Cliff Stephen of Dunedin asks :-

Why is the sea usually so much calmer in the early morning compared to later in the day?

Tony Trewinnard, a meteorologist with Blue Skies Weather and Climate Services Ltd, responded.

There are two types of waves which can turn a flat sea into a rougher one - swell waves and wind waves.

Swell waves (often just called "swells") are long period waves. A wave's period is the length of time it takes for the peaks or troughs of a wave to pass. Swell waves have periods between about 6 seconds and upwards of 30 seconds. They can be seen in relatively calm conditions as a gentle rising and falling of the sea surface. Swell waves produce the familiar up and down motion experienced in medium sized or large boats. Swell waves can travel a very long way from the place where they are generated and last for many days. It is not uncommon for swell waves to hit the east coast of New Zealand which have been generated near South America a week earlier.

Wind waves have a much shorter period, anywhere between a half and five seconds. They make the surface of the sea choppy and rough, and produce the unsettled movements often experienced in small boats. Wind waves don't travel far, and are usually generated near to where they are experienced. Wind waves are caused by the action of the wind on the ocean surface and usually dissipate within a few hours.

Swell waves can arrive at any time of day, but because wind waves are generated by the wind, they only develop when the wind begins to blow steadily. Since wind speeds are often low at night, and increase during the daytime, wind waves often die out during the night, leading to a relatively flat sea (perhaps with swell waves) in the early morning. During the day, the wind waves increase in size as the wind speed increases, leading to a rougher, more choppy, sea surface during the afternoon and evening.