Q: Can pollution affect the weather?

A: It can and often does in several ways, says Brent McRoberts of Texas A&M University. One of the most obvious ways is in the form of smog, which occurs when a layer of cold air traps a layer of warmer air underneath. "If there is any pollution in the area, it will get trapped too," McRoberts explains. "When heavy smog develops, many people can suffer breathing problems. There is also growing evidence that pollution affects the monsoon rains in Southeast Asia. Many scientists believe heavy pollution in the area has weakened the intense rains that occur in monsoon-prone regions, such as India and Indonesia."

Q: What other ways does pollution affect the weather?

A: We know beyond question that pollution can cause acid rain, which happens when pollutants in the air mix with rainfall, McRoberts adds. "Acid rain can be caused by the burning coal or other industrial products, vehicle exhaust or power generating plants. Some types of acid rain occur naturally, such as dust emitted from volcanic eruptions, while most is man-made. Acid rain can affect vegetation, especially trees, and can cause large fish kills in lakes. In general, the most polluted cities in the world are Beijing, Shanghai, Calcutta and Mexico City, while in the United States, Los Angeles, Houston, Atlanta and the Washington, D.C.-Baltimore areas are heavy regions of pollution."

Weather Whys is a service of the Department of Atmospheric Sciences at Texas A&M University.