Custom wind scale in bft

Normally i prefer wind force unit in bft.
When i want to have a fixed custom scale from 0 to 8 bft shown, then i have to enter a max value of 19 bft in the Edit Scales dialogue.

It seems to happen a miscalculation in this dialogue when wind force unit is set to bft.

It works as expected when wind force unit is set to kt. But there is also a bug when setting the manual scale in unit kt and afterwards switching to unit bft.

bft to kt table

Source Wikipedia

conversion formulas between kt and bft

v in kt
B in bft

v = 1.624 * B^(3/2)
B = ( v / 1.624 )^(2/3)

B rounded to a natural number gives the Bft wind force

Proofed at edge-values of above bft to kt table.

 v/kt  |    B        | Rounded 

 34    |    7.556    |    8
 37    |    8.037    |    8    
 40    |    8.465    |    8

 41    |    8.606    |    9
 44    |    9.021    |    9
 47    |    9.426    |    9

 48    |    9.560    |    10



Shux @tiwag, are you asking me to remember how I did Bft over 2 years ago :slight_smile:

I think I implemented it as ranges. I didn’t know there was an equation for it. I’ll put the bug on my to-do list.

Cheers, Duane.


I was just thinking it’s easier to have formulas for conversion…


@tiwag, you were thinking right. I think the formula is the easier and more reliable way to go.


Today I found another bug with the Bft wind scale.
While watching the tornado Dorian I found the wind scale 0 6 10 12 12!!!
See screenshot

I fear you have to use a real Bft scale with linear growing Bft values and not just display the calculated Bft value instead of the linear growing wind velocity values


Looks like it needs a bit of work. I do plan to work on scales in the bug/polish phase.