Tides - Heights & Streams

Question 1

What type of moon is on Springs and Neaps tides and how long do they last for?

Answer 1

Springs occur on full moon and no moon.

Neaps occur at the start of each half moon.

Each type of tide lasts two weeks.

Question 2

How are spring and neap tides caused?

Answer 2

Moon goes around earth & it’s gravity pulls the earth and sea towards it.

Sun also has gravitational pull that increases pull or decreases it depending on it’s position relative to the moon.

If sun is line with moon even if on opposite sides then effect causes higher tides. Known as spring tides.

If sun is 90’ with moon then the pulling is opposed and tides are smaller. Known as neap tides.

Question 3

What are the definitions of the following terminology?

1. Mean
2. MHWS
3. MLWS
4. MHWN
5. MLWN
6. HAT
7. LAT
8. Bridges, Cables and HAT
9. Range
10. Chart Datum
11. Drying Height
12. Height of Tide
13. Rise of Tide
14. Fall of Tide
15. Charted Height

Answer 3

Mean: When referring to tides, the mean is the average height taken over a period of around 19 years.

MHWS: Mean High Water Springs. Is the mean high water height of spring tides.

MLWS: Mean Low water springs Is the mean low water height of spring tides.

MHWN: Mean High Water Neaps is the mean high water height of neap tides.

MLWN: Mean Low Water neaps is the mean low water height of neap tides.

HAT & LAT: Is the ‘Highest or Lowest Astronomical Tide’ that can be expected to occur under any combination of astronomical conditions and average meteorological conditions.

Bridges, Cables and HAT: The clearance heights for bridges and cables are USUALLY given above HAT. Information for HAT heights can be found on the charts and in the almanac page 12.

N.B. Not all cables and bridges are measured above HAT some still use MHWS. To find which is used on a particular chart check the chart information box.

HAT and LAT can be affected by meteorological conditions which can cause a higher or lower levels, respectively.

Range: The difference between HW and LW. For Example HW 4m and LW 1m gives a range of 3m.

Chart Datum: Zero depth on the chart (0.0m). Charted depth and height of tide are both measured from this datum.

Drying height: The amount that a drying area - an area that covers and uncovers with the rise and fall of the tide - will show above chart datum (an underlined number in the green area of the chart).

Height of tide: Is measured from chart datum to sea level.

Rise of tide: Is measured from low water to sea level when the tide is rising.

Fall of tide: Is measured from the previous HW to sea level when the tide is falling.

Charted height: Is measured above Mean High Water Springs (MHWS). Lighthouses, spot heights etc. are measured in this way.

Question 4

Datum of Soundings