How can fit curve on this log data?

Question

I have a problem with a fit curve on this data:

On x axes we have a data

Answer 1

The problem is the model itself. The RHS of the model can be written:

k * exp(-1/2 * x.wind / sigma^2) * exp(1/2 * mu /sigma^2)

so k and the rightmost factor play the same role. The parameters are not uniquely determined.

To fix this omit k or the last factor from the model.