Spring tide

Topic: Movements of Ocean Water: Waves, Tides and Currents · NCERT: Class 11, Ch 13 "Movements of Ocean Water"

Meaning

A spring tide is a tide with the highest high tides and the lowest low tides. It comes twice a month, at new moon and full moon, when the Sun, Moon and Earth stand in a straight line and their pulls add together [2]. It matters because it gives the largest tidal range (the gap between high and low water) of the month. This range affects ports, fishing, coastal flooding and tidal power.

Explanation

How it forms: cause → process → result

• Cause: the three bodies line up
• At new moon and at full moon, the Sun, Moon and Earth come into one straight line.

• This line-up is called syzygy.

• Process: the two bulges sit on top of each other

• The Moon pulls the sea into a bulge of water. The Sun makes its own, smaller bulge.

• In a straight line, the two bulges fall in the same place, so the pulls add together.

• Result: the largest range

• High tides go higher and low tides go lower.
• The tidal range is the biggest of the month [2].

The two positions that give a spring tide

• New moon: the Sun and Moon are on the same side of Earth.
• Full moon: the Sun and Moon are on opposite sides of Earth.
• Both are straight-line positions, so both give a spring tide. That is why it comes twice a month.

Why the Moon leads and the Sun only helps

• The Sun is about 27 million times heavier than the Moon, but about 390 times farther from Earth [3].
• Tide-making force gets weaker with the cube of distance (distance × distance × distance). So the Sun's tidal pull is cut by about 59 million times [3].
• Result: the Sun's tide-making force is about half the Moon's [3]. (NCERT puts it as: the Moon's pull is more than twice the Sun's.)
• The Moon controls the tide. At a spring tide, the Sun simply adds its smaller pull to the Moon's.

The monthly cycle and distance effects

• Cycle: spring tide → about 7 days → neap tide → about 7 days → spring tide [2]. This follows the lunar month of about 29.5 days.
• "Spring" is not the season. The word comes from water "springing up". Spring tides happen all year round.
• Distance changes the size of a spring tide. The Moon's orbit and Earth's orbit are ellipses (stretched circles), so distances keep changing:
• Perigee (Moon closest to Earth, about 363,300 km) [5]: a spring tide at this time is extra large.
• Apogee (Moon farthest from Earth, about 405,500 km) [5]: the range is smaller.

• Perihelion (Earth closest to the Sun, around 3 January): ranges are larger.

• Perigean spring tide ("king tide"): a new moon or full moon falls close to perigee [4].

• It comes about 6–8 times a year [4].
• High tides can be more than a foot (about 30 cm) higher than during apogean spring tides (spring tides when the Moon is at apogee) [4].

Where it occurs

• Everywhere in the world's oceans, twice every lunar month. Spring tides are not tied to any one region or season.
• Along India's coast, where most places have semi-diurnal tides (two highs and two lows a day of about equal height). On both the Arabian Sea and Bay of Bengal sides, spring tides make these highs and lows more extreme.
• Gulf of Khambhat (Cambay) and Gulf of Kachchh (Kutch), Gujarat:
• These gulfs are funnel-shaped. They narrow inland, so the incoming tide is squeezed and piles up higher.

• Spring tides here give very large ranges. This is why the estimated tidal power potential is about 7,000 MW (Khambhat) and 1,200 MW (Kachchh) [6].

• Sundarbans (Gangetic delta), West Bengal:

• Tidal potential is about 100 MW [6].

• Low delta land here is at risk when king tides meet storm surges (sea water pushed onto land by a cyclone's winds).

• Coastal Odisha and Andhra Pradesh (Bay of Bengal): these low coasts face extra flood risk when a perigean spring tide comes together with a cyclone landfall.

Don't confuse with

• Neap tide: the Sun and Moon are at right angles (90°) to Earth, at the first and last quarter. Their pulls partly cancel, so neap tides give the smallest range. They come about 7 days after each spring tide [2].
• Spring season: a spring tide has nothing to do with the season. It happens twice a month, all year.
• King tide (perigean spring tide): every king tide is a spring tide, but not every spring tide is a king tide. A king tide needs the spring tide to fall near perigee, and it comes only about 6–8 times a year [4].
• Perigee vs perihelion: perigee means the Moon is closest to Earth. Perihelion means Earth is closest to the Sun (around 3 January). Exam questions often swap the two.

Prelims Hooks

• Spring tide → new moon and full moon → Sun, Moon and Earth in a straight line (syzygy) → largest tidal range [2].
• Spring tides come twice a month. A neap tide follows about 7 days after each spring tide [2].
• Trap: "Spring tides occur only in the spring season" → wrong.
• The Sun's tide-making force is about half the Moon's, because the Sun is about 390 times farther away [3].
• Spring tide + perigee = perigean spring tide / king tide, about 6–8 times a year [4].
• Tidal power potential, highest to lowest: Gulf of Khambhat (~7,000 MW) > Gulf of Kachchh (~1,200 MW) > Sundarbans (~100 MW) [6]. Output is highest at spring tides.

Mains Points

• Astronomy and coast shape together decide the tidal range (GS-I):
• The Sun–Moon line-up makes spring tides. Perigee and perihelion make them even larger.
• Funnel-shaped gulfs like Khambhat raise the range further.

• This matters for when ships can enter ports, for fishing, and for planning ship traffic in narrow gulfs and estuaries (river mouths where fresh water meets the sea).

• Spring and king tides as a coastal hazard (GS-III, disasters):

• Perigean spring tides add to seasonal sea-level highs and can cause minor flooding of low coasts [4].
• When they meet a cyclone's storm surge, they can flood deltas such as the Sundarbans and the coasts of Odisha and Andhra Pradesh much more.

• Tide calendars should be part of cyclone early warning and evacuation planning.

• Tidal energy (GS-III, energy):

• India's estimated potential is about 8,000 MW, mostly in Gujarat's gulfs [6][7]. Ocean energy was declared renewable energy in 2019 [8].
• Output rises at spring tides and falls at neap tides. So tidal power is predictable but not steady.
• Barrages (low dams across a bay) are also costly and can harm mudflats, mangroves and fisheries.

Related concepts

• Semi-diurnal tide
• Diurnal tide
• Mixed tide
• Neap tide
• Perigee
• Apogee
• Ebb and flood tide

Read more

• Types of tides: by frequency and by the sun-moon-earth positions