The snow-to-liquid equivalent is the amount of liquid precipitation that is produced after melting snow. The temperature profile of the troposphere and the surface temperature are important factors that determine this value. The “average” snow-to-liquid ratio is 10:1. This is saying that if 10 inches of snow fell and that snow was melted it would produce 1 inch of liquid precipitation in the rain gauge.
Wet Snow with Above-Freezing Ground
Wet snow is snow that has a high liquid content as it reaches the surface. It needs to be at least 50% made of ice or it will have more characteristics of being a raindrop instead of a snowflake. It gets this liquid content by partially melting before it hits the ground. The wetness of the snowflakes makes it easier for snowflakes to stick together as they fall, thus a wet snow will often have large snowflakes and a smaller number of snowflakes. If the ground temperature is above freezing, the snowflakes will continue melting. In these situations a snow event can occur but with no snow accumulation on the ground. The precipitation might only accumulate in grassy areas and the precipitation rate will need to exceed melting rate for it to accumulate.
Wet Snow with At or Below Freezing Ground
In this situation the snow will easily accumulate on the ground. This is the best situation for producing snow where the making of snowballs is the easiest. The snow is sticky due to its high partial liquid content. The ratio for wet snow will be less than 10:1. For example, a 5:1 ratio may occur in which it takes 5 inches of snow to produce 1 inch of liquid equivalent.
Heavy Snow – Regardless of the ground surface, this snow will quickly accumulate. Snow is highly insulating, thus heavy snow will accumulate and slow the rate that it melts. If the ground is above freezing, a heavy snow will accumulate faster than it melts. This will accumulate a slushy snow that is good for making snowballs. When heavy snow occurs with a below freezing surface and with below freezing temperatures aloft, accumulation will be immediate and significant.
Dry Snow – A dry snow has little to no liquid water content thus this snow will be less dense than average. Less dense meaning there will be a lot of air pockets between the snow crystals. Dry snow is not sticky and thus it is difficult to make snowballs with it and the wind blows it around substantially even after reaching the surface. The ratio for dry snow will be greater than 10:1. In extreme cases it can be 30:1 or greater. Dry snow occurs when the temperatures throughout the troposphere are well below freezing and the surface temperature is below freezing. Since dry snowflakes are less sticky they are less inclined to stick together as they fall, thus a dry snow will often be composed of a large number of small snowflakes. These snows are the hardest to remove from the road since they are difficult to melt away. It takes a snow plow to remove this snow in the short term.
Pitfalls Involving Snow-to-Liquid Equivalent
A pitfall is a forecasting problem that is not considered or complicates the weather situation. Watch for these pitfalls during a snow event:
- WHERE IS THE ACCUMULATION?- This pitfall occurs when the precipitation is light or quickly melts when reaching the surface. An extreme case is the expectation of snow and yet rain occurs. The accumulation is a bust in this case. In another case, snow accumulation can be lost to the precipitation being too light. If the melting rate is greater than the snow rate then what will be observed is just a melting snow as it falls. When the ground is too warm, accumulation may only occur as the snow falls. Once the snow stops, then the accumulation will quickly melt. If the snow is too wet, it will not maintain the crystal configuration as it falls and hits the ground. Snowflakes will be seen in the air somewhat but will immediately melt once they hit the ground.
- I KEEP SEEING SNOW FALLING YET THE REPORTS ARE FOR CLOUDY SKIES- This pitfall occurs since many observations are automated. In high snow-to-liquid ratios in which the precipitation is light, the automated station may not pick up that snow is falling. The snow is so fluffy that a tipping bucket gauge is not able to record it. The visibility is not low enough to detect the precipitation. In these cases a human observer will think it is obviously light snow but the automated station will just keep reporting cloudy skies and no precipitation. I’ve seen half an inch of dry fluffy snow fall throughout the day yet the automated station records nothing but cloudy skies hour by hour. The pitfall comes into play when the observer only goes by what the automated station is recording. Make sure to look outside to see what is happening!
- THE MODEL SAYS 0.3 INCHES, THUS 3 INCHES IS EXPECTED!- This pitfall assumes the snow-to-liquid equivalent will be 10-1. This is a common forecasting mistake since the average occurs less than what happens in reality. As a forecaster, ask if the situation is more likely to be less than 10:1 or greater than 10:1. In wet snow situations (above freezing ground, significant melting aloft), the snow-to-liquid equivalent will often have a lower snow accumulation than the 10:1 ratio. With all the melting, the snow-to-liquid equivalent may only by 5:1 in grassy areas and with there being little accumulation on the roads. In these wet snow situations, often the snow will accumulate best in the grassy areas and melt significantly on the roads. Wet snow is especially prone to compaction by gravity. A portion of the accumulation may appear lost several hours after it stops snowing due to the snow compressing (losing some air pockets between ice crystals due to the weight of snow).
- THE MODEL HAS 0.5 INCHES RAINFALL AND THE TEMPERATURE SUPPORTS SNOW AT THIS VALID TIME! IT IS GOING TO SNOW 5 INCHES!- This is a common pitfall. A forecast model liquid equivalent accumulation is typically for 6 hours or 12 hours. The temperature at the end of the 6 or 12 hours can be much different than what occurred previous to this time. The atmosphere may only support rain for much of the time and may only change over to snow close to the valid time. If several hours of the accumulation are lost to rain or another precipitation type then there will be a small accumulation of snow. The atmosphere MUST support snow over the entire time period that precipitation occurs in order for all the precipitation to be snow. This may sound easy enough but it is a common mistake. Entire events can be misdiagnosed as snow when the model is really forecasting rain or another precipitation type in which the precipitation ends just as temperatures become cold enough to support snow.
Keep all these issues in mind mentioned in this writing when the next snow event is expected. Pay attention to the temperature structure of the troposphere and how it evolves during the precipitation event, the ground temperature, the expected snow-to-liquid ratio (if it is leaning toward a wet snow or a dry snow), what portion of the precipitation event will be snow, the snow intensity, and how the snow accumulation is being measured.
Next: Basic ice mycrophysics considered when forecasting snow