Map 1: The earlier of two images on acid rain – 1985
Map 2: The latter of two images on acid rain – 2008
Time-series GIS mapping of a 25 year study of acid rain frequency and intensity in the United States shows a decrease over time.
Acid rain is rain or snowfall with a pH of <5.0 (neutral pH is 7.0 – Hydrochloric acid is ~pH 1 and humans are slightly alkaline at pH 7.4). Acid rain is often caused by industrial emissions of sulphur dioxide and nitrogen oxide. These chemically react with rainwater and become acidic. Over time acid rain degrades most things it comes in contact with – built assets, waterways, wildlife and marine life.
A long term study undertaken by the National Atmospheric Deposition Program, Illinois State Water Survey allowed the use of time series GIS mapping. The maps show that Acid Rain has decreased between the two time periods. The researchers attribute this to the 1990 clean air act.
Time-series mapping can be undertaken in a number of ways. In this case the Survey has chosen to publish a map of Acid Rain in the US from 1985 and another from 2008, and then relies on the viewer to to make a visual comparison. Another approach would be to use the GIS to compare the images and produce a third map representing the change that has occurred between the two time periods.
Further reading: https://www.sciencedaily.com/releases/2011/11/111116162244.htm