Normal rain is somewhat acidic, having a pH of about 5.6. This acidity results from carbon dioxide in the atmosphere reacting with water vapor to produce carbonic acid. Acid rain has a pH below 5.6. It results from sulfur dioxide gas and gases of nitrogen oxides reacting with water vapor to form sulfuric acid and nitric acid respectively. Sulfur dioxide is the product of combustion of coal. Nitrogen oxides are the products of combustion of gasoline. Acid rain can seriously affect some lakes, especially in the spring when a pulse of acid enters the lake with melting snow. Lakes have buffering systems that keep the pH of the water near 7 or above, but, over a number of years, acid rain can deplete the buffering capacity of the lake, causing the water to become acidic. Lake organisms vary in their sensitivity to acid waters. Often it is the larval stages of fish and insects that are most vulnerable. As the acidity of the lake increases, the most sensitive fish species disappear first, such as trout and bass, followed by less sensitive species, such as northern pike. Ultimately, the result is a beautiful blue lake with very few species.