Decide
When do marés vivas (spring tides) occur?
The lesson in plain words
- Springs and neaps
- The table also shows the fases da Lua.
- Near new and full Moon the range is large: marés vivas (springs), with the highest high waters and lowest low waters.
- Near the quarters the range is small: marés mortas (neaps).
- Springs bring the shallowest low waters, so check shoals and moorings most carefully then.
- Pressure
- The table assumes average weather.
- The IH notes that a change of 10 hPa moves the level about 0.09 m: low pressure raises the water and high pressure lowers it.
- The water responds to the average pressure over a wide area, not instantly.
- Wind and seiches
- A strong wind blowing onshore piles water up and gives higher levels than predicted.
- An offshore wind lowers them. Wind also shifts the times.
- Sudden weather changes, such as a deep low or an active front, can set up seiches: oscillations of 5 to 30 minutes and up to 70 cm in some harbours.
- Rivers and bars
- In estuaries and rias, river floods raise the level and droughts lower it.
- The IH also expects levels about 0.1 m above the table, because the sea has risen since the datum was fixed.
- None of this guarantees more water: weather can make it lower.
- On Portuguese harbour bars, check the AMN estado das barras (bar status: open, conditioned or closed) before going out.
Key data
- Marés vivas
- Springs: near new and full Moon, large range
- Marés mortas
- Neaps: near the quarters, small range
- Pressure effect
- 10 hPa ≈ 0.09 m (IH); low pressure raises the level
- Onshore wind
- Raises the level above the prediction
- Seiches
- 5–30 min oscillations, up to 70 cm
Key points
- Springs near new and full Moon; neaps near the quarters.
- Low pressure and onshore wind raise the level; high pressure and offshore wind lower it.
- Estuaries also react to river floods and droughts.
- The table is a prediction: keep a margin and check the bar status.
Check what you have read
Atmospheric pressure has stayed 10 hPa below average. What happens to the water level?
True or false?
Decide each statement before you see the answer.
High pressure gives more water than predicted.
The table is exact, so a 10 cm margin is enough.
The table is a prediction: keep a margin and check the bar status.
Low pressure and onshore wind raise the level; high pressure and offshore wind lower it.
Estuaries also react to river floods and droughts.
Marés vivas means the water is rough.
Practice case & exam keys
Situation
It is two days after full Moon. A high-pressure system of 1,033 hPa has sat over the coast for days, about 20 hPa above average, with a fresh offshore wind. You plan to cross a shoal near low water with the margin you used at neaps. Is that wise?
Show the answer
No. Two days after full Moon you are at springs, with the lowest low waters. The high pressure lowers the level by roughly 0.18 m (2 × 0.09 m), and the offshore wind lowers it further. All three effects take water away. Cross nearer high water or choose another route, and add margin.
Exam keys
- Marés vivas (springs): new and full Moon; marés mortas (neaps): quarters.
- Low pressure raises the level (≈ 0.09 m per 10 hPa); high pressure lowers it.
- Onshore wind raises the level; offshore wind lowers it.