Tides Flashcards
What is the Diurnal tidal period?
24h50m
What is the semi-diurnal tide period?
12h25m
What do diurnal tides feature?
1 high water and 1 low water tides per period
What do semi-diurnal tides feature?
2 high water and 2 low water tides per day
What are mixed (diurnal or semi) tides?
These tides are of unequal height.
E.g. 2 lower high water tides versus 2 higher low water tides
What is the origin of tides?
Gravitational attractive force acts as centripetal force that keeps Earth and Moon in mutual orbit around their common centre of mass
What does the magnitude of force vary with?
It varies with distance e.g. smaller when separation is larger
What does the equilibrium tidal theory consider?
– Spherical Earth covered with uniform ocean
– Action of gravitational force on fluid elements in the ocean
What is the Tide-generating force?
the NET force acting on a fluid element
What is the equation for tide generating force (TGF)?
= mass of tide-raising body
–––––––––––––––––––––––––––
distance to tide-raising body^3
How do tidal bulges occur?
Gravity and inertia act in opposition on the Earth’s oceans, creating tidal bulges on opposite sides of the planet.
They are aligned with the tide-raising body
What is lunar declination?
Angle of Moon’s orbit with equator
What tides are featured in high latitudes?
Diurnal
What tides are featured in low latitudes?
Semidiurnal
What tides are featured in mixed latitudes?
Mixed
What does the form of tides depend on?
Depends on latitude and lunar declination
What are the different tide raising bodies ?
Moon – LUNAR tide
Sun – SOLAR tide
What is the fortnightly cycle of tidal variations known as?
Spring-Neap cycle
What is true of spring tides?
– Sun, Earth and Moon make straight line in space
– Occurs during New Moon or Full Moon (syzygy)
– Tidal bulges add together
– Higher HW and lower LW
––––– = INCREASED tidal range
What is true of Neap tides?
– Sun, Earth and Moon make right angle in space
– Occurs during the Moon’s first quarter or third quarter (quadrature)
– Tidal bulges work in opposition
– Lower HW and Higher Lw
–––––––– = DECREASED tidal range
What are the features of the Equilibrium tidal theory?
– Explains origin of tides
– Correctly predicts key features e.g. timescales
– CANNOT capture details of real tides
What are the features of the Dynamic tidal theory?
– Constraining influence of land masses
– Frictional influence of rotating earth
– Deflection of tidal flows by Coriolis effect
How do rotary tidal systems occur/work?
– Assumed closed basin in Northern hemisphere
- Water flowing in basin deflects to right and hugs coast
——– HW travels counterclockwise around basin
——– Maximum tidal range at coast
——– Minimum tidal range in cente
————– Rotary wave
—————— Kelvin Wave
———————- Amphidromic system
What can the amphidromic system achieve?
it can create maps summarising tidal variations in a region
How do amphidromic systems work?
Draw lines joining locations of equal tide PHASE
– Where HW occurs at the same time
–––– Cotidal or cophase lines
Draw lines joining location of equal tidal RANGE
– Corange lines
How are tides predicted using harmonic analysis?
Splits measured tidal signal into set of sinusoidal constituents
Periods of each constituent corresponds to a different, specific astronomical motion
e.g. orbit of the Earth around the Sun, orbit of Moon around Earth
~390 different constituents identified
