A snow ratio is how much water would there be if you melted the snow. The old saying was on average for every 10 inches of snow, that would equal an inch of water (10:1). However, there are a lot of factors that can adjust the ratio.
To start off, remember that the higher the ratio, that means you’ll need more snow to fall to equal one inch of rain.
Some factors that can drastically change the snow ratio, is the depth of a warm layer of air from the ground to the cloud. The closer this layer is to the freezing mark of 32°F, the lower the ratio will be. If there is more super-cooled water droplets in a cloud, the ratio will be lower. The ratio could be higher if the cloud contains more ice crystals. Wind can also break up the snowflakes falling and lead to higher ratios. Also, it will take more snowfall to equal an inch of water if the air temperature is very cold.
You can easily find out the snow ratio at your house during a snow storm. Bring in your rain gauge filled with snow, and let the snowfall melt naturally. Once it is melted, measure the water in the gauge to the nearest .01 of an inch.
You can also add warm water to melt it quicker. Just make sure to measure the warm water before you pour it in. Then subtract the warm water amount from the total you find.
Recently, it has been found that the average ratio may actually be closer to 12:1 compared to the 10:1 ratio.