Wave height is the vertical distance from trough to crest. Significant wave height, or H1/3, is the average of the highest one-third of all wave heights. Offshore of the Hawaiian islands the seas are moderately rough, with significant wave heights of 3-14 ft (1-4 m), varying seasonally with the intensity of the trade winds, which are greatest in winter and spring. Northwest shores are exposed to large swells from storms in the northwest Pacific in winter while south shores receive relatively smaller swells (3-9 ft or 1-3 m) from southern hemisphere storms in summer. The north shore of Oʻahu is famous for its large winter surf where wave heights over 50 ft (15 m) are occasionally observed.