Pollution, Mountains and Rainfall

Pollution seems to be affecting rainfall in mountainous areas, there have been many reports of this over the last few years but now there is some hard evidence for this effect.

Daniel Roesnfeld of the Hebrew University in Jerusalem has been studying rainfall, and the amount of pollution on Mt Hua, a sacred mountain in China, over the last few decades. He has compared this rainfall with an area of plains a few km away. As the pollution has increased the amount of rain the mountain gets compared to the plain has decreased.

Rain is produced on mountains (orthographic rainfall) is produced when humid air is blown into the mountain, so it has to rise, this causes it to cool, and the moisture condenses onto bits of dust etc into small drops, then these collide and grow to the point they fall out of the cloud as rain.

If the air is clean, not very many drops form to start with, so they grow quite large and it doesn't take them long to grow big enough to fall as rain, however if the air is polluted, there is much more dust, so far more drops form to start with, so they are much smaller and it takes them many more collisions and much longer to grow large enough to fall as rain, by which point the air is often down the other side of the mountain.

This could be a big issue for many polluted areas that are dependent on mountain rains for their water.