You probably have heard people say 1″ of rain is 10″ of snow. This is called a 10 to 1 (10:1) snow to liquid ratio. It is not always 10:1. It could be as low as 5:1 (a very wet snow) and as high as 60:1 (a very powdery snow). When we forecast snow amounts this is one thing we look at closely. It is also a big challenge.
This means how much snow is going to accumulate with a given amount of liquid equivalent water. The ratio can be variable and is dependent on temperatures over many thousands of feet and where the maximum lift or vertical motion falls within those temperatures. The ratio can commonly vary from 10 to 20 to 1 and infrequently get even higher than that! This means a liquid equivalent of a quarter inch could vary anywhere from 2.5 inches of snow up to 5 inches or more. Generally the colder the temperatures the higher the snow ratios. Heavy, wet snows tend to have ratios closer to 10 to 1, while the lighter, fluffy snow will accumulate at higher ratios of 15 or 20 to 1.
The map below shows the average snow to liquid ratio for different parts of the country. On average our snow to liquid ratio is 12:1 or 13:1.
I went back to look at all of the daily snow totals (0.5″+) for Waterloo and calculated what the snow to liquid ratios were. It was a wide range from as low as 8:1 to as high as 50:1.