere, on the chemical composition of the aerosol, the shape of its crystals and its other properties. For example, volcanic ash aerosol differs from forest fire aerosol.

The presence of aerosol in low and high cumulonimbus clouds also affects the ability of clouds to rain down differently. Let's say, the "mushroom" from an atomic explosion is a cloud filled with solid particles crushed by the explosion of the area. And this aerosol cumulonimbus cloud up to 20 kilometers in height just sheds heavy downpours.

Scientists have known this for at least 60 years.

It is also known that in an aerosol cloud, in contrast to a "pure" cloud, there are more condensation centers - exactly by the number of aerosol particles. This means that the volume of water in the cloud is divided into a larger number of drops. It follows from this that there are more droplets in an aerosol cloud, and each of these droplets is smaller than in a cloud without aerosol, with the same volume of water in both clouds. In this case, a cloud with an aerosol may not give rain due to the too small size of its droplets.

The more droplets in a cloud, the worse it transmits sunlight and the better it reflects it. That is, the aerosol cloud also cools the surface over which it hovers.

And the novelty of the work of American researchers lies in the fact that for the first time they measured the effect of aerosol from forest fires on the probability of rainfall over a specific region - the western United States. Scientists have studied the smoke from wildfires in relatively low cumulus clouds - at an altitude of two to three kilometers.

To find out why smoke aerosol from fires interferes with the formation of raindrops in cumulus clouds above forest fires, American scientists decided by sampling air with droplets from cumulus clouds. They did this while aboard a research plane during the 2018 wildfires in the western United States.

It turned out that a cumulus cloud of smoke aerosol contained five times more raindrops than a "pure" cumulus cloud. But the size of raindrops in a cloud of smoke was half that of a cloud clear of smoke. Water droplets in a typical cumulus cloud grow to a diameter of about 8 micrometers (0.08 mm). And in a cumulus cloud with smoke aerosol from forest fires over the western United States, the average droplet diameter turned out to be about 4 - 5 micrometers. Such a small size of the droplets prevented them from falling to the ground with rain, the authors of the study believe. Recall that the diameter of raindrops usually ranges from 0.5 to 7 millimeters.

"We were surprised how efficiently these predominantly organic [aerosols] particles form droplets in clouds and how much they have an impact on cloud microphysics," says study lead author Cynthia Twohy of the Scripps Institute of Oceanography, University of California, San Diego. …

In high clouds, the addition of more aerosol particles can invigorate the cloud and cause rain. But for low cumulus clouds, the opposite is true, according to the results of an experiment by American researchers, set out in a published article.

“What really excited me about this article was the connection to the hydrological cycle,” says Ann Marie Carlton, a chemist at the University of California, Irvine who was not involved in this study. in precipitation, and the formation of clouds definitely affects the hydrological cycle."

The microphysics of clouds is complex, and the researchers, in addition to the smaller size of the raindrops, in their article also note other factors in the overall effect of smoke on the regional climate. For example, in small clouds, more numerous and smaller droplets reflect sunlight more strongly and thus cool the surface of the earth above which they are.

The study by American scientists focused on small cumulus clouds that cover about a quarter of the western United States in summer. However, other types of clouds, such as high cumulonimbus clouds carrying thunderstorms and squally winds, may exhibit different properties.

As summer rains in the western US diminish, Tui believes that drought-inducing effects are taking precedence over rain-causing phenomena such as the formation of rain clouds.

“Over the past couple of decades, summer rainfall in the region has decreased and temperatures have increased. Cloud effects are likely to be an important part of this process. I hope that our results will serve as an impetus for detailed modeling of regional [atmospheric] phenomena that will help us assess the impact of smoke on the clouds and the climate in the region, "notes Tui.

Earlier, we talked about how scientists measure cloudiness and how the new Russian device will help. We also wrote that UN experts consider climate change "unambiguous" and "unprecedented", and described what will happen to the environment if agricultural land appears in the Arctic.

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