How Do Tides Work?
Tides are the rise and fall of sea level caused by gravitational pull from the moon (primary force) and the sun (secondary). Most US coastlines experience semidiurnal tides — two highs and two lows every 24 hours 50 minutes. Spring tides (new and full moon) produce the largest tidal range. Neap tides (quarter moons) produce the smallest.
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Key Takeaways
- Moon's gravity drives tides. Two highs and two lows per day. Spring tides (full/new moon) have the biggest range; neap tides the smallest.
- Spring tides (full/new moon) swing 20-30% more than average. Neap tides (quarter moon) swing 20-30% less. Both affect wave shape at your break.
- Check tide charts daily. High tide shifts 50 min later each day. Spring tides amplify effects at your break; neap tides moderate them.
The Short Answer
Tides are the rise and fall of sea level caused by gravitational pull from the moon (primary force, ~2/3 of tidal effect) and the sun (secondary, ~1/3). Most US coastlines experience semidiurnal tides — two highs and two lows every 24 hours 50 minutes (the extra 50 minutes is because the moon orbits Earth, shifting the cycle daily). Spring tides (new and full moon, when sun-moon-earth align) produce the largest tidal range. Neap tides (quarter moons) produce the smallest. NWS predictions are accurate to within minutes.
The Lunar Cycle and Tidal Range
The moon's gravity creates a tidal bulge — water is pulled toward the moon on the near side of Earth, and centrifugal force creates a second bulge on the far side. As Earth rotates, coastlines pass through these bulges, experiencing two high tides and two low tides daily.
Spring tides happen twice per month (new moon and full moon) when sun and moon align. Their combined gravity produces tidal ranges 20-30% larger than average. In San Francisco, spring tidal range can exceed 7 feet. Neap tides happen at quarter moons when sun and moon pull at right angles, partially canceling each other. Neap ranges are 20-30% smaller than average. For surfing, spring tides mean bigger swings between low and high — which can expose more reef at low tide or flood breaks more at high tide. Neap tides produce more moderate, stable conditions.
“Spring tides (full/new moon) swing 20-30% more than average. Neap tides (quarter moon) swing 20-30% less. Both affect wave shape at your break.”
What This Means for Your Session
Check Quiver's tide chart before every session — it shows the exact curve with high/low times and heights. Plan around the mid-tide window for most beach breaks (see our Best Tide for Surfing guide). During spring tides, be extra cautious at reef breaks — extreme low tides expose hazards. During neap tides, conditions are more forgiving because the tidal swing is smaller. The tidal cycle shifts ~50 minutes later each day, so if high tide is at 8 AM today, it's at 8:50 AM tomorrow. Plan your week around this shift — sometimes the mid-tide window aligns perfectly with dawn patrol, and sometimes it doesn't. Moon phase calendars help you anticipate spring vs. neap weeks ahead of time.
Frequently Asked Questions
Why does high tide shift 50 minutes each day?+
Because the moon orbits Earth, advancing about 12.2 degrees per day. Earth has to rotate an extra 50 minutes to 'catch up' to the moon's new position, which delays each tidal cycle. This is why tide times are different every day.
Do all coastlines have two tides per day?+
Most US coastlines have semidiurnal tides (two highs, two lows). But the Gulf Coast (Texas, Louisiana, parts of Florida) has mixed or diurnal tides — sometimes just one high and one low per day. Coastal geometry and basin shape affect the tidal pattern.
How do I know if it's a spring or neap tide?+
Check the moon phase. Full moon and new moon = spring tide (big swings). First quarter and third quarter = neap tide (small swings). Spring tides happen about every 2 weeks. Most tide apps and Quiver's charts show this information.
Can tides create currents?+
Yes. Tidal currents flow as water moves to fill or drain coastal areas during tide changes. These are especially strong near inlets, harbors, and narrow bays. In open coastline surf, tidal currents are weaker but can affect rip current strength.
