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FLY FISHING DICTIONARY
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acid rain  English
  Rain with high acidity. Rain water with a hydrogen-ion density (pH) of 5.6 or less is generally called acid rain.
  Because fluid acidity is called acidic when its chemistry is less than pH 7.0, the question arises why rain of pH 5.7-6.9 is not called acid rain. It is because carbon dioxide that exists naturally in the air melts into rain water, and the pH becomes about 5.6, even without air pollution. Therefore rain water of a pH 5.7-6.9 is considered a normal range.
  The sulphur dioxides (SOx) and the nitrogen oxides (NOx) in exhaust gases that are discharged from factories or cars cause a photochemical reaction with solar light in the atmosphere that changes them into a strong acid, such as sulfuric acid and nitric acid. When these acids melt into rain, its pH falls and it becomes acid rain. Acid rain brings about acidification of soil, lakes and rivers, and therefore has a damaging effect on freshwater fish. When the pH level of the water becomes pH 6.0 or less, preservation of the species becomes difficult, when the pH level falls to 5.0 or less, fish cannot survive. It is believed that salmon and trout are especially sensitive to water acidification. In northern Europe such as in southern Norway and Sweden, high lake water acidity has already caused the total eradication of salmonidae. The problem of acid rain has made Sweden the origin of a measure to now protect the earth environment, and submitted from Europe, the measure has been adopted on an international scale.
hydrogen-ion density
 
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Waterside of mountain stream. Japan.
Waterside of mountain stream. Japan.
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