The Snow-to-Liquid Ratio (SLR) compares the depth of newly fallen snow to the liquid water it contains. This ratio is a metric of snow density, indicating the moisture locked within the frozen precipitation. Understanding this relationship is important for accurate weather forecasting and predicting water runoff. Meteorologists and hydrologists rely on the SLR to determine the total water supply contained in winter precipitation, which is necessary for managing water resources and anticipating flood risks during the spring melt.
The Baseline Snow-to-Liquid Ratio
The most commonly cited figure for the snow-to-liquid ratio has been 10-to-1. This means that, on average, ten inches of newly fallen snow yields one inch of liquid water. This 10:1 ratio functions as a convenient standard for quick, generalized estimates and initial forecast models worldwide. For example, 20 inches of snowfall is expected to contain two inches of liquid water. However, atmospheric conditions rarely align perfectly with this average, meaning the 10:1 figure is often inaccurate for a specific storm event.
Meteorological Factors That Cause Variation
The true ratio of snow to liquid water is constantly changing, driven primarily by the temperature profile of the atmosphere where the snow crystals form and fall. Snow density, which dictates the ratio, varies widely because snow is a combination of ice crystals and trapped air. Deep cold air promotes higher ratios because the snow crystals are smaller and drier, accumulating with significant air space.
In extremely cold and dry climates, the resulting fluffy powder can have a ratio of 20-to-1, or even as high as 30-to-1. This light snow is sometimes referred to as “cold smoke” because it contains very little water, making it easy to clear. Conversely, when air temperatures are near the freezing point, the snow crystals are wet and sticky.
This warmer snow packs together more efficiently, containing less trapped air and resulting in a lower ratio, sometimes falling between 5-to-1 and 8-to-1. This high-density snow is heavy and wet, placing stress on trees and power lines. The shape of the ice crystals also plays a role; complex structures like stellar dendrites trap more air, leading to a lighter snow and a higher ratio, while simpler plates and columns result in denser accumulation.
How Scientists Measure Liquid Equivalent
Scientists use specialized tools and techniques to accurately measure the liquid equivalent during a storm. One common automated method involves using heated precipitation gauges or specialized devices known as snow pillows. These instruments continuously measure the weight of the accumulated precipitation, allowing the direct calculation of the liquid water content.
When automated equipment is unavailable, meteorologists rely on manual measurement techniques, such as snow core sampling. This process involves pressing a tube, often the outer cylinder of a standard rain gauge, down through the fresh snowfall to collect an intact column. The collected snow sample is then brought inside and gently melted to prevent evaporation.
The resulting liquid is poured into a narrow measuring tube to determine its precise depth, typically measured to the nearest hundredth of an inch. This direct measurement confirms the actual liquid equivalent, providing the data necessary to calculate the storm’s real-time snow-to-liquid ratio.