You are right Tim. It's just that rounding down to the nearest 5 kts is arbitrary. You can make an argument for rounding to the nearest 5 kts. And you can find an argument for rounding upward to the nearest 5 kts. But rounding down to the nearest 5 kts is just arbitrary. Not that it would matter much. Simply hard to argue in favor of.

-- Chris.

There isn't an "operational" reason, it's just my understanding of how wind barbs are used.

If we were to make a decision to round wind values to the nearest 5 knots and then draw the barbs, a wind value that's given in the What's Here menu as 33 knots would have a barb of 35 knots, which would be equally "inaccurate". I use quotes on that word because we are talking forecasts here, and these differences are both negligible in magnitude and relatively meaningless considering the forecast values are likely to differ in reality by more than the rounding under discussion.

But that is the opposite of how SD does it.

Is there an operational reason why it would be more relevant for the pilot to have the wind speed rounded down to the next lowest multiple of 5 instead of rounded to the closest multiple of 5?

Usually, you would prefer the "nearest" rounding as it would minimise roundoff errors in calculations. The only real exception is when it is important to always round in a "safe" direction, e.g. when checking operating minima.

I guess the barbs represent the closest value (in steps of 5 kts) that is equal to or larger than the actual windspeed. So 51 kts would be represented by a pennant (50 kts) and a half-barb (5 kts) even though just a pennant would be closer to the value. This must be to give the pilot a firm number he can rely on and be sure that the wind will not be stronger than it. There probably is a set definition of how to convert wind speed to barbs. Tim, do you know?

-- Chris.

Is this really such an issue? After all is said and done, what we are talking about here is forecast wind and, as we all know, actual wind can vary from the forecast wind by a margin far greater than 5kts.


Mike
_________________________________________

It is exactly my question. In terms of accuracy, I prefer to see 35 kt with barbs when the wind is 34.6 kt.
Maybe my pictures are not very clear. The white boxes correspond to the graphical barbs of the left of the boxes.
The observed behavior is that, when I see a wind of 35 kt in numeric value (maybe 34.6 kt with more precise value), I only see three 10 kt barbs. In summary, in these case in SkyDemon, numeric value = 35 kt and barbs = 30 kt.


Alexandre
Based at LSGL (Lausanne - La Blécherette, in Switzerland)

Maybe I'm dense, but why is it better to show 34.6 with three 10 knot barbs than with three 10 knot barbs + one 5 knot barb?