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My rating system for some South Australian snowfalls (snow events).
This is my solution to the surprisingly difficult challenge of creating a simple and practical rating scale for South Australian snow events, from those with only one credible report of snow falling or seen on the ground at the bottom of the scale, to snowfalls which I'm defining as "may be one of the five biggest snow events in our recorded history".
One problem is that snow has a high preference for falling on the upper reaches of mountains and hills in South Australia rather than on lower flatter land where most of the towns are. A second difficulty is that there are no simple rain guage equivalents that can accurately record daily snowfalls. I've never seen in newspaper reports any accurate measurement of the amount of snow that's fallen anywhere in South Australia. I'm always thankful when someone reports the depth of snow they observed on the ground away from wind drifts. A third problem is that there's been no systematic means of reporting snow in South Australia such as there is with rainfall. There are no observers tasked with reporting details of snowfalls within defined areas of responsibility, nothing beyond post offices which are part of the rainfall recording network also reporting whether it snowed or not.
Now back to my rating scale. Imagine making up a rating scale from 1 to 10 for all snow events in our recorded history from 1838 to 2016 inclusive. We calculate a rating for each snow event, based on the number of locations that report snow, the distribution of locations reporting snow, the volume or amount of snow that falls, the area(s) of any snow cover on the ground, the purity of any snow that falls, and any other physical factors we deem relevant.
As we clearly don't have anywhere near the information necessary to rate each of the snowfall events with any precision on our scale, we make do with the limited information we do have to take a best guesstimate of the rating for each event. Fortunately for us, ten categories is only a handful really given the extreme difference between our tiniest and biggest snow events.
Here's my rating scale, with guides to what several of the numbers represent. I've currently restricted its use to the years 2000 to 2016 inclusive, and 1900 to 1911 inclusive, to assist with an eventual comparison of the two periods.
The top rating "10" is "a snowfall which may be one of the five biggest snow events in recorded history and may be a candidate for the biggest snow event in recorded history". "10" is not the theoretically biggest snow event that could happen given our particular climate.
1 = credible report(s) of snow falling or on the ground at one location.
2 = credible reports of snow falling or on the ground at 2 or 3 locations.
3 = (more snow than for rating 2 and less than for rating 4)
4 = (more snow than for rating 3 and less than for rating 5)
5 = reports of snow from a few locations in at least two of the following districts: Southern Mt Lofty Ranges, Central Mt Lofty Ranges, Mid-North, Southern Flinders Ranges, Northern Flindes Ranges, South-East. Usually would include up to a few reports of snow cover observed from a distance on upper reaches of local hills or a local mountain. Usually doesn't include reports of snowfall described as heavy.
6 = As for category "5" but includes more than one report of snowfall described as heavy.
7 = (more snow than for rating 6 and less than for rating 8)
8 = (more snow than for rating 7 and less than for rating 9)
9 = a snowfall which may be one of the ten biggest snow events in recorded history from 1838 to 2016 inclusive.
10 = a snowfall which may be one of the five biggest snow events in recorded history and may be a candidate for the biggest snow event in recorded history.
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