A swell as it passes through shallow water. These swells travel far out in advance of cyclones and are a fore shadow of possible stormy weather. In coastal areas, Shore dwellers can use ground swells (the time in between each one and size) as an indication of approaching bad weather. If the swells increase in size and shorten in time as each wave arrives, it is a reliable indicator that stormy weather might come.

an obvious change of public opinion or political sentiment that occurs without leadership or overt expression; "there was a ground swell of antiwar sentiment"

Long wave formations during calm or light air formed by waves running into shoaling water

a broad and deep undulation of the ocean