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Effects. Significant wave height in open sea.

Smoke will rise vertically.Sea like a mirror. H_s =0.0 m

Rising smoke drifts, weather vane is inactive.Ripples with the appearence of scales are formed, but without foam crests. H_s =0.1-0.2 m

Leaves rustle, can feel wind on your face, weather vane is inactive. Small wavelets, short but pronounced; crests have a glossy appearance, but do not break. H_s =0.3-0.5 m.

Leaves and twigs move around. Light weight flags extend. Large wavelets, crests begin to break. Foam of glossy appearance. Perhaps scattered white horses. H_s =0.6-1 m.

Moves thin branches, raises dust and paper. Small waves, becoming longer; fairly frequent white horses. H_s =1-1.5 m.

Moves trees sway.Moderate waves, taking a more pronounced long form; many white horses are formed. Chance of some spray. H_s =2-2.5 m.

Large tree branches move, open wires (such as telegraph wires) begin to "whistle", umbrellas are difficult to keep under control. Large waves begin to form; the white foam crests are more extensive everywhere. Probably some spray. H_s =3-4 m.

Large trees begin to sway, noticeably difficult to walk. Sea keeps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind. Spindrift begins to be seen. H_s =4-5.5 m.

Twigs and small branches are broken from trees, walking into the wing is very difficult. Moderately high waves of greater length; edges of crests break into spindrift. The foam is blown in well marked streaks along the direction of the wind. Spray affects visibility. H_s =5.5-7.5 m.

Slight damage occurs to buildings, shingles are blown off of roofs. High waves. Dense streaks of foam along the direction of the wind. Sea begins to roll. Visibility affected. 
H_s =7-10 m.

Large trees are uprooted, building damage is considerable. Very high waves with long overhanging crests. The resulting foam is in great patches and is blown in dense white streaks along the direction of the wind. On the whole of the surface the sea takes a white appearence. The rolling of the sea becomes heavy and shock-like. Visibility is affected. H_s =9-12.5 m.

Extensive widespread damage. These typically occur only at sea, and rarely inland. Exceptionally high waves. Visibility affected. Sea completely covered with long white patches of foam lying in direction of wind. Everywhere edges of wave crests are blown into fret. H_s =11.5-16 m.

Extreme destruction on land. On sea air filled with foam and spray. Sea white with driving spray; visibility very seriously affected. H_s>15 m.