Photo of a recently retrieved frost tube during the Snow Depth and Snow Water Equivalent (SWE) measurement. Sleepers River Research Watershed, Danville, Vermont.
Soil frost measurements have been made at Sleepers River Research Watershed starting in 1983. Measurements were made by U.S. Army Cold Regions Research and Engineering Laboratory from 1983 to 1993, followed by the U.S. Geological Survey from 1993 to the present. Measurements started at 5 sites and has increased to 10 sites. Each site has 2 to 8 frost tubes. Sites range in elevation from 225 to 670 meters and are in a mix of field and forest types. Soil frost measurements are made using clear flexible polyethylene tubes are filled with dilute methylene blue solution suspended in 25 mm PVC casing; on freezing the methylene blue remains in the liquid phase, yielding clear ice that marks the depth of soil frost. Soil frost measurements typically are made 2 to 4 times a month between November and May.