Acid rain is no longer the problem it once was because of laws that have been passed since the 1980s. So I guess in that sense it is not an invisible threat because it is not such a threat anymore.
However, in cases where it is a threat, it is an invisible one. Back when acid rain was a problem, it is not as if the rain fell and you could see or feel that it was acid rain -- it didn't dissolve things it hit right before your eyes. Instead, it seemed like normal rain, but gradually it did dissolve things and it did contaminate lakes and such.
So when acid rain is a threat it is invisible because people are not able to know when the rain is acidic or see the damage it causes in "real time."
Acid rain is a popular term used for rain snow or other such precipitation from the atmosphere which have been polluted by some acids present in the atmosphere. These acids mostly include sulphuric acid and nitric acid, which are formed by water reacting with polluting smoke and exhausts released in the atmosphere by motor vehicles, power plants, factories and other establishment and equipments that burn fossil fuel such as coal petrol, diesel and gas. Substances like sulphur dioxide and nitrogen oxides in these exhaust combine with water to produce the acids of acid rain.
In addition to acid mixed with water the acidic substances present in the atmosphere may also reach the earth surface without rain. Bot acid rain plus dry acidic substances are together called acid depositions. These depositions pollute lakes, rivers, and streams killing fishes and plan living is such waters.
Sulphuric acids and nitric acid are the two main acids, which then dissolve in the water in the atmosphere and fall to the ground as acid rain or may remain in atmosphere in clouds and fogs.
Acidification of environment is a man-made phenomenon. This acid rain is infact a cocktail of sulphuric acid and nitric acid. The acid rain problem has dramatically increased due to industrialisation. Acid rains create complex problems and their impacts are far reaching. They increase soil acidity, this affecting land flora and fauna, cause acidification of lakes and streams thus affecting aquatic life, affects crop productivity and human health.
Acid rain occurs when chemicals accumulated in the water cycle return to earth as a toxic substance. The rain has elevated levels of hydrogen ions which gives it an acidic quality. It can be formed as a result of natural decay but the depth to which it has formed in today's society is directly related to the use of fossil fuels which emit sulfur dioxide and nitrogen oxides.
Acid rain has had and continues to have a strong negative effect on the environments on earth. It has resulted in the over acidification of lakes and streams and other water systems. Forests in high elevations have been killed or become diseased by the precipitation. The rain itself does not kill the trees but weakens them to the point that they are vulnerable to disease and insects.
Acid rain has a decomposing effect on marble and limestone which has been used in buildings and structures. Its effect on them can be seen by placing a piece of chalk in a bowl of water overnight. The chalk will begin to dissolve as the water starts to break its outside coating down.
Acid looks, tastes, and feels like rain that is non-acidic, so it is hard to identify it from other rain. At present it is not known to have any direct effect on humans. However, the emissions caused by the same agents that create acid rain are also harmful to humans.