well actually dispite what annoymous has said the specific gravity of snow would be a good indicator of how much snow per inch of rain fall. as its a direct weight comparison between snow and water and its a measurement done by volume so its the weight of snow based on weight for a specific volume.
1 inch rain :6 inches of snow, would be a good although rough guide for snow
alternating factors would be wet snow to dry snow, which would vary the compaction, with wet snow being more like a 1:4-5 and dry snow being more 1:6-7
i think 1:10 is something that is just being thrown around and not based on anything factural, seems like a rather too nicely rounded value to be true. got proof? let me know
It really does depend---5 inches of very wet snow (almost to the point of sleet) is about an inch of liquid water, whereas "dry" snow, especially powder, which precipitates at colder temperatures can sometimes be 16 or even 20 inches of snow per inch of liquid water. Generally 8 to 12 inches of snow per inch of water is a pretty good conversion.
or you could just 'catch' a couple inches of snow and let it melt then measure how much water it is
That is definitely a simple and sensible way to make a measurement. Just need to be sure the container has a flat bottom so the depth measurements are valid.
As for the well-known "10-to-1 rule" for the snow/water ratio, here is an explanation of its historical origin:
". . . the 10-to-1 rule appears to originate from the results of a nineteenth-century Canadian study. Potter (1965, p. 1) quotes from this study: 'A long series of experiments conducted by General Sir H. Lefroy, formerly Director of the Toronto Observatory, led to the conclusion that this relation [10 to 1] is true on the average. It is not affirmed that it holds true in every case, as snow varies in density. . . .' The 10-to-1 rule has persisted, however, despite the almost immediate warnings concerning its accuracy."
The U.S. National Weather Service (NWS) uses a conversion table for estimation, with snow/water ratios increasing from 10:1 to 100:1 as surface temperature decreases:
Surface | Snow/water
Temperature | Ratio
28-34 °F | 10:1
20-27 °F | 15:1
15-19 °F | 20:1
10-14 °F | 30:1
0-9 °F | 40:1
-20 to -1 °F | 50:1
-40 to -21 °F | 100:1
"New Snowfall to Estimated Meltwater Conversion Table"
In fact, the NWS table was criticized in the same paper quoted earlier:
Deficiencies of NWS Table 4-9
"'The table’s temperature dependence of density is not based on actual measurements but rather on general impressions in the eastern Tennessee and western North Carolina areas. Hence, the reality of the apparent temperature dependence is uncertain.' As was shown in the previous section, this temperature dependence is, in fact, inadequate."
For a really detailed study, see the following very informative report (127 pages) at the Meteorology Education & Training website of the University Corporation for Atmospheric Research:
Here's what the author says about the ten-to-one rule:
"Ten-to-one rule (10:1) Despite the fact that this rule constitutes the operational tool most frequently used by meteorologists, there is not much more to be said about it. Several climatological studies clearly demonstrated its inaccuracy in about 50% of all cases."
The report proposes a classification of snow into six main categories, with corresponding mean snow/water ratios as follows:
4:1 - very heavy snow
7:1 - heavy snow
10:1 - average snow
15:1 - light snow
20:1 - very light snow
25:1 - ultra light snow
One interesting and important piece of information in the report is that snow is least dense (i.e., "fluffiest") at temperatures around -15°C (+5°F). For this reason, the NWS table also gets criticized by the study:
"In practice, as well as in theory, it has been clearly shown that there is an optimal range of temperatures (around –15°C) where lower densities are observed. At lower temperatures (e.g., –20 or –25 °C), a return to higher density crystals occurs. This peak does not appear in Table 21 [the NWS table]. As a result, this kind of conversion table will tend to overestimate snow/water ratios at low temperatures, and therefore to overestimate snow accumulations."
The next time your local snow forecast turns out to be way off the mark, you may at least understand better why that happened!