Line plots show the tidal height, water speed and direction for yesterday, today and tomorrow. Hover your mouse over the lines to see the heights at specific times.
Tidal height is the level of the sea surface compared to the ocean at rest, what is called the geoid. The largest positive value is the high tide, whilst the lowest negative value is low tide.
The period between a high tide and a low tide, when the tide is going out, is called an ebb tide. Whereas the time between low and high tide, when the tide is coming in, is called a flood tide – this can be seen clearly in the direction plots.
In Plymouth, the cycle of one low and high tide occurs every 12.4 hours, but not all tides are the same. At some points the difference in elevation between low and high tides becomes larger. These are called spring tides at which times the currents are also stronger – this can be seen in the water speed plots. Smaller differences in tidal height and weaker currents occur during neap tides. Spring and neap tides depend on the phase of the moon – the biggest (spring) tides occurring when there is a full or new moon. The speed of the water also changes with depth, near the bottom the water is slowed down due to friction with seabed.
Contours plots show the water speed, temperature and salinity at depths up to 16m for a 72 hour period.
The speed of the water changes with both the tides and depth. Near the bottom the water is slowed down because of “friction” with the sea bed. Whereas at the surface, the wind can alter the speed of the flow. The colour represents the speed of the water – the darker the green, the faster the water is flowing.
Temperature changes throughout the day, warming in the sun and cooling at night. Where waters of different temperatures meet, such as in winter when cold river meets warmer oceanic waters, the tides mix and move the different waters around.
Salinity changes due to the mixing of oceanic water with lower salinity waters from the estuaries. Darker colours represent lower salinity waters and clear tidal patterns can be seen and differences between offshore and upstream sites.
Animations show the changes in surface salinity and temperature over a 72 hour period. Lower salinity waters entering from the estuaries and the effects of tides can clearly be seen.