V. Using Central Ideas to Explain Intriguing Phenomena Involving Local Weather at the Beach
Emily van Zee and Elizabeth Gire
Students in our area of the country can go to the beach on sunny days. The sand is warm; brrr, the water is cold! How can that be? Later in the day, however, the sky often clouds over and a breeze comes up from over the water. Why would that happen?
A. Considering the influence of the properties of materials on thermal effects
Question 3.8 Why is the sand warm and the water cool at the beach if the Sun has been shining on both in the same way for the same time?
- Use central ideas about the properties of sand and water, such as their thermal conductivities, specific heats, and reflectivities, to explain why the sand is hot and the water cool at the beach even though the Sun has been shining on them in the same way for the same time.
1. Example of student work explaining about hot sand and cool water at the beach
At a sunny day at the beach, the top of the sand is warm. The radiation from the Sun heats up the surface of the sand, but sand has a low thermal conductivity, so this energy stays at the surface of the sand. When you dig your feet into the sand it is cool below because the energy from the Sun was not transferred below the surface of the sand.
Water has a higher thermal conductivity, meaning the energy from the Sun is quickly transferred throughout the water, a big area. This means the heat will be even throughout the water, taking a lot more energy than is supplied to finally heat up the entire ocean. The water also feels cool because water has a bigger specific heat than sand. Despite the water receiving the same amount of energy for the same amount of time as the sand, the water needs more energy to change one gram of the water one degree Celsius. It takes 1 calorie of energy from the Sun to change one gram of the water one degree Celsius, while sand only needs 0.2 calories of energy from the Sun to change the same amount. This means it will take longer for the water to increase in temperature because water needs more energy to do so.
Another factor contributing to water temperature is the angle of the Sun. If the Sun is shining on the water from an angle, some of the light is reflected. When the light is reflected the energy is not being absorbed, so the temperature is not increasing as much. When the Sun is high in the sky above the water, the energy will be absorbed because the light is not being reflected.
Physics student, Spring 2016
B. Considering the influence of light and thermal effects on weather at the beach
A wonderful sunny day at the beach can turn cloudy and somewhat chilly with a breeze coming off the water late in the afternoon. What causes this shift in the weather?
Question 3.9 Why do clouds and sea breezes often form in the afternoon after a sunny day at the beach?
- Draw a diagram that illustrates why sand is warm, water cool, clouds often form in the sky, and breezes blow in from the sea in the afternoon after a sunny day at the beach.
- Explain these aspects of local weather at the beach in terms of transfer of energy processes such as radiation from the Sun, reflection, absorption, conduction, evaporation, condensation, and convection.
- To explain a complex sequence of physical processes, start with radiation from the Sun and discuss each step with your group members. What is the influence of differences in thermal conductivity and specific heat between sand and water? What roles do conduction, evaporation, condensation, and convection play in setting up a circulation of air above the sand and water?
- Monitoring what one is doing can be helpful when developing complicated explanations. Every so often, step back and consider: What are we doing? Why are we doing this? How is doing this helping us?
Complete your explanations before looking at an example of student work. Next consider nuances about the influence of light and thermal phenomena on local weather at the beach. Then enjoy reading about how three young children learned to interpret a sea breeze diagram like the one below.
1. Example of student work explaining about cloudy skies and sea breezes forming in the afternoon after a sunny day at the beach
This student created a clear diagram of the convection cycle underlying sea breezes:
The diagram is labeled Convection Cycle and as a diagram illustrating explanation for sea breezes. The student drew and labeled a Sun whose rays shine on both the land and sea, heating the sand and water. The warm sand warms the air above by conduction, and the heating air rises, represented by the gray upward arrow. Some water evaporates forming cool moist air above the water, which blows toward the shore forming a sea breeze, represented by the gray arrow toward land. A cloud forms in which Condensation into cloud occurs as moisture in the warm moist air condenses in the cooler upper atmosphere. The cooling air sinks, represented by the gray arrows to become cool moist air over the water, which then blows toward shore, represented by the gray arrow toward the land.
The student interpreted the diagram as follows:
As we have learned in the past few weeks, light leaves a source such as the Sun and travels in all directions, (including) to the sand and to the water. Sand is a poor conductor of heat so when the Sun warms the sand, the heat is not distributed evenly, making the surface hot. Also the specific heat of sand is lower than that of water so when both are exposed to the same amount of light, the sand at the surface will have a higher change in temperature.
Water is a good conductor of heat so the heat from the sunlight is evenly distributed throughout the water. Also water has a higher specific heat than sand so it will not warm as much when exposed to the same light.
Over the water, some of the light energy is used to change the state of some water, evaporating it into a gas state. This makes the air over the water cooler moist air.
The air above the sand continues to heat, because the sand heats at a higher rate than the water. As the air (above the sand) warms, it rises. The cool moist air above the water begins to move toward the sand, (forming the sea breeze).
The hot sand heats the cool moist air in a process called conduction, causing the air to rise, (cool), and the gaseous water to form a cloud through condensation. As the air cools, it sinks back toward the water. As the warm and cool air moves, it is called “convection.”
Physics Student Fall 2015
2. Nuances about explaining changes in the weather during a sunny day at the beach
There are many sea breeze diagrams available on the Internet. Some are quite detailed, such as http://www.yachtingmonthly.com/sailing-skills/understand-sea-breeze-49027 ). This is intended for people interested in sailing in coastal waters. Some diagrams show land breezes that occur at night as well as sea breezes that occur during the day, such as http://climate.ncsu.edu/edu/Breezes . This website refers to differences in air pressure that are related to changes in the density of air. As the land warms by energy radiated from the Sun, the air immediately above it also warms by conduction; as the warm air expands, it become less dense than the cool air above it and the warm air rises. This forms a low pressure region over the land. The cool moist air over the water is more dense. This forms a high-pressure region over the water. The cool moist air then flows toward the land, which forms the sea breeze. As this moist air warms over the warm land by conduction, it also expands and rises. This sets up a convection current of warm moist air rising and cool air flowing from the ocean over the land up into the upper atmosphere and back down to the water. The upper atmosphere is cool and as the rising warm moist air cools, the moisture condenses into tiny droplets of water, forming clouds. This complex cycle is an example of the interactions of several of Earth’s systems, the geosphere (rock, soil, and sediments including sand), hydrosphere (water in the ocean and atmosphere), and atmosphere (air over both land and sea).
3. Example of learning and teaching about sea breezes with friends and/or family members
Question 3.10 What happens when teaching friends or family members about the physical phenomena underlying changes in the weather at the beach?
The student who drew the diagram in Fig. 3.22 chose to discuss sea breezes with three children for whom she baby sat: Lucie-7, Ava-4 and Ruby-4:
I did this project with some of the children I babysit, Lucie, Ava, and Ruby. I asked the girls if they had ever heard about sea breezes and what they think when they hear that phrase.
Lucie, said: “um is that like breezes from off of the sea?” Ava and Ruby asked: “is that like when you can see the wind moving leaves and stuff?” I then showed the girls a diagram of sea breezes and asked them their initial thoughts.
Mind you the girls are 4 and 7 so the answers I got are interesting.
Me: Which do you think is warmer when you go to the beach, the water or the sand?
R: “The sand!”
A: “The water….. I mean the sand!”
L: “ the sand, cause sometimes the sand is super hot and burn-y and you have to dig down to where it isn’t as hot.”
Me: Okay so do you think the air above the water is warmer or colder than the air above the sand?
L: “The air above the sand is probably warmer, cause it is being warmed up by the sand.”
Me: Okay good. Does warm air sink or rise do you know?
A:” Um, I think warm air rises.”
A: “ Well cause in our room during the summer it was always way hotter on my top bunk than it was on Ruby’s bottom bunk. And it was way hotter at the top of the house than in the downstairs and dad said its cause hot air likes to go up.”
Me: That’s really good thinking Ava. So do you think that the warm air at the beach warms and rises too?
A: “Well yeah probably.”
Me: Okay so say there is wind that comes and blows the air above the sea over to the sand, what would happen?
L: “well the air would get warmer, cause it would be with the warm sand. Then it would rise up like the hot air in the house does.”
Me: Ruby can you show me on the diagram where you think this is happening?
Ruby pointed to where the arrow points up on the diagram and goes towards the cloud.
R: Right here is where it is happening. It goes up and into a cloud.
Me: Good job Ruby. Okay so when the hot air goes up something called condensation happens, which is like when you have a cold water bottle and the water drops collect on the outside of the bottle. This is because the water inside the bottle is colder than the water in the air outside the bottle. This is also how clouds form. The different air temperatures mix and the water drops form into a cloud because a cloud is actually a mist and a lot of water drops. Now on the diagram what looks like it happens to the cloud?
L: The cloud goes back out to the ocean.
Me: Okay and as the air goes back to the ocean it gets colder, what happens to colder air?
R: Colder air goes down!
Me: Good job Ruby, Ava can you point to where the colder air is going down?
Ava pointed to the arrow of cold air descending back to the top of the water.
Me: Okay now what do you think will happen to the air when it gets back above the water?
L: I think it will keep going in this circle like the diagram says. Cause it will get pushed by sea breezes; then it will get all hot again over the sand. And that will make it rise up into the air and be a cloud. Then it will get pushed back out to sea and sink again when it gets cold. And start again and again.
One thing that I think Lucie in particular learned about was patterns, which described by the article are: “Patterns. Observed patterns of forms and events guide organization and classification, and they prompt questions about relationships and the factors that influence them.” Lucie was able to say that she thought that the air would continue in a cycle and would follow the sea breeze cycle in a pattern that the cold air would always sink and the hot air would always rise.
I also think that the girls participated in engaging in argument from evidence. When Ava explained to me why she thought that the hot air would rise she used evidence she had from her own life to explain what she thought would be true. She was engaged in forming an argument, and backing it up with what she already knew.
Also the girls worked on asking question and on finding a good way to answer the questions that I proposed to them, to think about what the other girls had said, and to make a guess or to answer with that in mind.
I also learned that I am not completely comfortable explaining this phenomena, mostly because I am not sure that I fully understand how this cycle is taking place yet so I was not sure how to phrase questions. I also didn’t do the best job of having the girls try and figure out the diagram on their own. This is something for me to work on in the future when I work with other students.
Physics student, Fall 2015