Definition
What if the rain falling from the sky were as sour as vinegar or lemon juice? Acid rain is rain that has become more acidic than normal rain after pollutants from power plant and factory smokestacks or car exhaust meet water vapor in the air. It is far more acidic than ordinary rain, so it can dry out and kill forests and slowly dissolve marble statues.
Why does rain turn sour like vinegar?
Coal-fired power plants, factories, and cars burn fossil fuels such as coal and oil. When they do, pollutants called sulfur oxides (smoke from burning sulfur) and nitrogen oxides (gases formed when nitrogen in the air combines with oxygen at high temperatures) are pumped up into the sky.
These smoke particles drift through the air and meet the water vapor in clouds. Once they combine with the vapor, a chemical reaction turns them into dilute sulfuric acid or dilute nitric acid. The very acids you learn are dangerous in a science lab end up being made inside clouds.
Rain that falls from clouds loaded with these acids is acid rain. It doesn't come only as rain. It lands on the ground as snow, fog, and dew too. In short, the smoke people send up goes through chemical changes in the sky and comes back down as harmful rain.
To be a bit more precise
In fact, even natural rain falling through very clean air is not perfectly neutral. Carbon dioxide that is already in the atmosphere dissolves in rainwater and forms carbonic acid. So even clean, unpolluted rain is weakly acidic (about pH 5.6).
Scientists reserve the name acid rain for rain that is more acidic than that, rain with a pH below 5.6. On the pH scale, every drop of 1 in the number means the acidity is 10 times stronger.
So if rain with a pH of 4.6 falls, it is 10 times more acidic than normal rain, and at pH 3.6 it is acid rain 100 times stronger. Near heavily industrial cities, rain nearly as strong as vinegar or lemon juice has actually been recorded.
It dissolves marble and leaves forests parched
When acid rain falls, good nutrients in the soil such as calcium and magnesium get washed away. In their place, harmful substances like aluminum dissolve out and reach plant roots, and the trees slowly wither and die. When the water runs into lakes, fish eggs fail to hatch, and the aquatic ecosystem is destroyed.
Buildings and cultural heritage suffer badly too. The calcium carbonate in marble and limestone dissolves when it meets acidic substances. In many countries, famous statues that had stood for hundreds of years have been losing the outlines of their faces to acid rain.
What's more, the pollutants ride the wind across borders and travel thousands of kilometers. Smoke released in one country destroys the forests and lakes of its neighbors, so many countries have worked together to cut emissions by fitting filters on factory smokestacks. As a result, acid rain in Europe and North America has dropped sharply since the 1980s.
Common misconceptions
If you get caught in acid rain, your hair falls out right away and you go bald.
The acidity of acid rain is usually weaker than orange juice or soda, so there is no evidence that getting rained on makes your hair fall out. The real danger of acid rain is that it slowly destroys whole ecosystems and environments, such as forests, lakes, and buildings.
Where you meet it
Centuries-old marble statues and ancient stone monuments in Europe have been dissolved by acid rain until their shapes blurred.
Lakes in Northern Europe and North America became acidified and turned into dead lakes where fish cannot live.
In one sentence
Rain formed when smoke meets water vapor in clouds and turns into acids, which does great harm to ecosystems and buildings.