Why does the sea have waves even when there is no wind?

We have a saying, "No wind without waves." On days when there is no wind, the lake surface is always calm, but the sea will also have waves on a windless day. Why is this?

This is because in the vast sea, if the sea water at a certain place is blown by the wind, the waves will not only fluctuate in place, but also continuously spread to the surrounding sea areas. Because the sea is vast and vast, there are waves even in places where there is no wind in the distance.

The wavelength of this kind of wave is very long. Even if the wind stops, the wave will not disappear immediately, but will fluctuate for a while. In addition, geological changes on the seabed, such as submarine earthquakes, can also cause rough waves.

Generally speaking, if the sea surface is wide, the wind speed is high, the wind direction is stable and lasts for a long time, the waves must be very strong. For example, in the westerly belts of the northern and southern hemispheres, the waves are often rolling; In the subtropical calm zone of the Northern Hemisphere, although the water is open, the waves are generally very small due to weak winds and uncertain wind directions.

Under different wind speeds, wind directions and topographic conditions, waves vary greatly. Usually the period is a few tenths of a second to tens of seconds, the wavelength is tens of centimeters to hundreds of meters, and the wave height is a few centimeters to more than 20 meters. In rare terrain, the wave height can reach more than 30 meters.