Several terms are used when talking about how much moisture is contained in a certain amount of snow. The most frequently used term is often liquid equivalent. This is the depth of water that would result from melting a sample of snow.
Liquid equivalent is the amount of measurable moisture if the snow were to have fallen as rain. This is where the infamous “10-to-1” ratio has its roots. The “10-to-1” ratio is the assumption that for every 10 inches of snow that falls, there is roughly 1 inch of actual moisture. This ratio is actually only an estimate and is based on snow forming in the 28-34 degrees F range.
If temperatures are colder, say in the 10-15 degree F range, estimates can be as high as 30-to-1 (30 inches of snow equal to 1 inch of moisture/precipitation). This is a simplified estimation because snow liquid equivalent is also subject to temperature and humidity above the surface as well. (Figure 1).
Often, with automated gauges, snow blows out of the gauge before it has a chance to melt. Therefore, estimates can be greatly under-measured. While we do have some heated rain gauges, they only activate once snow covers the sensor. If strong winds continually blow the snow around, it doesn’t have a chance to trigger the heater sensor and thus, is not measured.
Snow that is melted (either by the heater or by warmer temperatures the next day on non-heated rain gauges) and drips into the rain gauge is measured as liquid. The data from these gauges is the liquid equivalent of the accumulated snow.