Latest Score:


in 2019

score trend is up over time
Weight: 10%

Acid rain was first noted as a threat to forests in the mid 20th century1. Acid rain harms forests by physically damaging leaves and roots, as well as leaching important nutrients from soils. Some species such as red spruce (Picea rubens) and sugar maple (Acer saccharum) are particularly sensitive to acid rain, causing observed long-term decline. Due to these declines, the Clean Air Act and subsequent amendments were enacted; since then, there has been a steady increase in the acidity of rain. Here, acid deposition is assessed as the mean annual pH of precipitation in Underhill, VT. The pH scale runs from zero to 14, 7 being neutral. The pH lower than 7 is acid and higher than 7 is base. Note that normal, unpolluted rainfall has a pH of about 5.6 on this logarithmic scale. The current year is scored as the distance between the minimum and maximum values.

1Likens, G.E., Wright, R.F., Galloway, J.N. and Butler, T.J., 1979. Acid rain. Scientific American, 241(4), pp.43-51.

-- Expert interpretation for Precipitation Acidity is not available--

The score is calculated using a target value and the historical range of the the entire long-term dataset. The higher the score, the closer this year's value is to the target.

Once the score is computed for each year, the trend in scores over time is calculated. If the trend is significantly positive or negative, the long-term trend is marked as increasing or decreasing respectively.

Component Description
Scored as

Distance between minimum and maximum (scaled 1-5)

Target value


Directionality of scores

Higher values in the data are better.

Minimum value used in scoring

Data minimum - 10% of range

Maximum value used in scoring


Data on the pH of precipitation were accessed from the National Atmospheric Deposition Program (NADP)1. Data were collected from the Underhill, Vermont, station. We set a target for precipitation acidity of 5.6, based on a previously established value2. The annual score was computed as the difference between the lower scoring bounds (minimum value in the data minus 10% of the range) and the target value (5.6). This difference was then scaled between 1-5. Values above the target receive a 5.

1 National Atmospheric Deposition Program. 2017. Available at:
2 Vermont Department of Environmental Conservation. 2018. Available at: