How much moisture is in that snow?

Share Tweet Email

The latest Drought Monitor reports that over 60 percent of Kansas is under some form of drought with the remainder of the state experiencing abnormally dry conditions. At times like these, every precipitation event, whether it’s rain or snow, becomes important. The predominant form of precipitation falling the last several weeks has been in the form of snow. This begs the question, “How much moisture is actually in that snow?”

Several terms are used when talking about how much moisture is contained in a certain amount of snow. In Kansas, the most frequently used term is often liquid equivalent. This is the depth of water that would result from melting a sample of snow. Liquid equivalent is the amount of measurable moisture if the snow were to have fallen as rain. This is where the infamous “10-to-1” ratio has its roots. The “10-to-1” ratio is the assumption that for every 10 inches of snow that falls, there is roughly 1 inch of actual moisture. This ratio is actually only an estimate and is based on snow forming in the 28-34 degrees F range. If temperatures are colder, say in the 10 to 15 degree F range, estimates can be as high as 30-to-1 (30 inches of snow equal to 1 inch of moisture/precipitation). This is a simplified estimation because snow liquid equivalent is also subject to temperature and humidity above the surface as well. Historically, average Kansas snow ranges from 12-14 inches per 1 inch of moisture (Figure 1).

 

 

Figure 1. Average amount of snow per inch of water, 1971-2000. Graphic by Marty Baxter, Saint Louis University, courtesy of UCAR Comet Program – www.comet.ucar.edu.

In the mountains, however, there is more interest in the snow water equivalent. This is the amount of water stored in the entire snowpack not just the most recent snowfall. It is determined by the snow density or the specific gravity of the snow sample. Freshly fallen snow usually has a snow density of 7-15 percent while values as low as 0.4 percent have been measured. The amount of moisture stored in the snowpack is important for predicting runoff, reservoir refill, and flood potential. Unlike in Kansas, snow accumulates all winter and rapidly melts in the spring/summer contributing to issues. Thus far in 2018, snow packs are very low in southwest United States, Oregon, and Colorado with only 9.8-20 inches (Figure 2). Still, snow water equivalents in these mountainous regions are substantial when considered to the little snow Kansas typically receives.

 

 

Figure 2. Current North America snow water equivalent moisture in the existing snowpack. Graphic source: www.nohrsc.noaa.gov.

 

 

Figure 3. Snow survey. Photo courtesy of the NRCS.

Finally, snow is very difficult to measure, especially with a typical automated rain gauge. With manual gauges, samples are taken and then melted.  The latest storms, as reported at the Manhattan NWS Coop station, show the differences that can occur with similar snow depths but slightly different temperatures (Table 1).

 

Table 1.  Daily records from Manhattan Coop station (NWS)

 

Often, with automated gauges, snow blows out of the gauge before it has a chance to melt. Therefore, estimates can be greatly under-measured. This is the case with the Kansas Mesonet. While we do have some heated rain gauges, they only activate once snow covers the sensor. If strong winds continually blow the snow around, it doesn’t have a chance to trigger the heater sensor and thus, is not measured. Snow that is melted (either by the heater or by warmer temperatures the next day on non-heated rain gauges) and drips into the rain gauge is measured as liquid. The data from these gauges is the liquid equivalent of the accumulated snow.  You can see the current measured liquid equivalent or rainfall on the Kansas Mesonet here: mesonet.ksu.edu

 

 

 

Mary Knapp, Weather Data Library
mknapp@ksu.edu

Christopher “Chip” Redmond, Kansas Mesonet
christopherredmond@ksu.edu

Tags: