ROCHESTER, NY (WROC) – Snow ratio is the amount of water content within a column of snow. It can be measured by collecting snow in a large rain gauge and then melting that snow to see how much liquid is in the gauge. Calculating the ratio comes from comparing the amount of snow produced to one inch of rain, so if five inches of snow fell and it melted down to 1″ of water, that would be a 5:1 ratio.
A general rule of thumb is that when temperatures are closer to freezing, snow ratios will be lower because some of the molecules within a snowflake may be closer to a water droplet than ice. These are usually ice needles or plates. As the air gets colder, ratios start to increase as more of the snowflakes can fall as ice (called dendrites) that will take up more space and end up piling high. Their geometry is important in deciding how high the snow pile will end up getting.
Most models forecast using the general standard of 10:1. That is ten inches of snow equals one inch of water equivalent.
All snowfall can be troublesome for drivers on the road, but it may take only a few inches of heavy, wet snow to cause slick roadways. Three inches of very light and fluffy snow may cause less of an impact as it does not stick as easily to surfaces. According to climatology, Rochester averages a snow ratio of about 15:1.
Put together by St. Louis University, this is climatology data from 1971-2000 of snow events that measured at least 2″ and at least .11″ liquid equivalent. Source
In order to forecast snow ratios, we must look at the temperature at which snow starts to form. That gives us a key to which type of snowflake will fall. Below is a chart that shows the type of crystal that is formed based on temperature. The dendrites correlate with higher snow ratios.
From here, we need to use a chart that shows what snow ratios can be based on the temperature. The temperature to use is important when it comes to forecasting and there are three general ways to find which temperature to use within the atmosphere. You can find sounding profiles here.
- Using the surface temperature. This is convenient because you often have that with you. It can provide a rough estimate, but not the most accurate.
- Using the warmest temperature within the column of air. Ideally this layer is also saturated, so there is some snow growth and anything falling out of that layer will freeze, so it will remain as that shape.
- Finding the saturated layer with the most lift. This assumes that there will be ice crystal formation at the point of lift as that later is also saturated. This will create the most accurate snow ratio forecast. An algorithm to calculate possible snow ratio using this method through the entire column of air is known as the Cobb method after Daniel Cobb at the National Weather Service office in Caribou, Maine.
This “all important” graph is your guide for forecasting snow ratios that can help with winter weather forecasting. Snow ratios can vary hour-by-hour, storm-to-storm, and location-to-location even within a single storm. Happy forecasting!