Tides, Waves & Storm Surge — Explain tides, waves, and...

SECTION 1

Historical Context & Motivation

For thousands of years, people who lived near the ocean noticed that the water level rose and fell in a regular rhythm. Ancient sailors in Greece, China, and Polynesia depended on understanding these patterns to navigate safely. They watched the tides (the regular rise and fall of sea level) and waves (moving ripples of energy across the water's surface) to decide when to set sail or pull their boats ashore. Meanwhile, coastal communities learned to fear the deadly walls of water that hurricanes could push onto land — what we now call storm surge.

~150 CE

Ancient Greek Observations

The Greek geographer Ptolemy recorded that tides were connected to the position of the Moon, though he could not explain why.

1687

Newton's Law of Gravitation

Sir Isaac Newton published the law of universal gravitation, finally explaining that the Moon's and Sun's gravitational pull causes tides on Earth.

1900

Galveston Hurricane

A catastrophic storm surge of over 4.5 meters struck Galveston, Texas, killing more than 8,000 people. This disaster showed scientists and engineers the deadly power of storm surge and led to new coastal defenses.

1960s

Modern Tide Prediction & Wave Models

Computers allowed scientists to build mathematical models that predict tides, wave heights, and storm surge with much greater accuracy, saving countless lives along coastlines around the world.

Understanding tides, waves, and storm surge matters today more than ever. With rising sea levels and stronger storms linked to climate change, coastal communities depend on this science for safety. The big question this lesson addresses is: What forces create tides, waves, and storm surge, and how are they different from one another?

SECTION 2

Core Principles & Definitions

Before we dive deeper, let's nail down the three main ideas. Each one involves water moving, but they are driven by completely different forces and behave in different ways.

Tides

Tides are the predictable, regular rise and fall of ocean water caused mainly by the gravitational pull of the Moon and, to a lesser extent, the Sun. Most coastlines experience two high tides and two low tides each day.

Waves

Ocean waves are energy moving through water, usually generated by wind blowing across the surface. The water itself mostly moves in circles — it is the energy, not the water, that travels forward.

Storm Surge

Storm surge is an abnormal rise of water pushed ashore by a storm's winds and low pressure. Unlike tides, storm surge is not predictable far in advance and can raise water levels several meters in just hours.

Gravitational Pull

Gravity is the invisible force of attraction between any two objects that have mass. The closer or more massive an object, the stronger its gravitational pull. The Moon is close enough to Earth to tug on the ocean and create tidal bulges.

✦ KEY TAKEAWAY

Think of a bathtub. Tides are like slowly tilting the whole tub so water sloshes gently from one end to the other — that is gravity at work. Waves are like blowing across the surface to make ripples — that is wind energy. Storm surge is like slamming the water with your hand during a storm — a sudden, powerful push that floods the edge.

SECTION 3

Visual Explanation — How Tides Work

This diagram shows Earth with two tidal bulges — one facing the Moon and one on the opposite side. The near-side bulge forms because the Moon's gravity pulls the water toward it. The far-side bulge forms because Earth itself is pulled slightly toward the Moon, leaving the water behind. As Earth rotates, any point on the coast passes through both bulges, creating two high tides and two low tides roughly every 24 hours and 50 minutes.

Notice how the tidal bulges line up with the Moon. The cycle is not exactly 24 hours because the Moon also orbits Earth, so each day the Moon has shifted a little. That is why high tide arrives about 50 minutes later each day. When the Sun, Moon, and Earth line up during a full moon or new moon, we get extra-large spring tides. When the Sun and Moon are at right angles to each other (first or third quarter moon), the tides are smaller — those are called neap tides.

SECTION 4

How Waves Work — Energy in Motion

When wind blows across the ocean surface, friction transfers energy from the air to the water. This energy creates ripples that can grow into full-sized waves if the wind keeps blowing. Three factors control how big the waves get: wind speed, wind duration (how long it blows), and fetch (the distance over open water that the wind travels without obstruction).

Parts of a Wave

Every ocean wave has identifiable parts. The crest is the highest point of the wave. The trough is the lowest point. The wave height is the vertical distance from trough to crest. The wavelength is the horizontal distance from one crest to the next. Finally, the wave period is the time it takes for two consecutive crests to pass the same point.

WAVE SPEED

Wave Speed = Wavelength ÷ Wave Period

Wave speed is measured in meters per second (m/s). Wavelength is in meters (m), and wave period is in seconds (s). A wave with a wavelength of 100 m and a period of 10 s travels at 10 m/s.

🌊 Important Concept

Water particles in a wave move in circular orbits. They go up, forward, down, and backward — ending up almost exactly where they started. A floating cork bobs in a circle as a wave passes, but it does not travel with the wave. It is energy, not water, that moves across the ocean.

When waves approach a shoreline, the bottom of the wave drags against the sea floor. This friction slows the base while the top keeps moving, causing the wave to steepen and eventually break. That is why you see white, crashing surf near the beach but smooth rolling swells farther out at sea.

SECTION 5

Types of Tides, Waves & Storm Surge

This diagram compares normal sea level, a normal high tide, and storm surge. The yellow arrow shows the surge height — the extra water pushed ashore by a storm. If a storm surge happens during high tide, the combined water level can be devastating.

Types of Tides

Common tidal patterns around the United States

Tide Type	Description	Example Location
Diurnal	One high tide and one low tide per day.	Gulf of Mexico
Semi-diurnal	Two roughly equal high tides and two low tides per day.	U.S. Atlantic coast
Mixed	Two high tides and two low tides per day, but they are unequal in height.	U.S. Pacific coast

What Makes Storm Surge So Dangerous?

Storm surge is the single greatest threat to life during a hurricane. Three factors make it worse: stronger winds push more water, shallower coastal waters pile up more water because there is less room for it to spread, and the shape of the coastline can funnel water into bays and estuaries. If the storm arrives at high tide, the surge rides on top of the already elevated water, making flooding even worse.

SECTION 6

Worked Example — Calculating Wave Speed

Let's apply what we know about waves to a real problem. We will use the wave speed formula to find how fast a wave is traveling.

Finding the Speed of an Ocean Wave

Step 1 — Read the ProblemA surfer notices that ocean wave crests are about 80 meters apart. She times the waves and finds that a new crest passes her every 10 seconds. What is the speed of these waves?

Step 2 — Identify Given ValuesWavelength = 80 m (distance between crests). Wave period = 10 s (time between crests passing the same point).

Step 3 — Write the FormulaWave Speed = Wavelength ÷ Wave Period

Step 4 — Substitute and SolveWave Speed = 80 m ÷ 10 s = 8 m/s

The wave is traveling at 8 meters per second.

Step 5 — Check and Interpret8 m/s is about 29 km/h — roughly the speed of a person on a bicycle. This is a typical speed for ocean swells. The answer makes sense because longer wavelengths or shorter periods mean faster waves.

SECTION 7

Comparing Tides, Waves & Storm Surge

Students often confuse tides, waves, and storm surge because all three involve changes in water level. The table below highlights their key differences side by side.

Side-by-side comparison of tides, waves, and storm surge

Feature	Tides	Waves	Storm Surge
Cause	Gravitational pull of Moon and Sun	Wind blowing across the water surface	Storm winds and low atmospheric pressure
Predictability	Very predictable — predicted years in advance	Somewhat predictable from weather forecasts	Only predictable a few days before a storm
Time Scale	~12.4 hours between high tides	Seconds to minutes per wave	Hours — rises quickly during a storm
Water Movement	Entire ocean surface rises and falls	Energy moves forward; water orbits in place	Large volume of water pushed onto land
Danger Level	Low (but strong tidal currents can be risky)	Moderate — large waves can be hazardous	Very high — leading cause of hurricane deaths

✦ KEY TAKEAWAY

Here is an easy way to remember the differences. Tides are like a clock — steady and predictable. Waves are like ripples on a jump rope — energy traveling along without the rope itself moving forward. Storm surge is like someone scooping a bathtub's worth of water and dumping it on your front yard — sudden, massive, and dangerous.

SECTION 8

Connection to Advanced Topics

The concepts you have learned here are the foundation for more advanced oceanography topics. As you move into higher-level Earth science or even college courses, you will encounter more detailed models. The table below shows how each concept scales up.

From basic concepts to advanced oceanography

Basic Concept (This Lesson)	Advanced Extension
Tides caused by Moon and Sun gravity	Harmonic analysis breaks tides into dozens of frequency components (called tidal constituents) for precise prediction.
Wave Speed = Wavelength ÷ Period	Deep-water wave speed depends on wavelength: v = √(g × λ / 2π). Shallow-water wave speed depends on depth: v = √(g × d).
Storm surge from wind and low pressure	Computer models like SLOSH and ADCIRC simulate surge using fluid dynamics equations, bathymetry data, and real-time weather inputs.
Spring and neap tides	Tidal resonance in certain bays (e.g., Bay of Fundy) can amplify tides to over 15 meters due to the natural period of the basin.

🔬 Looking Ahead

Climate change is raising sea levels, which means even a moderate storm surge will push water farther inland than it did decades ago. Scientists now combine tide prediction, wave models, and storm surge simulations to create total water level forecasts that help communities plan evacuations and build better coastal defenses.

SECTION 9

Practice Problems

PROBLEM 1 — CONCEPTUAL

Explain why there are two tidal bulges on Earth — one on the side facing the Moon and one on the opposite side. Use the idea of gravitational pull in your answer.

PROBLEM 2 — BASIC CALCULATION

An ocean wave has a wavelength of 150 meters and a period of 10 seconds. What is the wave speed? Show your work using the formula Wave Speed = Wavelength ÷ Period.

PROBLEM 3 — INTERMEDIATE

During a spring tide, the tidal range (difference between high and low tide) at a beach is 3 meters. During a neap tide, the tidal range drops to 1.2 meters. (a) Which arrangement of the Sun, Moon, and Earth produces each type of tide? (b) Why is the spring tidal range larger?

PROBLEM 4 — APPLIED

A Category 3 hurricane is approaching a coastal town. The normal high tide is expected to raise water 1.5 meters above mean sea level. The National Weather Service predicts a storm surge of 3.5 meters. (a) What could the total water level above mean sea level be if the surge arrives at high tide? (b) Name two factors that could make the storm surge even worse than predicted.

PROBLEM 5 — CRITICAL THINKING

Imagine you are a coastal city planner. Sea levels have risen 0.3 meters over the past century and are projected to rise another 0.5 meters by 2100. How would this sea-level rise change the threat posed by (a) normal high tides, (b) storm surge, and (c) everyday wind-driven waves? Which of the three concerns should most influence your building codes for new construction near the coast? Justify your reasoning.

SUMMARY

Lesson Summary

Tides are the predictable, regular rise and fall of the ocean caused by the gravitational pull of the Moon and Sun. Most coasts see two high tides and two low tides each day. When the Sun and Moon align, we get extra-large spring tides; when they are at right angles, we get smaller neap tides. Waves are energy traveling through water, generated mainly by wind. Their size depends on wind speed, duration, and fetch. Remember: the water particles move in circles — it is the energy, not the water, that travels across the ocean. Wave speed equals wavelength divided by period.

Storm surge is an abnormal rise in water level driven by a storm's strong winds and low atmospheric pressure. It is the deadliest hazard during a hurricane, and it becomes far more dangerous when it coincides with high tide. Factors such as a shallow continental shelf, a funnel-shaped coast, and rising sea levels all increase the severity of storm surge. Understanding these three ocean phenomena helps us predict coastal flooding, design safer infrastructure, and protect lives.