Tides in a Nutshell
Presented to
IWAI
© Ocean Froggie 2003-2012
What causes Tides?
 Moon’s Gravity


28 Day Lunar Orbital Cycle
Bulge in Oceans
 Earth’s Rotation


Causes 200 mph tidal wave only 1” high
Interfered with by Continental Land Masses
• Water heaps up against coasts

25 Hour Cycle
• Moon has moved a little
Spring Tides
Sun
Moon’s
Gravitational
Effect
Neap Tides
Sun
New Moon
Full Moon
Every 28 days
Spring Tides
Moon and Sun in Line
Sun
Moon’s
Gravitational
Effect
Neap Tides
Every 28 days
Half Moon
Sun
Earth 24 hr
Rotating on
it’s Axis
28 Day Lunar Cycle
Half Moon
Tides around Ireland
 28 Day Lunar Orbit – New Moon


2 Spring Tides (every 14 days)
2 Neap Tides (every 14 days)
 Twice a Day – 25 hrs


High Tide = HW x 2
Low Tide = LW x 2
• Semi-diurnal
 HW every 12 ½ hours approx
Tidal Terms
MHWS
MHWN
Spring
Range
Neap
Range
Height
of
Tide
Depth
of
Water
MLWN
MLWS
Charted Depth (Below CD)
CD = Chart Datum
Minimum Depth
LAT = Lowest astronomical tide
 Depth in Meters
 Charted Depth = Depth below CD – Chart Datum
 Tide Increases Depths
 Depth of Water = CD + HOT (Height of Tide at a specific time)
Tidal Terms
 CD – Chart Datum
Lowest Astronomical Tide – LAT
 Tides are measured as heights above CD
 Navigation charts give depths below CD

 Mean Datum Levels

MHWS, MLWS
• High and Low water spring tides

MHWN, MLWN
• High and Low water neap tides
Tidal Terms
 HW / LW


High Water above CD
Low Water above CD
 Tidal Range

Difference between HW and next LW
• Springs ranges are greater
• Neaps ranges are less
Tide Tables
 Limerick July 2005

Time is Local
 HW time
 HW height
 BA Tables

UT Time Zone
• Ireland + UK
• E.g Galway, Cobh


HW + LW times
HW + LW heights
 Tidal Streams
 Flood Tide = Height Increasing
 Ebb Tide = Height Dereasing
TIDAL STREAMS
 TARBERT RACE
 7kts in Springs
Effects on Tide
 Carioles effect in channels

Greater Tidal Range
• Welsh and French Channel Coasts
– 14 meters in Bristol Channel
– 12 meters on French Channel Coast
• Caused by Earths Spin
 Wind and Pressure Effect
 Almost no Tide in
Mediterranean!
 Little tide mid ocean

Lack of land mass
Tidal Terms
 Equinoxes


Twice a year
Spring and Autumn
• March 21st and Sept 23rd approx

Maximum Spring Tide Ranges
• Highest and Lowest tides of year
Tides and Tidal Streams
Have Fun!
© Noel Griffin 2003-2012