There are many challenges presented to meteorologists in forecasting snow. Some of these challenges are a result of the tools and data that we have at our disposal. Let’s look at one important aspect in forecasting snow: the snow to liquid ratio.
When looking at numerical models, precipitation is forecast in liquid totals. In other words, snow, ice, and other frozen precipitation is melted down and the precipitation values shown in numerical models is the total of the liquid equivalent of the precipitation. Some sites will show snow total graphics from a numerical model, however these graphics are usually based on some assumption that isn’t always valid.
The snow to liquid ratio describes how much liquid would be obtained by melting a certain amount of snow. For example, a ratio of 10:1 means for 10 inches of snow there would be one inch of water, if it is completely melted down. Unfortunately, life isn’t always that easy. The snow to liquid ratio can depend greatly on a variety of factors, such as geography, time of year, and various meteorological conditions. To explain the meteorological conditions, let’s begin by looking at crystal behavior.
Below is a graphic explaining the different types of crystals that can develop at various temperatures.
Types of Ice Crystals
Notice in the diagram that the peak of the graph is between -12°C and -18°C. This temperature range is known as the Dendritic Growth Zone. Dendritic growth is favored in this region because the saturation vapor pressure with respect to ice is lower than the saturation vapor pressure with respect to liquid. As a result, ice crystals will form more quickly than liquid water droplets. Typically for dendritic growth to occur, the relative humidity should be at least 80%. In addition, ascent within the Dendritic Growth Zone should be assessed. Stronger ascent within the Dendritic Growth Zone will produce more snow than those environments with weaker ascent. Meteorologically, the snow to liquid ratio (at least partially) is dependent on the temperature profile of the atmosphere and the ascent, especially in the Dendritic Growth Zone.