Key concepts
Physics
Air pressure
Density
Heat capacity
Temperature

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 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 beach to find out—so you will know why the sea breeze is blowing!

Background
If you were asked to describe wind, you might say 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 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. Surfaces with a low heat capacity will heat and cool quickly but those with a high heat capacity take much longer to do so. Water has a higher 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 warms 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 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 whereas 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 denser and a local high air pressure zone.

The difference in air pressures above the beach and ocean is what causes the air movement 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 replace the rising air. The outcome of this airflow is the cool breeze that blows from the ocean toward 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, because 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 airflow pattern reverses. Want to see for yourself? Create your own sea breeze in this activity—no real ocean or beach required!

Materials

  • Two same-size baking dishes, preferably glass
  • Sand (enough to fill one baking dish)
  • Workspace that has no wind
  • Pot
  • Stove
  • Incense stick
  • Lighter
  • Adult helper
  • Ice water
  • Hot water
  • Freezer
  • Large cardboard box (optional, if needed to shield any breezes)
  • Thermometer (optional)


Preparation

  • Put the sand in a pot, and with the help of an adult heat it on the stove until it is very hot to the touch (about 70 to 80 degrees Celsius).


Procedure

  • Place the hot sand in one of the baking dishes. Carefully feel the sand with your fingers. How hot is the sand?
  • Fill the other baking dish with ice water. Feel the temperature of the water with your fingers. How cold is the water?
  • 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 airflow.
  • 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 toward 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. Are the temperatures of both very different?
  • 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 to 15 minutes. While the sand is cooling down heat water on the stove to about 70 to 80 degrees 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 which direction the smoke is moving. Does it move toward the sand or the water? Can you explain why?
  • Extra: Repeat this activity 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?
  • Extra: Would your observations be different at a pebble beach? Replace the sand with pebbles and repeat the activity to find out!
  • Extra: How would your results be affected by external winds? Remove your wind shield or repeat the activity in a windy area to see how your air flow is affected.

Cleanup
Make sure to fully extinguish the incense stick. Dispose of the sand outside if you have permission or in the trash. Clean your baking dishes with warm water and soap.

Observations and results
Can you see how you created a little beach model? The dish with sand represented the beach and the one 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 airflow 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 and becomes denser, 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 test, no airflow will be created. The air above the sand and 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 airflow. In the last test you created a land breeze from the sand to the water, which happens at night when the air flows from beach to ocean. In this case the sand is cooler than the water, which reverses the airflow due to the changes in air pressure above both the beach and ocean.

More to explore
Under an Ocean of Air Pressure, from University of Illinois Extension
Where Does Wind Come From?, from Scientific American
A Change in the Winds: Studying Bernoulli's Principle, from Science Buddies
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies