This technical note provides guidance on how to convert Height of New snowfall Water equivalent (mm) to depth of new snow (cm). It should be read after reading the Learning Zone Article: Snowfall Data User Guide.
Technical Note: How to convert from Height of New snowfall Water equivalent (mm) to depth of new snow (cm)?
A general guideline to convert Height of New snowfall Water equivalent (HNW) (mm) to depth of new snow (cm) is the “10:1” ratio, which assumes that 1 mm of HNW corresponds to approximately 1 cm of new snow depth1. This ratio is only a general approximation. Actual snow density can vary greatly, not only across Canada, but even from one snowfall event to the next at the same location, as it depends on temperature and surface conditions at the time of deposition on the ground. Hence, relying on a single fixed ratio can lead to misleading results.
To better reflect this variability, two complementary approaches are presented: one based on long-term regional averages, and one that shows the typical range of event-to-event variation across Canada.
Approach 1: Region dependent conversion ratios
Using these ratios, the conversion can be done in two steps:
- Step 1: Convert the projected HNW (mm) into an initial snow depth (cm) by applying the standard 10:1 assumption, where 1 mm of HNW corresponds to about 10 mm or 1 cm of new snow depth.
- Step 2: Adjust for regional differences in new snowfall density by dividing this depth by the regional ratio (ρ) in Figure 2 of Mekis and Vincent (2011).
For example, if HNW for a given snowfall event is projected to be 5 mm, depending on the region, the conversion ratio values can be applied as follows:
- In St. John’s, NL, where the regional ratio (ρ) is 1.5:
- Step 1: 5 mm HNW → 5 cm snow (10:1 assumption)
- Step 2: 5 cm snow ÷ 1.5 = 3.3 cm snow
- In Vancouver, BC, where the regional ratio (ρ) is 0.75:
- Step 1: 5 mm HNW → 5 cm snow (10:1 assumption)
- Step 2: 5 cm snow ÷ 0.75 = 6.7 cm snow
Therefore, in the case of the very wet, dense snow that often falls in Atlantic Canada, 5 mm of HNW would represent 3.3 cm of new snow depth. In the case of the lighter, fluffier snow that often falls in western Canada, 5 mm of HNW would represent 6.7 cm of new snow depth.
Limitations of Approach 1: Estimates of conversion ratio derived from historical snow density may not represent the actual snowfall density in a changing climate and may require some further investigation depending on the specific project goal or application. Conversion ratios, such as those shown in Figure 2 of Mekis and Vincent (2011), are interpolated from weather stations and may not represent HNW to new snow depth conversion ratios at locations that are farther from stations. In addition, these ratios reflect long-term regional averages and do not capture the full event-to-event variability that can occur at any given location. As such, these regional conversion ratios should only be used as general indicators rather than precise predictors of snowfall depth at a location.
Approach 2: Min/max conversion ratios (independent of the region)
Regional ratios like those shown in Approach 1 are helpful for estimating average or total snowfall depth over long periods of time. However, these ratios do not capture variability in snowfall density. Snowfall density can vary considerably at the same location, meaning a regional average will not represent the full range of snowfall densities that can occur at a location, both historically and in the future.To capture that range, Approach 2 proposes to use 15:1 for very fluffy snow to 5:1 for very wet snow, as described by Environment and Climate Change Canada in its Weather Tools: interesting facts page, “How the water equivalent of snow is calculated”1. Approach 2 uses this range for all regions of Canada.Using these ratios, the conversion can be done in two steps:
- Step 1: Convert the projected HNW value in millimetres to snowfall depth in millimetres for very wet snow (i.e., 5:1) and very fluffy snow (15:1). The results describe the range in possible snow depths that correspond with a HNW value.
- Step 2: Because these ratios are in mm of snow per mm of water, convert the snow depth values from mm to cm by dividing by 10
For example, for a small amount of 5 mm HNW:
- If the snow were very wet (5:1 ratio):
- Step 1: 5 mm × 5 = 25 mm
- Step 2: 25 ÷ 10 = 2.5 cm snow
- If the snow were very fluffy (15:1 ratio):
- Step 1: 5 mm × 15 = 75 mm
- Step 2: 75 ÷ 10 = 7.5 cm snow
The above example starts with the same 5 mm of HNW used in Approach 1 and shows that the 5:1 to 15:1 range (2.5 cm to 7.5 cm) encompasses both the results for St. John’s (3.3 cm) and Vancouver (6.7 cm). In other words, Approach 2 captures the range of plausible snowfall depths for a given amount of water, making it more representative of potential variability in snowfall depth.To show the influence of snowfall density on snowfall depth, here is the same calculation for a heavy one-day snowfall of 40 mm HNW:
- If the snow were very wet (5:1 ratio):
- 40 mm × 5 ÷ 10 = 20 cm snow
- If the snow were very fluffy (15:1 ratio):
- 40 mm × 15 ÷ 10 = 60 cm snow
Approach 2 better captures the extremes in snow depth that can be associated with a single HNW value and is useful for contextualizing regional conversion ratios.
Limitations of Approach 2: While this approach captures a broader range of possible snowfall densities, it does not account for regional climate differences. Using the same 5:1 to 15:1 range everywhere may over- or under-estimate variability in a region. In reality, the average range of snowfall density over a region is likely to be narrower, reflecting local climate characteristics. Similar to approach 1, this method should not be treated as a precise predictor of snowfall depth; instead, it provides indicative bounds that illustrate the uncertainty associated with snowfall depth conversion ratios.
To learn about the snowfall data on ClimateData.ca, please read the Snowfall Data User Guide.
