January 12, 2017

Beaufort and Rubrics

A quick post about the Beaufort scale for wind speed, see below, as paradigm of a rubric.

Rubrics are really important in evaluation.

The Beaufort Scale is just a beautiful example of clearly defined, easily observable criteria. I love the way different features like smoke (lower wind speeds) become important at different wind speeds. At force 6, umbrella use becomes difficult - umbrellas are not mentioned elsewhere but it gives you a great idea of what force 6 feels like and so helps to anchor the scale.

In particular, I love the way the individual statements are mostly relatively objective, or rather, inter-subjective, i.e. they are likely to be understood by different people from different backgrounds in a similar way. In the social realm it is often hard to be so objective and so rubrics in project evaluation often include formulations like effective”, reasonably good overall” or just about adequate” which are again not inter-subjective and so in a way beg the question.

This must have been a big improvement for the Navy before the advent of mechanical windspeed measurement.

Another fascinating thing is that it manages to break a quantity like wind speed into no less than 12 different levels. In evaluation, we mostly see rubrics limited to 4-7 levels.

Wikipedia says the Beaufort scale was extended in 1946, when forces 13 to 17 were added, but they are only used in typhoon countries.

I always thought it would be hard to really distinguish 10, 11 and 12 though. And on the other hand, numbers are still needed beyond 12 especially in the tropics.

I wonder if that is because the scale is linearly anchored to windspeed. I would have thought that perception of windspeed, like most other things like light and sound, would be logarithmic so that there should be more frequent divisions at the lower ends and bigger jumps at the higher extremes. If the number/windspeed ratio was logarithmic, 10 11 and 12 would cover ever increasing spreads of windspeed and would neatly extend to cover just about any hurricane.

An example of where assuming a linear relationship with a physical quantity has unfortunate consequences. The developers of the scale would probably have been better advised to ignore the physical windspeed and concentrate on what can be inter-subjectively distinguished. It is not even obvious that the span of each rubric in terms of physical windspeed, or its logarithm, or indeed in terms of anything else, need to be equal at all - even subjectively. That all depends on what the rubric-based rating is going to be used for.

Beaufort number Description Sea conditions Land conditions
0 Calm Sea like a mirror Calm. Smoke rises vertically.
1 Light air Ripples with the appearance of scales are formed, but without foam crests Smoke drift indicates wind direction. Leaves and wind vanes are stationary.
2 Light breeze Small wavelets, still short but more pronounced; crests have a glassy appearance and do not break Wind felt on exposed skin. Leaves rustle. Wind vanes begin to move.
3 Gentle breeze Large wavelets. Crests begin to break; scattered whitecaps Leaves and small twigs constantly moving, light flags extended.
4 Moderate breeze Small waves with breaking crests. Fairly frequent whitecaps. Dust and loose paper raised. Small branches begin to move.
5 Fresh breeze Moderate waves of some length. Many whitecaps. Small amounts of spray. Branches of a moderate size move. Small trees in leaf begin to sway.
6 Strong breeze Long waves begin to form. White foam crests are very frequent. Some airborne spray is present. Large branches in motion. Whistling heard in overhead wires. Umbrella use becomes difficult. Empty plastic bins tip over.
7 High wind, Sea heaps up. Some foam from breaking waves is blown into streaks along wind direction. Moderate amounts of airborne spray. Whole trees in motion. Effort needed to walk against the wind.
8 moderate gale, Moderately high waves with breaking crests forming spindrift. Well-marked streaks of foam are blown along wind direction. Considerable airborne spray. Some twigs broken from trees. Cars veer on road. Progress on foot is seriously impeded.
9 near gale High waves whose crests sometimes roll over. Dense foam is blown along wind direction. Large amounts of airborne spray may begin to reduce visibility. Some branches break off trees, and some small trees blow over. Construction/temporary signs and barricades blow over.
10 Gale, Very high waves with overhanging crests. Large patches of foam from wave crests give the sea a white appearance. Considerable tumbling of waves with heavy impact. Large amounts of airborne spray reduce visibility. Trees are broken off or uprooted, structural damage likely.
11 fresh gale Exceptionally high waves. Very large patches of foam, driven before the wind, cover much of the sea surface. Very large amounts of airborne spray severely reduce visibility. Widespread vegetation and structural damage likely.
12 Strong/severe gale Huge waves. Sea is completely white with foam and spray. Air is filled with driving spray, greatly reducing visibility. Severe widespread damage to vegetation and structures. Debris and unsecured objects are hurled about.

Source: Wikipedia


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