Douglas sea scale

The Degree (D) value has an almost linear dependence on the square root of the average wave Height (H) above, i.e., ${\textstyle D\simeq \beta +\lambda {\sqrt {H}}}$. Using linear regression on the table above, the coefficients can be calculated for the low Height values (${\textstyle \lambda _{L}=2.3236,\beta _{L}=1.2551}$) and for the high Height values (${\textstyle \lambda _{H}=2.0872,\beta _{H}=0.6091}$). Then the Degree can be approximated as the average between the low and high estimations, i.e.:

$${\displaystyle D\simeq \left[{\tfrac {1}{2}}\left(\lambda _{L}{\sqrt {H_{L}}}+\lambda _{H}{\sqrt {H_{H}}}\right)+{\tfrac {1}{2}}\left(\beta _{L}+\beta _{H}\right)\right]}$$

where [.] is the optional rounding to the closest integer value. Without the rounding to integer, the root mean square error of this approximation is: ${\textstyle RMSE\leq 0.18}$.

Wavelength

• Short wave 100 m –
• Average wave 100–200 m
• Long wave 201 m +

Wave height

• Low wave 2 m –
• Moderate wave 2–4 m
• High wave 4.01 m +