Mountain Rain

Mountain Rain Thunder Lake, Rocky Mountain National Park
Photo Credit: NPS.gov

Pollution from cities could dry out nearby mountains.


How pollution leaves hills high and dry. I'm Bob Hirshon and this is Science Update.

Air pollution from cities can deprive nearby hills and mountains of rainfall. This according to meteorologist Daniel Rosenfeld of Hebrew University in Jerusalem. Looking at fifty years of data from a mountaintop in China, he and his colleagues found that while air pollution has steadily increased, rainfall has decreased.


Not only that, but when we look into the day-to-day variability in the amount of rain over the mountain, it is going together with the day-to-day variability in pollution.

Normally, moist air deflects up a mountainside, then cools and condenses into rain over the mountaintop. But if that moisture condenses on air pollutants, the drops don't get big enough to fall. Rosenfeld says this effect could dry up alpine watersheds, causing problems for the very cities creating the pollution. I'm Bob Hirshon, for AAAS, the science society.

Making Sense of the Research

When you think of air pollution, you probably imagine smog hanging over a city, or smoke belching out of a factory. This study demonstrates that air pollution doesn't just stay local: it also affects seemingly pristine areas nearby. What's more, the air pollution affects mountain areas in ways that could threaten the livelihood of the cities causing the pollution.

The air pollution in question includes super-small particles of dust, ash, and other substances, coming from a wide variety of sources, from car exhaust to burning trash. Prior to this study, satellite imaging had shown that clouds tend to form smaller water droplets when they're downwind of major cities. As you heard, that's because water droplets in clouds normally condense on naturally occurring particles in the air. Polluted air has a lot of extra particles, which makes the cloud water spread itself too thin. As a result, they form into rain droplets much more slowly.

But would this actually result in less rain on the ground? That's what Rosenfeld's team wanted to find out. They decided to study mountain rainfall, since clouds naturally form in the foothills of mountain ranges, and condense into rain as they pass up the slopes and drift toward the peaks. As a result, mountaintops normally get a lot of rain and snow. However, the clouds that form over mountains are short-lived, and sometimes evaporate before the raindrops get big enough to fall. Therefore, Rosenfeld reasoned, these kinds of clouds would be most vulnerable to the drying effects of pollution particles. The researchers chose a location in China because China's air pollution has increased rapidly in the last few decades.

Overall, the data showed that visibility from the mountain is now only half of what it was in 1954, with visibility being a pretty good indicator of air pollution. During that time period, rainfall has also decreased by 20 percent. However, the day-to-day relationship between mountain rainfall and pollution was most convincing. On cleaner days, mountain areas normally get much more rain than the lowlands, because clouds constantly form over them and condense into rain. But on more polluted days, the rainfall on this Chinese mountain was much closer to that in the drier, lower altitudes. Similar trends have been observed in other dry, mountainous areas, like the Middle East and American Southwest.

As the report mentions, what goes around, comes around. Because mountains get so much rain, they're great sources of water, and gravity conveniently takes that water downhill in the form of rivers and streams. Often, mountain areas (like the Rocky Mountains) feed rivers (like the Colorado River) that provide the livelihood for heavily populated areas (like Arizona and Southern California). In fact, the Colorado River is drying up today, and Rosenfeld says air pollution is partly to blame. Rosenfeld notes that the tiny particles that cause this effect aren't generally regulated as strictly as other kinds of air pollution. His study suggests that these small pollutants may deserve bigger notice.

Now try and answer these questions:

  1. Why do mountains often get more rain than lowlands?
  2. How does air pollution reduce the rainfall from a given cloud? Why is mountain rainfall especially vulnerable?
  3. Why does it matter that daily rainfall varied with day-to-day pollution levels? What if they found only that pollution has increased overall since 1954, and rainfall has decreased?
  4. If this problem ultimately dries up watersheds for populated areas, what are some potential consequences?

You may want to check out the April 27, 2007, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: the science of deja vu, how pollution from cities affects rainfall on mountaintops, a machine that can make almost anything, diets that just don't work, and why celebrities don't make great salespeople.

Going Further

For Educators

In the National Geographic News article, Mountain Ecosystems in Danger Worldwide, read about how the pressures of modern life are causing permanent ecological damage to the world's mountain ranges.

Also from National Geographic News, Rainy Days Driven by Traffic Patterns describes a study that reports that summer rainfall in the Southeast United States peaks during workdays and declines on weekends due to air pollution from vehicle congestion.

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