Semi-diurnal tide
Topic: Movements of Ocean Water: Waves, Tides and Currents · NCERT: Class 11, Ch 13 "Movements of Ocean Water"
Meaning
A semi-diurnal tide is a pattern with two high tides and two low tides each day. Each high is about the same height as the other, and so are the lows. It happens because as the Earth turns, each place passes through two tidal bulges, one facing the Moon and one on the far side. A full cycle takes about 24 hours 50 minutes, so the tides arrive about 50 minutes later each day. This is the most common tidal pattern in the world.
Example
Most of India's coast has semi-diurnal tides. At the Gulf of Khambhat in Gujarat, for example, the sea rises and falls twice every day.
Don't confuse with
- Diurnal tide: only one high and one low each day, as in parts of the Gulf of Mexico.
- Mixed tide: also two highs a day, but the two are of clearly unequal height.