Snow-to-Liquid Ratio Overview
By Charlie Peachey

Collecting the Precip Can for measurements.
Forecasting and measuring precipitation in the winter is often a difficult process at the summit. The thermodynamics involved with the water cycle tend to become a lot more complicated once water freezes, so it can lead to greater uncertainties in a forecast which are hard to avoid. In order to try and overcome this issue, there are typically three different steps that meteorologists take when creating a forecast:
1) Determining how much liquid water a storm is expected to bring
2) Determining what type of precipitation there will be (snow, rain, freezing rain, etc…)
3) Applying the most accurate snow to liquid ratio

Diagram depecting the different snow-to-liqued ratios that are possible per one inch of rain. The temperatures are approximately where the shown ratio is produced, but not exactly true every time.
The snow-to-liquid ratio (SLR) is the amount of snow produced by a given amount of liquid water. It’s typically expressed as the number of inches of snow per inch of water, with the commonly cited ratio being 10:1 (10 inches of snow for every inch of water). However, this can vary depending on atmospheric conditions like temperature and humidity.

Diagram depicting the various possible snowflake shapes produced at different temperatures and humidities. These pictures were taken by Snowflake Bentley back in the early 1900’s.
Variation in SLR can range widely—from as low as 1.9:1 to as high as 100:1 in extreme cases. This means using a simple 10:1 ratio without considering factors such as temperature, wind, and snow crystal formation can lead to large errors in snowfall forecasts. The Mount Washington Observatory’s extensive record of synoptic weather data offers an opportunity to refine these ratios, particularly for forecasting in mountainous environments like Mount Washington, where the weather is highly variable and extreme.
We aim to figure out SLR climatology to improve the accuracy of snowfall forecasts and address the limitations of the common 10:1 rule. This project seeks to go beyond the simplistic 10:1 rule by establishing a climatology of SLR on Mount Washington. By doing so, the Observatory will have a more nuanced understanding of how different environmental parameters—such as temperature, and humidity—affect the snow-to-liquid ratio. This will not only improve our higher summits forecasts but could also be applied in avalanche prediction and outdoor safety initiatives in the surrounding area.
We will analyze historical weather data from the Mount Washington summit to determine SLR climatology. The project will leverage data collected at the summit weather station since 1950. Using a combination of synoptic precipitation, temperature, and relative humidity, the Observatory team will identify patterns and relationships that govern the SLR.
The goal is to apply the findings to improve the accuracy of forecasts at the summit. By creating an operational schema from the SLR climatology, the Observatory will enable its weather observers to make more precise snowfall forecasts. This will help outdoor recreationalists better prepare for the harsh conditions on Mount Washington and could provide critical insights for avalanche forecasting. Collaboration with the Mount Washington Avalanche Center is also being considered to further study SLR climatologies at different elevations to see if there is a correlation.
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