The amount of water contained within a layer of snow can vary dramatically, a fact often misunderstood outside of meteorology and hydrology. Snow is crystallized water, but the space it occupies is mostly air, making its density highly inconsistent. Understanding this liquid content is important for accurate weather forecasting, managing water resources, and predicting the risk of flooding. The ratio of snow depth to the liquid water it holds determines its density, which is a powerful metric for scientists tracking the winter landscape.
The Standard Snow-to-Water Ratio
The commonly cited average for snow density is the 10-to-1 ratio. This means ten inches of accumulated snow will melt down to yield one inch of liquid water. This ratio is a useful general guideline, suggesting the snow is about ten percent liquid water by volume. This benchmark provides a simple conversion widely used for initial snowfall estimates and quick calculations.
However, the 10-to-1 conversion is merely a mathematical average, not a universal law applied to every storm. Snow density refers to the mass packed into a given volume, and this property is rarely constant. The actual snow-to-water ratio can fluctuate significantly, even changing during a single snow event, requiring forecasters to look beyond the simple average.
Factors That Alter Snow Density
Physical and atmospheric variables cause snow density to deviate from the standard ratio. Temperature is a primary influence, as colder air generally produces lighter, drier snow with a higher ratio. When air temperatures are near \(0^{\circ} \text{F}\) to \(10^{\circ} \text{F}\), delicate snowflakes form, trapping air and resulting in ratios as high as 20-to-1 or more. In extreme cold and dry conditions, ratios can occasionally reach 50-to-1.
Conversely, snow falling near the freezing point, often between \(32^{\circ} \text{F}\) and \(35^{\circ} \text{F}\), is much wetter and heavier. This type of snow has a lower ratio, sometimes as low as 5-to-1 or 6-to-1, because the flakes are partially melted or denser. This heavier snow is often called “Sierra Cement” or “heart attack snow” due to its weight and high water content.
Wind also plays a role in decreasing the snow-to-water ratio. Strong winds batter the delicate structure of falling snowflakes, breaking them into smaller, more compact grains. This mechanical process reduces the air space between crystals, increasing the snow’s overall density. Furthermore, snow density increases over time after settling due to gravity compacting the snowpack.
Calculating and Utilizing Snow Water Equivalent
The scientific community uses a standardized metric called Snow Water Equivalent (SWE) to accurately measure the liquid content of a snowpack. SWE is defined as the depth of water that results if the entire snowpack were melted instantly. This measurement is preferred over simple snow depth because the water content determines the water resource value.
Hydrologists measure SWE using different methods, including manual sampling and automated technology. Manual measurements involve using a specialized snow tube to bore a core sample down to the ground, which is then weighed to determine the water content. Automated SNOTEL sites, which are remote weather stations, often use snow pillows—large, fluid-filled bladders—that measure the weight of the overlying snowpack.
SWE data is a fundamental tool for water resource management where snowmelt contributes significantly to the annual water supply. In the Western United States, for example, 60 to 70 percent of water supplies originate from snowpack. Accurate SWE forecasts allow managers to plan for irrigation, adjust reservoir levels, and make decisions about water allocations. This information is also important for predicting spring flooding, as a high SWE indicates a large volume of water poised to enter river systems.