Summary
Introduction
A hot summer day is the perfect time to go to the beach and cool down in the brisk ocean water. But it's not only the water that has a cooling effect at the beach. Have you ever noticed that there always seems to be a cool breeze blowing from the ocean to the shore? Where does the wind come from? In this activity, you will build a model of the ocean and the beach to find out—so next time you go to the beach, you will know why the sea breeze is blowing!Materials
- Two same-sized baking dishes, preferably glass
- Sand, enough to fill one baking dish
- Work space that has no wind
- Pot
- Wooden spoon
- Stove
- Large cardboard box
- Incense stick
- Lighter or matches
- Adult helper
- Ice water, hot water
- Freezer
- Optional: Thermometer
Prep Work
- Put the sand in a pot and with the help of an adult, heat it up on the stove until it is very hot to the touch (about 70–80°C).
Instructions
- Place the hot sand in one of the baking dishes. Feel the sand with your fingers.
- Fill the other baking dish with ice water. Feel the temperature of the water with your fingers.
- Put the dishes next to each other at a place where there are no external winds. You can use the cardboard box to shield the dishes from any air flow.
- Make sure there is no air movement above the dishes. Then, with the help of an adult, light the incense stick. Once it makes lots of smoke, hold it in between the sand and ice water dishes. Observe the movement of the smoke above the dishes for about one minute.
Where does the smoke go? Is it blowing towards the sand dish or the ice water dish? Can you explain why?
- Empty both dishes and let the sand cool to room temperature. Then, fill one dish with the room temperature sand and the other dish with room temperature water. Feel the temperature of the sand and the water with your finger.
- Place both dishes next to each other again in your wind protected area. Use the same incense stick and hold it between both dishes.
Where does the smoke go this time?
- Next, put the dish with the sand into the freezer for about 10–20 minutes. While the sand is cooling down, heat water up on the stove so it has about 70–80°C. Replace the room-temperature water with the hot water in the second dish.
- Once the sand is cooled and the water is heated, place the dishes next to each other again in your wind protected area.
- Hold the incense stick between the dishes again and observe where the smoke is moving.
Does the smoke move towards the sand or the water? Can you explain your observations?
Cleanup
What Happened?
Can you see how you created a little beach model? The dish with sand represented the beach, and the dish with water represented the ocean. You heated the sand on the stove like the sun heats the sand at the beach. When the sand is much warmer than the water, you should have seen the smoke of the incense stick steadily flow from the water to the sand dish. This air flow is caused by the temperature differences between the sand and water. The air above the hot sand heats up and starts to rise as it becomes less dense, creating a local low air pressure zone. Above the cold-water dish, the air cools down and becomes more dense, leading to a local high air pressure zone. To balance out the air pressure differences, the air moves from the high air pressure zone (the water dish) to the low air pressure zone (the sand dish) and the smoke is carried along with it.
If you remove the temperature difference, as you did in your second experiment, no air flow will be created. The air above the sand and the water have the same air pressure, which is why you shouldn't have seen the smoke move into a particular direction unless you had an external air flow. In the last experiment, you created a land breeze from the sand to the water, which happens at night when the air flows from the beach to the ocean. In this case, the sand is cooler than the water in the ocean, which reverses the air flow due to the changes in air pressure above the sand and the ocean.
Digging Deeper
If you were asked to describe wind, you might say that it is the movement of air from one place to another. But have you ever thought about what makes the air move? It all starts with the sun heating up Earth's surface. Not every surface heats up the same way: some surfaces take longer to warm, whereas others get hot right away. This is because different materials have different heat capacities. While surfaces with a low heat capacity will heat and cool quickly, those with a high heat capacity take much longer to do so. Water has a high heat capacity, which means it takes a lot of heat to increase its temperature. Beach sand, on the other hand, has a lower heat capacity and heats up pretty quickly in the sun, which you might have noticed when walking on the sand barefoot. But what do water and sand temperatures have to do with wind?
The air above the beach warms up because of the hot sand. As the air heats up, it becomes less dense and rises. The expanding air results in a decrease in air pressure (how much pressure the air exerts on Earth's surface at a certain location). Warm air rises, causing low air pressure, while cold, dense air results in high air pressure. Above the ocean, the air cools down due to the colder temperatures of the ocean water. This leads to the air becoming more dense and a local high air pressure zone.
The difference in air pressures above the beach and the ocean is what causes the air movement that we perceive as wind. In an attempt to balance the pressures in both areas, the air from the high-pressure zone (the ocean) will rush to the low-pressure zone (the beach) and replaces the rising air. The outcome of this air flow is the cool breeze that blows from the ocean towards the beach. The strength of the sea breeze depends on the difference in temperature between the land and ocean. Higher temperature gradients lead to stronger winds. During the night, this sea breeze turns into a land breeze, as after sunset, the sand will cool down much faster than the water due to its lower heat capacity. Once the sand becomes colder than the ocean, the air flow pattern reverses.
Ask an Expert
For Further Exploration
- Repeat this experiment with other temperature differences between the water and the sand. How high does the temperature difference need to be to observe a sea or land breeze?
- Would your observations be different at a pebble beach? Replace the sand with pebbles and repeat the experiment to find out!
- How would your results be affected by external winds? Remove your wind shield or repeat the experiment in a windy area to see how your air flow is affected.
- Vary the size of the two dishes. Is the effect more pronounced with larger dishes compared to smaller ones?
