The chemistry of rain

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Rain has an awful lot of suppositions and accusations connected to it. While it may seem pretty simple and straightforward to say, `when it rains, things get wet,’ is that really all there is to it? Not necessarily so, say many adults who `know’ when its going to rain. They will tell you they can `smell it’ and their `joints ache.’ And doesn’t every sensible almanac reader know that the leaves on trees turn over to catch the coming rain. Last, but not least, is the more general statement `rain gives nourishment to plants.’
Well, is it all so neatly cut and dried (excuse the pun) or are there some half truths to some of these statements? We once did find some truths in some other weather folklore, so how about rain?
First, lets look at the olfactory side of rain. It does seem that on summer days when it is hot and dry, with a thunderstorm brewing just over the next hill, you can `smell the rain.’ Well, you can smell something, but rain? Have you ever tried to smell this same rain in January when the ground is frozen solid? Not a chance, but when the ground and plants are warmer, you can smell something. What you are really smelling comes not from the air, but the ground! Plants release oils that enter the soil and blend with the other earthy odors. These odors are released into the air when the relative humidity at ground level exceeds 75 percent. Moist humid air will transmit odors far better than dry air. In these moist humid conditions we notice these odors more readily. And since rain is so often connected with moist humid air, we tend to associate one with the other.
Does rainy or damp weather really cause aching joints? This has been rather hotly debated between medical groups. Currently the accepted theory, and the one that seems to make the most sense, is that a change in barometric (air) pressure will cause some additional aches and pains to already afflicted areas. And, like humidity, rain tends to follow a drop in barometric pressure.
What about the leaves turning upside down? Well, there is a scientific explanation for this too. You may have heard of the term prevailing winds. Prevailing winds is a name given to the wind direction most often observed in an area. Here in Chester County, they are from the west, northwest in the winter, and southwest in the summer. Leaves that grow under prevailing westerly winds do strange things, when they are suddenly blown in a different direction, such as east, like turning over. And, you guessed it, a change in wind direction often preceeds rain!
The statement that rain brings nourishment to plants is certainly true; rain is necessary to most forms of life. It seems ironic then, to think of rain as a killer, but with pollutants added, rain can be a killer. When water vapor in the atmosphere mixes with certain pollutants the result is acid rain, a nowadays familar term. The key pollutants in acid rain are sulfur dioxides and nitrogen oxides. Only about 10 percent of these pollutants occur naturally from volcanoes, forest fires and natural organic decay. All of the rest come from human use, mainly that of automobile emmisions and burning of coal or other fossil fuels. These pollutants can travel hundreds of miles from the source before they actually fall to the ground as acid rain. High levels of acid rain can damage plants and water resources. Studies have shown that thousands of lakes in the US and Canada are so acidified that entire fish populations have been threatened. While Many countries have enacted strict regulations on pollution control, many developing countries are continuing to dump massive amounts of pollutants into the atmosphere.
There are myths and truths connected with rain but one thing is absolute. Rain should be celebrated, water is the most precious substance on the planet, and it is rain that circulates it. Whether you sing in it, jump in rain puddles, or complain about it, it is still the most vital resource. Let’s keep it clean!

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