VOA慢速英语--研究表明森林火灾产生的烟雾会使得大气降雨减少

A new study suggests that smoke from wildfires can change how clouds store water and could lead to less rainfall.

The research comes as western parts of the United States are currently experiencing drought conditions after several years of intense wildfire activity.

A team of scientists took to the skies in western U.S. states during the summer of 2018 to study the effects of wildfire smoke on clouds. They flew a C-130 research airplane through clouds with instruments to measure gas and particle levels.

In 2018, California suffered its most destructive wildfire season ever. Other western states, including Oregon, Colorado, and Washington, have also experienced intense wildfire activity in recent years.

Cynthia Twohy is an atmospheric¹ scientist at NorthWest Research Associates and Scripps Institution of Oceanography in San Diego. She led the new study, which recently appeared in Geophysical Research Letters.

Twohy said in a statement she decided² to center on cloud research because she wondered if there was a link between the increase in wildfires and a lack of rain.

"I started thinking, ‘What are the long-term effects of this? We have drought, and we have a lot of wildfires, and they're increasing over time. How do clouds play into this picture?'" she said.

Clouds contain a collection of water droplets⁴ that form from vapor⁵ in the atmosphere. When these droplets condense onto each other within a cloud, they become heavier and eventually fall as rain.

But the water condensation⁶ process requires a solid substance for the vapor to attach to. Different kinds of particles rising up from the Earth's surface can reach clouds and enable this process.

The new study found that smoke from wildfires contains small particles that affect the way droplets form in clouds.

The main effect was the amount of water droplets formed. The team reported the smoky clouds contained about five times the number of droplets as non-smoky clouds. But data also showed that the droplets in smoky clouds were much smaller, about half the size, as those in the clouds without wildfire smoke.

It is that size difference, the researchers say, that could reduce the amount of rainfall. This is because small droplets are less likely to grow and eventually fall to the ground as rain.

"Because of their small droplet³ sizes, these smoky clouds are expected to reflect more light and produce less rain than clouds in clean air," the study states.

The finding could mean that active wildfire seasons in the western U.S. may in fact lead to less rain and more drought, the researchers said.

"We were surprised at how effective these primarily organic particles were at forming cloud droplets and what large impacts they had on the microphysics of the clouds," Twohy said.

The researchers noted⁷ that past studies had also found similar changes in cloud droplets related to wildfire smoke in the Amazon. However, the new study points out that, in addition to the cloud droplet changes, "a number of complex and competing" elements might also be affecting climate in the U.S. West.

Twohy also noted that different kinds of clouds behave differently. The current study examined changes in small, cumulus clouds. Other kinds that sit much higher in the atmosphere can cause heavy storms, she said.

Twohy added: "I'm hoping these results will spur detailed⁸ regional modeling studies that will help us understand the net impact of smoke on clouds and climate."

Words in This Story

drought – n. a long period of time during which there is very little or no rain

vapor – n. a substance in a gaseous⁹ state as opposed to a liquid or solid

condense – v. to change from a gas to a liquid

reflect – v. to send back or bounce off

organic – adj. used in chemistry to describe chemicals that contain carbon

impact – n. the effect a person, event or situation has on someone or something else

cumulus (cloud) – n. a type of thick cloud that is rounded on top and has a flat base

spur – v. to urge someone to do something or something to happen

net -– adj. complete or total

1 atmospheric   

adj.大气的，空气的；大气层的；大气所引起的

参考例句：

• Sea surface temperatures and atmospheric circulation are strongly coupled.海洋表面温度与大气环流是密切相关的。
• Clouds return radiant energy to the surface primarily via the atmospheric window.云主要通过大气窗区向地表辐射能量。

2 decided   

adj.决定了的，坚决的；明显的，明确的

参考例句：

• This gave them a decided advantage over their opponents.这使他们比对手具有明显的优势。
• There is a decided difference between British and Chinese way of greeting.英国人和中国人打招呼的方式有很明显的区别。

3 droplet   

n.小滴，飞沫

参考例句：

• The rate of droplet growth under different conditions can be evaluated.可以计算在不同条件下的云滴增长率。
• The test results showed that increasing droplet size was associated with better stability.试验结果表明，增加液滴尺寸将使稳定性提高。

4 droplets   

n.小滴( droplet的名词复数 )

参考例句：

• Droplets of sweat were welling up on his forehead. 他额头上冒出了滴滴汗珠。 来自辞典例句
• In constrast, exhaled smoke contains relatively large water droplets and appears white. 相反，从人嘴里呼出的烟则包含相当大的水滴，所以呈白色。 来自辞典例句

5 vapor   

n.蒸汽，雾气

参考例句：

• The cold wind condenses vapor into rain.冷风使水蒸气凝结成雨。
• This new machine sometimes transpires a lot of hot vapor.这部机器有时排出大量的热气。

6 condensation   

n.压缩，浓缩；凝结的水珠

参考例句：

• A cloud is a condensation of water vapour in the atmosphere.云是由大气中的水蒸气凝结成的。
• He used his sleeve to wipe the condensation off the glass.他用袖子擦掉玻璃上凝结的水珠。

7 noted   

adj.著名的，知名的

参考例句：

• The local hotel is noted for its good table.当地的那家酒店以餐食精美而著称。
• Jim is noted for arriving late for work.吉姆上班迟到出了名。

8 detailed   

adj.详细的，详尽的，极注意细节的，完全的

参考例句：

• He had made a detailed study of the terrain.他对地形作了缜密的研究。
• A detailed list of our publications is available on request.我们的出版物有一份详细的目录备索。

9 gaseous   

adj.气体的，气态的

参考例句：

• Air whether in the gaseous or liquid state is a fluid.空气，无论是气态的或是液态的，都是一种流体。
• Freon exists both in liquid and gaseous states.氟利昂有液态和气态两种形态。