pH measures hydrogen concentration in water. pH is presented on a scale from 0 to 14. A solution with a pH value of 7 is neutral; a solution with a pH value less than 7 is acidic; a solution with a pH value greater than 7 is basic. Natural waters usually have a pH between 6 and 9. The scale is negatively logarithmic, so each whole number (reading downward) is ten times the preceding one (for example, pH 5.5 is 100 times as acidic as pH 7.5). The pH of natural waters can be made acidic or basic by human activities. For example, coal-burning power plants and heavy automobile traffic can produce nitrous oxides and sulfur dioxide, making the rain and snow highly acidic.