we get asked this question a lot by cruising students & new charter skippers, who get asked this by their crew members: LET US HELP YOU ~
Q: wow can I tell how strong the wind is just by looking at the sea?
A: the BEAUFORT wind force scale was devised in 1805 by british admiral SIR FRANCIS BEAUFORT
initially, the scale did not reference wind speed numbers but related to effects on the sails ~ after many years of evolution the scale is now an excellent visual guide to the wind conditions
by the way, don't go out on the water when you see the flags above - those two code flags together indicate a HURRICANE WARNING!