Precipitation Acidity

Latest Score:


in 2019

Weight: 8%
score trend is up over time

Precipitation with high acidity (a lower pH)—also known as acid rain—was first noted as a threat to forests in the mid-20th century1 and is harmful to trees as it damages leaves and leaches 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. These observations led to the passage of the Clean Air Act in 1970 and subsequent amendments. Since then, there has been a steady decrease in the acidity of rain. Here, acid deposition is assessed as the mean annual pH of precipitation in Underhill, Vermont. The pH scale runs from zero (acidic) to 14 (basic), 7 being neutral. Unpolluted rainfall has a pH of around 5.6. A high score means that measured precipitation pH is at or near this number.

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 value 2. 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 and 5. Values above the target receive a 5.

1 National Atmospheric Deposition Program. 2019. Available at:
2 Vermont Department of Environmental Conservation. 2018. Available at: